Title: Dr. Desouky
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Unit 6: Practical Skills in
# Physics II
# A2 Physics
# Dr. Mohamad desouky
# 202 4-202 5Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 1of 161
Graphing Data
Step What To Do How To Do It
1 Identify the
variables
a. Independent Variable -
(controlled by the experimenter)
>
Goes on the X axis (horizontal)
>
Should be on the left side of a data table.
b. Dependent Variable -
(changes with the independent variable)
>
Goes on the Y axis (vertical)
>
Should be on the right side of a data table.
2 Determine the
variable range.
a. Subtract the lowest data value from the highest data
value.
b. Do each variable separately.
3 Determine the scale
of the graph.
a. Determine a scale,
(the numerical value for each square),
that best fits the range of each variable.
b. Spread the graph to use MOST of the available space.
4 Number and label
each axis.
>
This tells what data the lines on your graph represent.
5 Plot the data points.
a. Plot each data value on the graph with a dot.
b. You can put the data number by the dot, if it does not
clutter your graph.
6 Draw the graph.
a. Draw a curve or a line that best fits the data points.
b. Most graphs of experimental data are not drawn as
"connect -the -dots".
7 Title the graph.
a. Your title should clearly tell what the graph is about.
b. If your graph has more than one set of data, provide a
"key" to identify the different lines. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 2of 161
Scientific Graphs
- Extrapolate: extending the graph, along the same slope, above or below measured data.
- Interpolate: predicting data between two measured points on the graph.
- Most scientific graphs are made as line graphs. There may be times when other types would
be appropriate, but they are rare.
- The lines on scientific graphs are usually drawn either straight or curved . These "smoothed"
lines do not have to touch all the data points, but they should at least get close to most of
them. They are called best -fit lines .
- In general, scientific graphs are not drawn in connect -the -dot fashion.
- Below are two examples of best -fit graph lines. One is drawn correctly, the other is not.
Best -Fit Line #1 Best -Fit Line #2 Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 3of 161
Accuracy and Precision
- The accuracy of the measurement refers to how close the measured value is to the true or
accepted value.
- For example, if you used a balance to find the mass of a known standard 100.00 g mass, and
you got a reading of 75.95 g, your measurement would not be very accurate.
- Precision refers to how close together a group of measurement actually are to each other.
- Precision has nothing to do with the true or accepted value of a measurement, so it is quite
possible to be very precise and totally inaccurate.
- In many cases, when precision is high and accuracy is low, the fault can lie with the
instrument. If a balance or a thermometer is not working correctly, they might consistently give
inaccurate answers, resulting in high precision and low accuracy.
- A dartboard analogy is often used to understand the difference between accuracy and
precision. Imagine a person throwing darts, trying to hit the bullseye. The closer the dart hits to
the bullseye, the more accurate the tosses are. If the person misses t he dartboard with every
throw, but all of their shots land close together, they can still be very precise but not accurate. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 4of 161
Errors in Calculated Quantities
Uncertainty (Errors)
- Any experiment will involve a series of measurements, and each of these measurements will
be made to a certain degree of accuracy.
- For example, the calculation of a velocity requires the measurement of the time taken and the
distance travelled.
- Using a stopwatch you may measure the time to the nearest tenth of a second and using a
metre rule, you may find the distance to the nearest millimetre.
- Experimental errors cause a measurement to differ from its true value
Uncertainties (errors) can be split up into t hree different categories:
1- Random uncertainties
- It is defined as an error result from unknown and unpredictable variations
- They occur in all the experimental measurement situations.
- They cannot be eliminated entirely.
- They can be reduced by repeat ing the measurement several times.
- The more measurements which are taken; the closer the mean value of the measurements is
likely to be to the true value of the quantity.
- Taking repeat readings is therefore a way of reducing the effect of random uncertainties (errors) .
2- Systematic error
- It is defined as An error that adds a constant amount to the measurement Or the constant
discrepancy between a measured value and the true value
- These can result due to
a fault in the equipment
design of the experiment
possible zero error such as not taking into account the resistance of the leads when
measuring the resistance of an electrical component or use of a ruler at a different
temperature from the one at which it is calibrated.
- The effect of these cannot be reduced by taking repeat readings.
- They cannot be eliminated entirely.
- The systematic error can be eliminated by redesign ing of the experimental technique or
theoretical analysis. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 5of 161
3- Personal errors
They come from carelessness, poor technique, or bias on the part of the experimenter.
The experimenter may measure incorrectly, or may use poor technique in taking a
measurement, or may introd uce a bias into measurements by expecting (and
inadvertently forcing) the results to agree with the expected outcome
Resolution
it is defined as the smallest quantity to which an instrument can measure .
Mistake
This is the misreading of a scale or faulty equipment.
Anomalous points
It is defined as These are points that lie well outside the normal range of results or they are
away from a line or curve of best fit . They often arise from mistakes in measurement. These
should be recorded and reason for discarding noted by the candidate.
Absolute and Relative uncertainty
Absolute uncertainty and relative uncertainty are two types of error with which every
experimental scientist should be familiar. The differences are important.
Absolute uncertainty :
- It is defined as the amount of physical uncertainty in a measurement .
- Lets say a meter stick is used to measure a given distance. The error is rather hastily made,
but it is good to 1mm. This is the absolute uncertainty of the measurement.
Absolute uncertainty = |Your Result - Accepted (True) Value|
Relative Uncertainty :
- Relative uncertainty gives an indication of how good a measurement is relative to the size of
the thing being measured.
- Lets say that two students measure two objects with a meter stick. One student measures the
height of a room and gets a value of 3.215 meters 1mm (0.001m). Another student measures
the height of a small cylinder and measures 0.075 meters 1mm (0.001m). C learly, the overall
accuracy of the ceiling height is much better than that of the 7.5 cm cylinder. Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 6 of 161
Percentage uncertainty (%U)
It is the difference between the accepted value (Or theoretical value) and the experimental
value divided by the accepted value and the result is multiplied by 100 to make it a percentage.
The percentage uncertainty obviously can be positive or negative; however, we will take the
abs olute value of the difference.
% =
>
or
% = | |
> ( )
Determining the uncertainties in derived quantities
1- Multiplying and dividing:
The percentage uncertainty in a quantity, formed when two or more quantities are combined by
either multiplication or division, is the sum of the uncertainties in the quantities which are
combined.
Example 1
- The following results were obtained when measuring the surface area of a glass block with a 30
cm rule, resolution 0. 1 cm
Length = 9.7 Width = 4.4
- Note that these uncertainties are estimates from the resolution of the rule.
- Use of half resolution = .
>
= 0.05 mm
- This gives the following percentage uncertainty :
Length: = 0.05
> 9.7
100 = 0.5%
Width = 0.05
> 4.4
100 = 1.1%
So the percentage error in the surface area , = 0.5% + 1.1% = 1.6%
Hence surface area = 9.7 4.4 = 42.68 cm 1.6 %
% =
value of thing measured 100
1.6 =
42 .68 100
= 0.685 0.69 2
- The correct result, then, is 42.68 0.69 cm Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 7of 161
Example 2:
- Let us assume we are to determine the volume of a spherical ball bearing.
After performing a series of measurements of the radius using a micrometre screw gauge, the
mean value of the radius is found to be 9.53mm 0.05mm.
% U in r = (0.05/9.53) x 100=0.5%
Thus, the percentage uncertainty in the radius is 0.5%.
The formula for the volume of a sphere is:
=
>
Using this formula, the value for the volume of the ball bearing is found to be 3625.50 mm 3.
=
>
So, % U in volume =3 % U in r =3 0.5% = 1.5%
The volume of the ball bearing = 3625.50 mm 3 1.5% or
% =
. =
.
= .
- The volume of the ball bearing =3625.50 mm 3 54.38 mm 3.Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 8of 161
Example 3:
The following determinations were made in order to find the volume of a piece of wire:
Diameter: d = 1.22 0.02 mm
Length: l = 9.6 0.1 cm
The percentage uncertainties are: pd = 1.6%; pl = 1.0%.
Working in consistent units, and applying the equation 2
4
dV l
= , we have:
V = 448.9 mm 3
The percentage uncertainty, pV = 1.6 2 + 1.0 = 4.2 % = 4 % (to 1 s.f.)
[Note that
and 4 have no uncertainties.]
So the absolute uncertainty u = 448.9 0.04 = 17.956 = 20 (1 s.f.)
So the volume is expressed as V = 450 20 mm 3.
2- Raising to a power (e .g. x2, x1,x )
- The percentage uncertainty in xn is n times the percentage uncertainty in x.
e.g. a period ( T) is as being 31 seconds with a percentage uncertainty of 2 %,
So T2 = 961 4%.
4% 961 = 40 (to 1.s.f)
So the period squared is expressed as T2 = 960 40 s.
- Note: x1 is the same as 1/x. So the percentage uncertainty in 1/x is the same as that in x. Can
you see why we ignore the sign?
- Note: the percentage uncertainty in x is half the percentage uncertainty in x.
3- Multiplying by a constant
In this case the percentage uncertainty is unchanged. So the percentage uncertainty in 3 x or 0.5 x
or
x is the same as that in x.
4- Adding or subtracting quantities
When two (or more) quantities are added or subtracted to calculate a new quantity, we add the
maximum probable uncertainties in each quantity to obtain the maximum probable error in the
new quantity.
Example 4:
- When heating water we may measure the starting temperature to be (35.0 0.5) oC and the
final temperature to be (85 0.5) oC.
- The change in temperature = (85.0 35.0) oC (0.5+0.5) oC or (50.0 1.0) oC. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 9of 161
Note:
In all cases, when the final % uncertainty is calculated it can then be converted back to an absolute
uncertainty and quoted 1 sig. figure. The final result and uncertainty should be quoted to the
same num ber of decimal places
How is the uncertainty in the measurement of a quantity estimated?
a. Estimation of uncertainty using the spread of repeat readings.
Example 1
The following results were obtained for the time it took for an object to roll down a slope.
4.5 s, 4.8 s, 4.6 s, 5.1 s, 5.0 s , 2.5 s
Step 1:
Ignore any anomalous readings (2.5 s is excluded)
Step 2:
The best estimate of the true time is given by the mean which is: 4.5 4.8 4.6 5.1 5.0 4.8s 5
t + + + += =
Step 3:
A reasonable estimate of the uncertainty is half the range :
=
>
, where max x is the maximum and min x the minimum reading of x
= 5.14.5
> 2
= 0.3
The percentage uncertainty, % = 0.3
> 4.8
100 = 6.25 %
Step 4:
The final answer and uncertainty should be quoted, with units, to the same no. of decimal places
and the uncertainty to 1 sig. fig i.e. t = 4.8 0.3 s
Note that
Even if the initial results had been taken to the nearest 0.01 s, i.e. the resolution of an electronic
stopwatch, the final result would still be given to 0.1 s because the first significant figure in the
uncertainty is in the first place after the decimal point. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 10 of 161
b. Estimation of uncertainty using the resolution of a device .
Example 2:
The student uses Vernier calipers to measure the diameter of the cylinder.
The reading she obtains is 24.0 mm.
Calculate the percentage uncertainty in the measurement of the diameter.
Answer:
- The resolution of Vernier callipers= 0.1 mm
- Use of half resolution = 0.1
> 2
= 0.05 mm
Uncertainty = 0.05
> 24
100 = 0.208 % or 0. 21%
2. Estimation of uncertainty from a single reading
- Sometimes there may only be a single reading.
- Sometimes all the readings may be identical.
- Clearly it cannot be therefore assumed that there is zero uncertainty in the reading(s).
- With analogue instruments, it is not expected that interpolated readings will be taken between
divisions (this is clearly not possible with digital instrument anyway).
- Hence, the uncertainty cannot be less than the smallest division of the instrument being used,
and is recommended it be taken to be the smallest division. In some cases, however, it will be
larger than this due to other uncertainties such as reaction t ime and manufacturers uncertainties
Using uncertainties in drawing conclusions
Often an experiment will require a comparison to a known value. This is when the uncertainty
can be used to assess whether the measured value is accurate or not. This can be achieved in
the following ways.
a. Calculating maximum and minimum values
- This method should always be used when the percentage uncertainty in the value is known.
The final uncertainty can be used to determine the range in which the measured value may lie.
If the known value lies within this range then we can say that the measured value is accurate.
Example 1:
- A student used a simple pendulum to obtain a value of g = 10.1 m s 2.
- The experimental percentage uncertainty was calculated as 4%.
- Minimum value of g = 10.1 (10.1 4%) = 9.7 m s 2 & Maximum value of g = 10.1 + (10.1
4%) = 10.504 m s 2 Since the accepted value of g = 9.81 m s 2 lies between the minimum value
& the maximum value , then we can conclude that the measured value of g is accurate. Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 11 of 161
b. Comparing %D with %U
- This method should always be used when the percentage uncertainty in the value is known .
If %D > %U so the measured value is not valid (inaccurate)
If %D < %U so the measured value is valid (accurate)
Example 2:
- A student used a simple pendulum to obtain a value of g = 10.1 m s 2.
- The experimental percentage uncertainty was calculated as 4%.
% (% ) =
= . .
. = . %
%D < %U so the measured value is valid (accurate)
c. Calculating a percentage difference
- If the measured value has been determined from a graph and there is no information about
the percentage uncertainty of the measured value, then percentage difference can be used to
comment on accuracy.
- If the percentage difference is less than 5%, then this is an indication that the result is
accurate.
Example 3:
- A student used a simple pendulum to obtain a value of g = 10.1 m s 2.
- The experimental percentage uncertainty was calculated as 4%.
% (% ) =
= . .
. = . %
As this is less than 5% we can conclude that the measured value of g is accurate.
Note:
No information about % to compare with
% is given or known we have to compare with (see example 2) Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 12 of 161
Percent age Difference (%D):
If the two experimental quantities, E1 and E2, neither of which can be considered the correct
value. The percent difference is the absolute value of the difference over the mean times 100.
% (% ) =
( + )
Example 4:
You are using the simple pendulum to find the value of g.
You got two values 8.20 m s -2 and 8.30 m s -2.
You can now compare both of your measured values of g to each other using a Percent Difference
% (% ) =
( + )
% (% ) = . .
( . + . )
= %
This last calculation shows that your two experimental measurements of g are very close to
each other (within 1%),
Note:
but both values are more than 15 % lower than the accepted value of 9.81 m s -2.
The percentage uncertainty, 9.81 8.20
> 9.81
100 = 16%
The percentage uncertainty, 9.81 8.30
> 9.81
100 = 15% Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 13 of 161
Example 5b: Oct 2019 -WPH13/01
The table shows the values recorded by the five students
The students suggested that they should include in their calculations all the values of t. The
teacher suggested the value recorded by student 4 should be discarded.
(i) Explain why both of these suggestions are reasonable
Answer:
Student
- The values of student are within range of 0.16 s.
- The human reaction time is in range of 0.2 s 0.3 s.
- Reaction time > range of values
Teacher
4 = 0.88 + 0.87 + 0.91 + 0.88
4 = 0.885 0.89
% (% ) = 0.89 0.75
0.89 100 = 15 .7%
% 0.75 4
Advice for Specific apparatus
Metre Rule
- Take the resolution as 1 mm. It should be remembered that all length measurements using
rules actually involve two readings one at each end both of which are subject to uncertainty.
- In many cases the uncertainty may be greater than this due to the difficulty in measuring the
required quantity, for example due to parallax or due to the speed needed to take the reading
- e.g. rebound of a ball, in which case the precision could be 1 cm
Standard Masses
- For 20 g, 50 g, 100 g masses the precision can be taken as being as being 1g this is probably
more accurate than the manufacturers [often about 3%].
- Alternatively, if known, the manufacturers uncertainty can be used.
Digit al meters [ammeters/voltmeters]
- The uncertainty can be taken as being the smallest measurable division.
- Strictly this is often too accurate as manufacturers will quote as bigger uncertainty.
[e.g. 2% + 2 divisions] Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 14 of 161
Thermometers
- Standard -10 oC to 110 oC take resolution as 1 oC
- Digital thermometers uncertainty could be 0.1C.
- However the actual uncertainty may be greater due to difficulty in reading a digital scale as an
object is being heated or cooled, when the substance is not in thermal equilibrium with itself let
alone with the thermometer.
The period of o scillation of a Pendulum/Spring
- The resolution of a stopwatch, used for measuring a period, is usually
0.01s.
- Reaction time would increase the uncertainty and, although in making
measurements on oscillating quantities it is possible to anticipate, the
uncertainty derived from repeat readings is likely to be of the order of 0.1 s.
- To increase accuracy, often 10 (or 20) oscillations are measured.
- The absolute uncertainty in the period [i.e. time for a single oscillation] is then 1/10 (or 1/20
respectively) of the absolute uncertainty in the time for 10 (20) oscillations
- Use half resolution = 0.01
> 2
= 0.005
- e.g. 20 oscillations: Time = 15.8 0 0. 05 s [0. 3%]
(Note: absolute uncertainty in 20 oscillations 0.005 10=0.05 s)
Period =15 .80 0.05
> 20
s = 0.790 0 0.00 25 s
Note that
The percentage uncertainty, % , in the period is the same as that in the overall time.
In this case, % = 0.05
> 15 .8
100 = 0.32 % (2 s.f.)
or , % = 0.0025
> 0.7900
100 = 0.32 %(2 s.f.)
Digit al vernier callipers/micrometer
The resolution is 0.01mm.
Measur ing cylinder / beakers/ burette
- the resolution is 1 cm, but this depends upon the scale of the instrument.
Note
Candidates must be careful to avoid parallax when taking these measurements, and should state
that all readings were taken at eye level. They should also measure to the bottom of the meniscus.
1-11 Sensitivity
- The sensitivity of a measuring instrument is equal to the output reading per unit input
quantity.
- Example
an multimeter set to measure currents up to 20 mA will be ten times more sensitive than one set
to read up to 200 mA when both are trying to measure the same unit current of 1 mA. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 15 of 161
1-12 Reliability
- Measurements are reliable if consistent values are obtained each time the same
measurement is repeated .
Example:
Reliable: 45 g; 44 g; 44 g; 47 g; 46 g Unreliable: 45 g; 44 g; 67 g; 47 g; 12 g; 45 g
1-13 Validity
Measurements are valid if they are of the required data or can be used to give the required data.
Example:
In an experiment to measure the density of a solid:
Valid: mass = 45 g; volume = 10 cm 3
Invalid: mass = 60 g (when the scales read 15g with no mass (zero error );
resistance of metal = 16 ( irrelevant)
1-14 Repeatability
The precision obtained when measurement results are obtained by a single operator using a
single method over a short timescale.
Example:
- A measurement is repeatable when similar results are obtained by students from the same
group using the same method.
- Students can use the precision of their measurement results to judge this.
1-15 Reproducibility
The precision obtained when measurement results are obtained by different operators using
different pieces of apparatus.
Example:
- A measurement is reproducible when similar results are obtained by students from different
groups using different methods or apparatus.
- This is a harder test of the quality of data. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 16 of 161
Glossary
1-Error is defined as The difference between the measurement result (experimental value)
and the true value if the true value is thought to exist
2- Random error is defined as An error result from unknown and unpredictable variations
3- Systematic error is defined as An error that adds a constant amount to the measurement
Or the constant discrepancy between a measured value and the true value
4- Resolution is defined as The smallest measuring interval and the source of uncertainty in a
single reading .
5- Mistake is defined as The misreading of a scale or faulty equipment .
6- Anomalous points is defined as These are points that lie well outside the normal range of
results or they are away from a line or curve of best fit .
7- Uncertainty is defined as The interval within which the true value can be considered to lie
with a given level of confidence or probability
8- Absolute uncertainty is defined as The amount of physical uncertainty in a measurement .
9- True value is defined as The value that would have been obtained in an ideal
measurement with the exception of a fundamental constant the true value is considered
unknowable .
10 - Percentage uncertainty (%U) is defined as The difference between the true value (Or
theoretical/accepted value) and the experimental value divided by the true value and the
result is multiplied by 100 .
11 - Sensitivity is defined as The sensitivity of a measuring instrument is equal to the output
reading per unit input quantity .
12 - Reliability is defined as Measurements are reliable if consistent values are obtained each
time the same measurement is repeated .
13 Validity is defined as Measurement is valid if it measures what is supposed to be
measuring - this depends on the method and the instrument .
14 - Accuracy is defined as How close the measured value is to the true or accepted value .
15 - Precision is defined as How close together a group of measurement actually are to each
other . Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 17 of 161
16 - Repeatability is defined as The precision obtained when measurement results are
obtained by a single operator using a single method over a short timescale .
17- Reproducibility is defined as The precision obtained when measurement results are
obtained by different operators using different pieces of apparatus .
18 - Significant figures (SF) is defined as The number of SF used in recording the
measurements depends on the resolution of the measuring instruments and should usually be
the same as given in the instrument with the fewest SF in its reading .
Micrometer
The micrometer is a precision measuring instrument, used by engineers. Each revolution of the
ratchet moves the spindle face 0.5 mm towards the anvil face. The object to be measured is
placed between the anvil face and the spindle face. The ratchet is turned clockwise until the
object is trapped between these two surfaces and the ratchet makes a clicking noise. This
means that t he ratchet cannot be tightened anymore and the measurement can be read.
> RATCHET
- Use the ratchet
- to avoid squashing the metal
and to reduce random errors Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 18 of 161
Using the first example seen below:
1. Read the scale on the sleeve. The example clearly shows 12 mm divisions.
2. Still reading the scale on the sleeve, a further mm (0.5) measurement can be seen on the
bottom half of the scale. The measurement now reads 12.5mm.
3. Finally, the thimble scale shows 16 full divisions (these are hundredths of a mm).
The final measurement is 12.5mm + 0.16mm = 12.66 Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 19 of 161
Vernier Calliper
The meter scale enables us to measure the length to the nearest millimeter only. Engineers and
scientists need to measure much smaller distances accurately. For this a special type of scale
called Vernier scale is used.
How to use the vernier calliper?
1- Find the millimeter mark on the fixed scale that is just to the left of the 0 -mark on the vernier
scale. (6mm on the fixed caliper Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 20 of 161
2- Look along the ten marks on the vernier scale and the millimeter marks on the adjacent fixed
scale, until you find the two that most nearly line up. (0.25mm on the vernier scale)
3- To get the correct reading, simply add this found digit to your previous reading. (10mm +
6mm + 0.25mm= 16.25 mm) Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 21 of 161
Questions
1- The width of a wooden block is measured using vernier calipers. What is the width of the block?
2- Vernier calipers are shown with the jaws closed. What is the zero error?
3- The diagram shows a vernier scale. What is the reading on the vernier scale?
> The width of the block=
> The reading=
> The zero error=
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 22 of 161
Digital Multimeter
Red meter lead i s connected to Voltage/Resistance or
amperage port
It i s considered the positive connection .
Black meter lead i s always connected to the common
port
It i s considered the negative connection .
Probes are the handles used to hold tip on the tested connection
Tips are at the end of the probe and provides a connection point
Once you know the approximate resistance of the device, then use the switch to select the
lowest range that will still accommodate the resistance of the device. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 23 of 161
Data Logger
- A data logger is an electronic recording instrument that monitors and reports various changes
in environmental conditions over time.
- Uses of Data loggers can measure temperature, relative humidity, light intensity, voltage,
pressure, displacement, acceleration, and velocity.
- Advantage and Disadvantages of Data logger
Advantages Disadvantages
Can be used in remote or dangerous
situations.
If breaks down or malfunctions, some data
could be lost or not recorded.
Can be carrie d out 24 hours a day, 365 days
of a year.
Expensive
Time intervals for collecting data can be very
frequent and regular, for example, hundreds
of measurements per second.
It will only take readings at the logging
interval which has been set up. If something
unexpected happens between recordings,
the data will not be collected.
Can be set up to start at a time in the future Sensors must be carefully calibrated
otherwise they could be taking the wrong
readings
No need to have a person present
Is more accurate because there is no human
error. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 24 of 161
Dat a logger versus liquid -in -glass thermometer to measure the temperature
Dat a logger Liquid -in -glass thermistor and
stopwatch
Number of
readings
Advantage
- Large number of readings
- Small time interval between readings
Disadvantage
- Large number of readings
- Small time interval between
readings
Thermal
Conductivity
Advantage
Metal is a good conductor of heat
Disadvantage
The thermometer is made of glass
and glass is a poor conductor of heat
Graph Advantage
Drawn automatically
Disa dvantage
Drawn manually
Breakages Disa dvantage
Breaks down or malfunction s because
some sensors are very sensitive to
high voltage
Disadvantage
The thermometer is easily broken
Power
Supply
Disadvantage
Power supply is needed
If power fails, some will be lost or not
recorded.
Advantage
No power supply is needed
Cost Disadvantage
expensive
Advantage
Cheaper
Transport Disadvantage
Not easily transported
Advantage
Easily transported.
Errors Disadvantage
May be zero or systematic errors
Disadvantage
May be random, systematic or
parallax errors Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 25 of 161
Dat a logger versus stopwatch to measure time the time taken for a ball to fall
Dat a logger stopwatch
Complexity Disa dvantage
- Needs training, setting up, alignment
issue
Advantage
- Simple to operate or to set up
Reaction
time
Advantage
No reaction time
Disa dvantage
Reaction time error
Precision Advantage
Readings to the nearest millisecond
Disadvantage
Insufficient precision for shortest
times
Graph Advantage
Drawn automatically
Disa dvantage
Drawn manually
Power
Supply
Disadvantage
Power supply is needed
If power fails, some will be lost or not
recorded.
Advantage
No power supply is needed
Cost Disadvantage
expensive
Advantage
Cheaper
Transport Disadvantage
Not easily transported
Advantage
easily transported.
Errors Disadvantage
May be zero or systematic errors
Disadvantage
May be random, systematic or
parallax errors Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 26 of 161
Dat a logger versus liquid -in -glass thermometer & stopwatch to measure the cooling or
heating rate
Dat a logger Liquid -in -glass thermistor and
stopwatch
Number of
readings
Advantage
- Large number of readings
- Small time interval between readings
Disadvantage
- Large number of readings
- Small time interval between
readings
Simultaneity Advantage
Simultaneous reading of temperature
and time
Disa dvantage
Reaction time error when stop watch
is used
Thermal
Conductivity
Advantage
Metal is a good conductor of heat
Disadvantage
The thermometer is made of glass
and glass is a poor conductor of heat
Graph Advantage
Drawn automatically
Disa dvantage
Drawn manually
Breakages Disa dvantage
Breaks down or malfunction s because
some sensors are very sensitive to
high voltage
Disadvantage
The thermometer is easily broken
Power
Supply
Disadvantage
Power supply is needed
If power fails, some will be lost or not
recorded.
Advantage
No power supply is needed
Cost Disadvantage
expensive
Advantage
Cheaper
Transport Disadvantage
Not easily transported
Advantage
Stop watch and thermometer are
easily transportable.
Errors Disadvantage
May be zero or systematic errors
Disadvantage
May be random, systematic or
parallax errors Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 27 of 161
Experiments with radioactivity
- A Geiger -Mller tube (G -M tube) attached to a ratemeter is used to detect radioactivity in the
school laboratory.
- It is used to measure the radioactive count -rate /counts per second (which is proportional to
the number of emissions per second).
Geiger -Muller tube (GM tube)
Errors/Uncertainties
They arise in the use of the GM tube because after it has registered one count, there is a short
interval (known as the dead time ) before it can register another, so any decays occurring in this
period are not registered. This problem is most noticeable when the count rate is high.
- Radioactive sources for experiments are always supplied in a holder, and have low levels of
activity.
- When not in use, they are stored inside a lead castle, in a wooden box; accordingly they will
prevent no danger when stored.
- Safety precautions must always be observed when handling radioactive sources:
1- Always lift with LONG forceps
2- Hold so that the open end is directed away from the body
3- Never bring close to the eyes
- In all experiments, it is necessary to first measure the background radiation for a short time
and obtain an average value.
- This value is then subtracted from all future readings to give the count rate due to the source
alone the corrected count rate .Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 28 of 161
Uncertainty/ Resolution of Specific apparatus
> Device Resolution
> Meter rule:
1 mm
> Micrometer screw gauge
0.01 mm
> Vernier calipers:
0.1 mm
> Standard Masses (1 g, 5 g, 10 g, 50 g, 100 g)
1 g
> Analogy stopwatch
T he smallest measurable division.
> Digital stopwatch
0. 01 s
> Liquid in glass thermometer ( -10 oC to 110 oC)
1oC
Or the smallest measurable division.
> Digital thermometers
0.1 oC
Or the smallest measurable division.
> meters [ammeters/voltmeters]
T he smallest measurable division.
> Measuring cylinder / beakers/ burette
1 cm
Or Smallest m easurable quantity
Explain how repeating readings helps to improve reliability.
1- Identify anomalie s OR identify wrong readings.
2- Allows an average to be taken.
3- Reduces random error OR reduces uncertainty.
Explain how plotting a graph should improve his result ( Discuss the advantages of using a graph).
1- Adding a line of best fit acts as an averaging method
2- Adding a line of best fit can identify anomalous values
3- The gradient value will ignore any systematic error
Or the line/intercept will identify any systematic error [e.g., zero error]
Advantages of f ilming to obtain the data
The human response time is neglected when measuring the time taken.
Can be watched multiple times as much as requires to.
Video can be slowed down and stopped to gather accurate readings. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 29 of 161
To Increase accuracy when data is collected
1- Repeat Measurement taking the average.
2- Consistent precision of timing (to the same number of significant figures/decimal places)
3- Seven or eight trails are needed to plot a graph.
4- Use a wide range of values
Criticise a set of measurements.
1- Inconsistent precision for extension, mass, timingetc. (reference to significant figures/
decimal places.)
2- Lack of precision of
Mass is measured to 1s.f. (1 g, 2 g, 3 g, )
Time is measured to 1 s.f. (1 s, 2 s, 3 s, )
Distance is measured to 1s.f. (1 cm, 2 cm, 3 cm, )
3- Missing unit
4- No evidence of repeat readings
5- Too few values
6- Small range
and are directly proportional
The student concludes that his readings support the relationship.
Explain why the students conclusion may not be correct.
Your answer should refer to the readings taken by the student.
1- There are not enough readings
2- The range of readings is too small
3- The (relationship predicts that the graph should be a straight line
through the origin
4-An accurate best fit line cant be drawn
Or
A straight line graph cant be confirmed Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 30 of 161
Question:
Apart from repeating her readings, state one prec aution she could take to ensure each
measurement is as accurate as possible.
Answer:
Check for zero error and correct it to eliminate systematic error
Measure the thickness at several places along the rule and find the mean to reduce
effect of random error .
Question:
In electric circuit questions, d escribe one safety issue and how it should be dealt with.
Answer:
1- The wire will get hot
2- Turn off the power supply between readings /a dd a protective resistor to the circuit
OR
1- There may be a short circuit
2- Add a protective resistor to the circuit
OR
1- There is a risk of electric shock (from the copper wire)
2- Use insulated wire
- The instruments you would use to measure width of a rule and its thickness are
Width: (Vernier) callipers
Thickness: Micrometer (screw gauge)
- Reason:
To obtain both measurements to 3 SF
- Procedure
1- First check the micrometer screw gauge or digital calipers for zero error.
2. Measure the thickness in four different places and thin divide by 4 to find the average
thickness.
3. Measure the width in four different places and thin divide by 4 to find the average width.
Question:
Apart from repeating her readings, state one prec aution she could take to ensure each
measurement is as accurate as possible.
Answer:
Check for zero error and correct it to eliminate systematic error
Measure the thickness at several places along the rule and find the mean to reduce
effect of random error
Experiment 1: To measure width and thickness of a rule Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 31 of 161
1. First check the micrometer screw gauge or digital calipers for zero error.
2. Take a stack of 20 sheets and compress to remove any air.
3. Measure the thickness of 20 sheets in four different places and thin divide by 4 to find the
average thickness of 20 sheets.
4. = Average thickness of 20 sheets
> 20
.
Student A uses m icrometer screw gauge /v ernier calipers and Student B used m eter rule .
Explain your choice of measuring instruments
Student A (higher accuracy)
Micrometer can measure to 0.01 mm Or 0.001 mm
This gives uncertainty of 0.01 / thickness of one sheet Or 0.001/ thickness of one sheet.
Vernier calipers can measure to 0. 1 mm
This gives uncertainty of 0. 1 / thickness of one sheet
Student B (lower accuracy)
Meter rule can measure to 1 mm
The i dea that uncertainty is 1 mm / thickness of one sheet.
Procedure
1- fold the metal sheet in half.
2- Continue to fold the metal sheet in half until it had been folded five times.
3- Use a micrometer to measure the total thickness of the folded sheet.
4- = 52 = 32
5- =
> 32
Question 1:
State one reason for measuring the thickness of the folded sheet rather than an unfolded sheet.
Answer:
To reduce the percentage uncertainty
Experiment 2 : To measure the thickness of a sheet of paper
Experiment 3: To measure the thickness of an aluminum sheet Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 32 of 161
Question 2
Explain one other technique she should use to ensure that the measurement of the
thickness of the folded sheet is accurate.
Answer:
Repeat at different places and calculate a mean
to reduce random errors
OR
Check for zero error on the micrometer
to eliminate systematic error
OR
Use the ratchet
to avoid squashing the metal and to reduce random errors
OR
Squeeze to remove air gaps
to eliminate systematic error
OR
Take measurements away from the folded edge
as the fold will be thicker and to reduce random errors
Student A: using micrometer screw gauge/vernier calipers
1. First check the micrometer screw gauge or digital calipers for zero error.
2. Measure the thickness of a coin piece in four different places and thin divide by 4 to find the
average thickness of the coin piece .
Student B: using the meter rule
1. Measure the thickness of 20 coin piece s using a meter rule
2. Repeat measurement and take the average
3. = Average thickness of 20 coins
> 20
.
Student A uses m icrometer screw gauge /v ernier calipers and Student B used m eter rule .
Explain your choice of measuring instruments
Student A:
Measuring one coin several times across different diameters gives an average
One coin may be abnormal
Student B:
Measuring 20 gives average for all coins
All coins may not be identical
Experiment 4: To measure the thickness of a coin piece Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 33 of 161
Question 1:
State why the micrometer screw gauge is the most appropriate instrunment to be used to
measure the thickness of the coin.
Answer:
resolution of 0.01 mm is much less than thickness of the coin .
Or resolution of 0.01 mm will give small %U
Question 2:
She takes measurements at different points on the coin.
Explain why this would make the mean value for the thickness more accurate
Answer:
Thickness of coins varies
Identifies anomalies and discard it.
Question 3:
Student A records the following values for coin X:
diameter/mm 25.9, 25.9, 25.9
thickness/mm 1.80, 1.84, 1.82
Use these measurements to calculate the mean value for the volume of coin X.
Answer:
= 1.80 + 1.84 + 1.82
3 = 1.82
= /4 (25 .9 10 3 )2 1.82 10 3 = 9.59 10 7 3
Question 4:
Use the measurements to estimate the percentage uncertainty in the volume.
Answer:
= 1.84 1.80
2 = 0.02 % = 0.02
1.82 100 = 1.1%
= 0.1
2 = 0.05 % = 0.05
25 .9 100 = 0.195%
% = % %
% = (2 0.195% ) + (1.1% ) = 0.39% + 1.1% = 1.49% Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 34 of 161
Question 5:
She measures the mass of coin X as 7.08 g with negligible uncertainty. Calculate the density of
coin X.
Answer:
=
= . 10 3
9.59 10 7 3 = ( )
Question 6:
The student makes the same measurements for coin Y. The value of the density for coin Y is
6900 kg m 3. The percentage uncertainties in the measurements are the same for both coins.
Use these measurements to decide if the coins are made from the same material.
Answer:
% (% ) = 1 2
1
2 ( 1 + 2)
100 = 7380 6900
1
2 (7380 + 6900 )
100 = 6.7%
% = 2 1.9% = 3.8% (The percentage uncertainties are the same for both coins )
Since %D %U so probably not the same material
1- Measure the coiled length of spring with vernier cal ipers and divide by the number of loops
2- Repeat and average
3- Measure in different places/orientations
- Reason for using Vernier caliper :
To obtain both measurements to 3 SF
Remark
(DO NOT use of micrometer screw gauge)
Experiment 5: To measure the thickness of a coin piece Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 35 of 161
Q1: Explain why a metre rule is suitable to measure the length of each side of the sheet.
Answer: resolution is 1mm giving a small %U
Q2: Describe how the student should use this rule to make the measuremen ts as accurate as
possible.
Take repeat readings for both dimensions and average
Q3: In order to determine the thickness, the student is told to fold the sheet in half five times.
Explain why this technique would make the value for the thickness of the sheet more precise.
Answer:
By measuring 32 times the original thickness the mean value of thickness is obtained
Measuring a larger thickness (32 times the original thickness) gives a smaller %U . Since
thickness is now greater and instrument precision is the same
Enables at least 2 SF measurement since thickness is now greater than is now greater
than 0.1mm
Q4: State what instrument the student should use to measure the thickness of the folded sheet.
Answer: Micrometer screw gauge Or digital Vernier calipers. (DO NOT accept vernier callipers)
Reason
resolution of 0.01 mm micrometer screw gauge is much less than thickness of the folder
sheet .
Or resolution of 0.01 mm will give small %U
Experiment 6: To determine the density of a metal in the form of a thin sheet of Aluminum Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 36 of 161
Procedure
1. Sit on a chair positioned sideways next to a desk or table.
2. Your forearm should lay flat on the desk with fingers extending over the edge. The wrist and palm
should stay on the table to prevent downward motion of the hand. The thumb and index finger
extend straight out from the edge of the table with the middle finger, ring finger, and little finger
closed tightly against the index finger.
3. You should keep your thumb and index finger 2 cm apart.
4. Your lab partner stands in front of you, holding the ruler by its 30 -cm end and dangling the 0 -cm
end of the rul er so that it is level with your thumb and index finger.
5. Without any warning, your partner releases the ruler and you try to catch it as quickly as possible.
6. Your partner checks the ruler just above the index finger to see how many centimeters it f ell.
7. Repeat this procedure three times and record the average distance (s) the ruler fell.
Calculations
= + 2
The ruler releases from rest = 0
= 2
= 2
Precautions/ sources of errors
Hold/drop the rule vertically
Release from rest
Ensure your fingers are just at the end of the rule
Practice first before you start collecting data
Repeat measurements taking the average
Use the same bit of your fingers for measurement
Your forearm should lay flat on the desk with fingers extending over the edge. The wrist and
palm should stay on the table to prevent downward motion of the hand.
Experiment 7: How do you measure reaction time without a stopwatch? Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 37 of 161
- Sensors connected to a computer can be used to record the position and velocity of an object
over time.
- The motion detector sends out a short burst of ultra sound. The computer measures the time
it takes for the sound to travel from the detector to an object and back.
- The computer also knows the speed of sound (about 350 m/s). Knowing the speed and time
allows you to calculate distance.
- With these detailed and accurate measurements, computer software can produce graphs of
the data automatically.
Notes
1- Measurements made with electronic sensors will be more precise and more accurate than
measurements made by people using stopwatches and rulers.
2- The electronic measurements will not suffer from human errors such as reaction time or
parallax error. This means that the results and any conclusions drawn will be more reliable.
3- Below are some common problems addressed when the motion sensor is used.
1. The motion detector does not work when objects are placed within 15 cm of the
detector. This is because it takes a certain amount of time to switch between "send" and
"receive" mode.
2. The motion detector can only see objects that are directly in front of it.
3. Sometimes other motion detectors, sound sources and/or irregular reflecting surfaces
can cause problems.
4. On most of the motion detectors, there is a switch on top that will enable you to switch
back and forth between "cart" and "person walking."
Experiment 8: Recording motion Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 38 of 161
1. Place a light gate on the track 1m from the trolley as shown in the figure below.
2. Connect the light gate to a suit able data logger and computer.
3. Measure length (L) of a card using ruler
4. place a card on the trolley.
5. Light gate (connected to computer / datalogger) to measure time (t) for the card of a known
length ( L) to pass the gate.
6. speed = ( )
>
Precautions:
The student thinks that the stopwatch method is less reliable than ICT method
Discuss what makes using a stopwatch less reliable
Answer:
1- Human error in using stopwatch/ no human error using ICT method
2- Different reaction time for different people/ on different occasions
Remark
- When the front edge of a card of a measured
length, l, breaks the beam the timer is started;
and when the back edge of the card passes the
gate, the beam is unbroken and the timer stops,
recording the time interval t.
Experiment 9: Measuring average speed
> Figure 1
> Trolley with card
> Light gate
Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 39 of 161
1. Measuring the initial velocity of trolley at the top of the runway ( ) as follow:
measure length (L) of a card using ruler
Light gate (connected to computer / datalogger) to measure time ( 1) for the card of a
known length (L) to pass light gate 1.
Determine the initial velocity = ( )
> 1
2. Measuring the final velocity of trolley at the bottom of runway ( )
Light gate to measure time ( 2) for the card of a known length (L) to pass light gate 2.
Determine the final velocity = ( )
> 2
3. measure distance ( ) between centers of gates 1&2 using ruler
6. calculate the acceleration:
= 2 2
2
1- the time between frames in a video is known.
2- markers placed on road/track a known stated/distance apart.
3- measure the distance moved over a number of frames.
4- Velocity =
>
5- frames known from the start to give the time to plot velocity against time graph
Experiment 10 : Measuring acceleration
> Figure 2
> Trolley with card
> Light gate 1
> Light gate 2
Experiment 11 : Explain how a velocity time graph can be produced from the recording of a video
camera that is used to film a moving car Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 40 of 161
Method
1. A ball -bearing is held by an electromagnet at height, s, above a trapdoor switch
2. A small iron ball is released by operating the switch, which also starts the electronic timer.
3. When the ball hits the timing gate the lower circuit is broken and the electronic timer stops
4. We can measure the vertical distance traveled by the ball, s, using meter rule and record
the time taken, t.
5. The initial velocity , is zero . The acceleration, , is the acceleration due to gravity, g.
Calculations
Method 1 : Substituting in the equation = + 2 & = 0 gives = 0 + 2
Rearranging this gives: = 2
> 2
- Method 2:
Even better is to repeat the measurements for a range of different
heights and plot a graph.
Plotting on the y -axis and 2 on the x -axis
Comparing the equation = 2 + 0 with the equation for a
straight line = + . The graph will be a straight line through
the origin (since the intercept c = 0) with a gradient equal to
= or = 2
Experiment 12 : Measuring the acceleration of free fall, g Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 41 of 161
Quantities to be measured & explain your choice of measuring instrument
Time & distance
quantity Device used advantage
Time Electronic timer reading to 0.01 s or 0.001 s
Distance (height) Meter rule reading to 1 mm
Independent and dependent variables
Time is independent variable (along x -axis) and the height is dependent variable (along y -axis)
Identify the main source of uncertainty and systematic error :
1- Systematic error: Zero error of the electronic timer.
3- Par allax error when the height is measured using the meter rule.
Explain how the data collected will be used to find the spring constant
Plot height against square of time
Comparing the equation = 2 + 0 with the equation for a straight line = + . the graph
will be a straight line through the origin (since the intercept c = 0) with a gradient equal to
= or = 2
Precautions:
1- The electronic timer is used to eliminate the reaction time to get more accurate measurements and
results.
2- Avoid Par allax error when the height is measured using the meter rule.
3- Re -zero the electronic timer.
Comment on safety
Risk from falling weights so use foot protection,
Random error that would lead to a low value of g (g is underestimated)
1- Air resistance / drag (use a small and dense ball to ignore air resistance)
2- parallax error when height from which the ball is released is measured using meter rule
(might also lead to a high value of g /g is overestimated )
Systematic errors that would lead to a low value of g (g is underestimated)
1- The timer ha s a zero error so t was too large giving a low value of g.
2- delay in releasing ball when the electromagnet is switched off (the electromagnet may retain
some magnetism when it is switched off so the time recorded by the timer would be longer so use
weaker electromagnet)
3- The rule is not vertical to the floor (use set square). Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 42 of 161
In this experiment the student releases a small steel ball in front of a meter rule and uses a
video camera to record its motion.
The camera captures ( n) images per second, which may be played back one image at a time.
Steps:
1- Prepare to drop the ball from near the top of the camera's field of view.
2- Open the camera shutter using 3 -2-1-0 countdown and drop the steel ball.
3- The steel ball is released from rest so the initial speed of the ball is zero (u=0).
4- Choose the first image of the ball after its release. You can define 'time zero'
to be the time at which this image was produced
5- Record the position on the rule for each frame.
6- Calculate the time between frames ( the time between each two
images= 1
> (# ( ))
)
t d
Between
1&2 1 = 1
1 = 1&2
Between
1&3 2 = 2
2 = 1&3
Between
1&4 3 = 3
3 = 1&4
.
.
.. = . .
Between
1&10 10 = 10
10 = 1&10
Calculations
= + 1/2 2 = 0 = 2
2
Plot distance against t 2
= 2
Experiment 13 : Multiflash Photographs of Free Fall
100 cm Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 43 of 161
Systematic errors which could arise
1- The camera has not been calibrated correctly (runs too fast/slow).
2- Ball released before the 1st image so u is not 0.
3- The rule is not vertical to the floor.
4- There is a parallax error from camera to object.
> Precautions
1- Darken the room and shine light on the falling object but not the background.
2- If air resistance is significant compared with the weight of the falling object, then the
gradient of the speed -time graph will decrease. This indicates a decreasing acceleration caused
by the increasing effect of air resistance matching the increasing speed of the object.
> 3-
Use a set square to make sure that the rule is vertical to the floor.
4- The steel ball should have the following properties to be suitable to use in this Experiment
It should be shiny so easy to see on the recording.
It should be streamlined to minimize drag a nd ensure laminar flow so does not affect its
acceleration.
It should be dense so the weight of the ball will be much greater than drag and does not
affect its acceleration.
It should have small surface area to minimize drag and does not affect its acceleration.
It should have smooth surface to minimize drag and ensure laminar flow so does not affect
its acceleration.
The advantages of using a video camera
The advantages of using a video camera to take measurements for this experiment rat her than
using a stopwatch are:
1- No reaction time so uncertainties will be Reduced Or (time recorded) more precise/accurate
2- Can be pa used /stopped to take readings.
Measurements taken at exact times
Positions against rule recorded more accurately.
Velocities can be calculated frame by frame.
3- Allows repeated playback
Allows values to be checked/confirmed
Values obtained are more reliable Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 44 of 161
Procedure
- A cylinder is dropped down a clear plastic tube so that it cuts
through a light beam.
- The length of the cylinder, l, and the height of the top of the
tube, h, above the light gate are measured, and the time, t, for
which the cylinder cuts the beam is recorded.
- The experiment is repeated several times, and an average
value of t is obtained.
Calculations
- The velocity, v, of the cylinder passing through the gate is calculated using
=
h t v v2
h1
h2
h3
.
.
.
h7
Plotting the graph
A graph of v2 on the y-axis and s on the x-axis should be a straight line through the origin.
Gradient =2 g
=
2
Explanation
Comparing v2 = u2 + 2 gh with y = mx + c
since u = 0 (so c = 0 )
and since a = constant (a = g).
v2 = 2 ah with y = mx if u = 0 stated
so v2 is directly proportional to h.
Note
The students value of g is less than the accepted value of 9.81 m s -2. Suggest why.
Experiment 14 : Measuring the acceleration of free fall, g
> v2(m 2/s 2)
> h(m)
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 45 of 161
Random error that would lead to a low value of g
1- Air resistance / drag
2- Collision with sides
3- Friction with side
4- Velocity calculated is an average rather than instantaneous
The experiment has been done in an area of the world where g is lower/ different.
Systematic errors that would lead to a low value of g
1- The timer had a zero error so t was too large giving a low value of g.
2- The distance is measured to the bottom of the light gate (not the middle of the light gate) so
s was too large giving low g.
Suggest what the student should do to obtain accurate values for t.
1- Middle of cylinder should be level with top of tube
2- Drop the cylinder from rest
3- Drop cylinder to prevent touching sides of tube
4- Arrange the tube vertically
5- Repeat readings of t for each value of s and average
6- Check light gate for zero error/ Reset light gate to zero.
7- Light gates should be perpendicular to the tube.
8- Use a longer cylinder
9- Exclude anomalous readings from repeat readings. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 46 of 161
1- Release a metal ball on the slope.
2- Uses a stopwatch to mea sure the time taken for the car to travel down part of the slope.
Question 1:
Suggest a suitable length l for the slope used in this school laboratory experiment.
Answer: 1.0 m -2.5 m
Question 2:
Suggest a suitable height h, above the laboratory bench, for one end of the slope.
Answer: 10.0 cm (h must be less than l/2)
Question 3:
(The ramp makes an angle with the bench as shown. Describe an accurate method to
determine the angle . You may add to the diagram if you wish.
Answer:
Measured height () using a set square (rule)
The length of the plane ( )
= sin 1(
)
Question 4:
Explain why your method gives good precision
Answer:
1- Distances measured are large and can be measured to 3 SF So small percentage uncertainty
2- If protractor is used, the precision of protractor is 1 o. so high percentage uncertainty.
Question 5
State three technique s that can be use d to reduce the uncertainty in this measurement
1- Making marks / lines on track for start and finish
2- Repeats and find average time
3- Constant starting positions
4- Not pushing ball
5- Use light gates / data logger / automatic timer for timing to reduce human reaction error.
6- Method for avoiding parallax error when judging start and finish point s by keeping eye level
with ball.
Experiment 15 : Measuring the acceleration of free fall, g using an inclined plane Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 47 of 161
Aim
The aim of this experiment is to find the centre of gravity of an irregular rod in this case a
broom.
Apparatus required
A broom
Retort stand, boss and clamp
Knife edge (A thin strip of metal can be used).
A set of slotted masses (1 kg in100 gm units)
String
Metre ruler
Experimental instructions
- Clamp the knife -edge in the clamp so that the fine edge is vertical. Balance the broom on the
knife -edge.
- The point on the broom where it balances is the centre of gravity of the broom (G). Mark a cross
on a piece of sticky tape and fix it to the broom at G.
> In order to balance the ruler on your finger, you must position it so
> that the point at which the moments of pull of gravity acting to the
> right and left of your finger are equal .When you position the
> irregular rod on your finger so that it is perfectly balanced, then you
> have found the centre of gravity.
Suspending the rod horizontally at the centre of gravity using a thin string or thread to find the
position of the centre of gravity.
Experiment 16 : Find the centre of gravity of irregular rod
> Method 1
> Method 2
> Method 3
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 48 of 161
Part 1:
1. Using the set up shown in the figure, the acceleration can be measured for various values of
the resultant force acting on the trolley while the mass is kept constant.
2. By plotting a graph of acceleration against resultant force, a straight line will show that
acceleration is proportional to the resultant force.
Total mass = mass of trolley + mass of string + hanging masses
Note:
To increase force and keep total mass constant we move masses from trolley to hanger.
Experiment 17 : Investigating the relationship between F, m and a
=
> acceleration
force
> masses
> masses
> es
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 49 of 161
Part 2:
3. Using the same set up shown in the figure, the acceleration can be measured by adding
masses to the trolley while the resultant force acting on the trolley is kept constant .
4. A graph could also be plotted for varying masses of trolley while the resultant force is kept
constant.
5. By plotting a graph of acceleration against 1
>
, a straight line will show that acceleration is
inversely proportional to the mass.
Note:
- It is clear from the first table that F is directly proportional to acceleration ( ).
- It is clear from the second table that a is inversely proportional to the mass ( 1
>
).
- These two relationships can now be combined:
>
or
- Another way to express this is: = and = 1
> acceleration
> 1
>
=
= Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 50 of 161
Procedure
1. A partner hold a meter stick vertically, with the zero end on the floor .
2. Hold the bottom of a ball at a given height from the floor (h i) and release it.
3. You must quickly note the height (h f) to which the bottom of the b all returns on its first
bounce .
4. Repeat three more times and take the average of the bounce height (h f).
Initial Height
( )
Bounce height
trial 1
Bounce height
trial 2
Bounce height
trial 3
Bounce height
average
( )
% KE
lost
Calculations
To find the percentage of energy lost after the bounce you can assume that the energy is
directly proportional to the height and that the percentage change in height will also express
the percentage change in kinetic energy of the ball.
Initial KE: = =
Final KE: = =
% =
100 =
100
% =
100
Precautions:
Use a set Square to make sure that the ruler is vertical to the floor to avoid parallax error.
Experiment 1 8: Determination the percentage loss of kinetic Energy of a bouncing ball Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 51 of 161
Galileos Pendulum experiment in which he determined that the pendulum would always
return to the same height illustrates the conservation of mechanical energy nicely.
By careful measurement of the height a pendulum rises and falls through its swing, we can
determine the gravitational potential energy ( = ) it loses and gains throughout one
oscillation.
This can then be compared with the kinetic energy ( = ) it
has as it passes through the lowest point.
This experiment will show that the energy is constantly being
transferred from kinetic to gravitational potential and back again.
Values should be known :
- Measure the mass of the pendulum bob using a balance.
- Measure the height from which the bob is released
- Measure the velocity using the light gate.
Experiment 19 : Pendulum Energy Exchange Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 52 of 161
Precautions:
1. The pendulum should be suspended from two points as shown for better results.
This helps keep the swing of the pendulum in the plane perpendicular to the photo gate.
2. Now adjust the position of the photo gate as accurately as you can so that the photo gate
beam strikes the center of the pendulum bob.
3. Avoid parallax error when the height is measured using the meter rule.
4. Make sure that the meter rule is perpendicular to the bench using a set square.
- By pulling the brick up at a constant velocity, and using a constant force.
- You can measure the force ( F) using the force meter.
- You can measure the velocity using the data logger.
- You could calculate the output power from =
- You can calculate the time taken ( ) to lift the brick vertically using a stopwatch.
- You can measure the vertical height ( ) using the brick rises vertically.
- You can measure the total mass of the brick ( ) and the card using a digital balance.
You could calculate the power from =
>
- =
>
100
Experiment 20 : Investigating power and efficiency Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 53 of 161
Remark
The efficiency is less than 100% because > due to the work done by the block against
the friction while sliding up.
1- A person climbs 'n' starts of height 'h' m
2- His/ her friend records the time
3- Let the man of the person be 'm' kg
4- Calculate the power =
>
Apparatus required
Digital stopwatch meter rule tape - digital balance
Quantities to be measured & explain your choice of measuring instrument
mass, height , time
quantity Device used advantage
Time Electronic timer Resolution is 0.01 s or 0.001 s
Distance (height) Meter rule Resolution is 1 mm
Mass balance Resolution is 0.1 kg
Comment on whether repeat readings are appropriate in this case
1- Student may become tired/veloc ity may change/power may change so r epeat after rest
2- Repeat height measurement to reduce parallax error when the meter rule is used to measure
height.
3- Difficulty of starting and stopping (visual or reaction time)
Explain how the data collected will be used to determine the power
1- The height of one stair is measured using the meter rule and the multiply by number of stais
he climbed to measure the total vertical height (h).
2- =
3- =
>
Experiment 21 : Investigating the power of a person Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 54 of 161
Identify the main sources of unce rtainty and/or systematic error
1- Reaction time
2- Difficulty of dealing with differen t starting and stopping points.
3- Parallax error when the height of stairs is measured using the meter rule.
4- Zero error in the balance.
Comment on safety
1- May be collisions if other people are on stairs, ensure that there are none.
2- Wear approp riate footwear to avoid falling or damage to feet.
3- Use of headw ear/kneepads to prevent injury when falling.
1- Measure the mass ( ) of the block to be raised using a balance .
2- Start the stopwatch and start the motor.
3- Measure the height ( ) through which mass is raised.
4- Record the time needed ( ) to raise the mass.
5- Measure voltage ( ) using voltmeter
6- Measure current ( ) using ammeter
7- Repeat for different masses
Explain how the data collected will be used to determine the efficiency (by calculation)
Output power: =
>
or output energy: =
Input power: = or input energy: =
% =
>
100 or % =
>
100
Experiment 22 : Investigating the efficiency of an electric motor Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 55 of 161
Explain how the data collected will be used to determine the efficiency (graphical method)
1- Output power: =
>
or output energy: =
2- Input power: = or input energy: =
3- Plot a graph of output power (change in GPE) on the y -axis against input power (Electrical
energy supplied) on the x -axis.
4- The gradient is the efficiency of the motor
Identify the main sources of unce rtainty and/or systematic error
1- Reaction time
3- Parallax error when the height is measured using the meter rule.
4- Zero error in the balance.
Comment on safety
Wear approp riate footwear to avoid falling or damage to feet.
Question 1 :
Power produced by the motor is much more than the power calculated. Explain why .
Answer:
- Some power is lost as heat and sound
- Energy transferred to surroundings as heat / sound due to friction between the moving parts of
the motor .
- Mass also has KE;
- Mass of string has been ignored
Question 2 :
The students repeated the experiment with a much larger mass and determined that the
efficiency of the motor was decreased.
Describe how they should collect and use data to determine the mass at which the efficiency
starts to decrease.
Answer:
1- Continue increasing the mass and extend the graph
2- Identify the mass/point at which the line starts to curve
3- Take smaller increments in mass around this point Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 56 of 161
- As the block falls it loses gravitational potential energy and the trolley and block together gain
kinetic energy.
Procedure:
1. Measure the mass M of th e trolley and the mass m of the falling block.
2. Pull back the trolley so that t he block is close to the pulley and release the trolley.
3. Measure the distance d fallen by the block.
4. Measure the time t it takes to fall.
5. The final velocity is given by = 2
>
6. Calculate the gravitationa l potential energy lost and the kinetic energy gained.
7. Divide the kinetic energy by the gravitational potential energy. This is the fraction required.
Quantity to be
measured
Instrument resolution of measuring
Masses, M and m Balance At least 0.1 g
Distance, d Metre rule 1 mm
Time, t Stopwatch 0.01 s
Question1 : Describe how you would measure d accurately
Answer:
1- Vertical rule with use of set square
2- Use of Set Square at bottom of block Or Eye level with bottom of block
3- Rule close to block Or Marker shown
Question 2 : Describe how you would make each measurement of t accurate
Answer:
1- Start stopwatch when block/trolley released and stop when block hits floor
2- Repeat several times and average
Question 3: Comment on any safety aspect of this experiment.
Answer:
1- Identifies an appropriate risk and suitable precaution .
2- Explain s why risk is insignificant.
Experiment 2 3: to find out what fraction of the gravitational potential energy becomes kinetic
energy Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 57 of 161
Making measurements & Explain your choice of measuring instruments
Meter rule measures original length ( ). (Resolution of rule is 1 mm to reduce percentage
uncertainty )
Digital balance to measure mass m (Resolution of digital balance is 0.1 g/0.0 1 g to reduce
percentage uncertainty ).
Add a mass m = 100 g to the end of the spring .
measure the new length .
Repeat several times by increasing the load gradually and record the new length produced
each time.
Recording and presenting your data
1) Record your values for m and x in a table of results.
Mass , / , / , /
3) Plot a graph of load (y-axis) against extension (x-axis).
Experiment 24 : To determine Spring Constant/ to Investigate Hooks Law /to determine
the relation between load and extension
> load/N
> x/m
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 58 of 161
Analyzing your data
1) Calculate the gradient (slope) of your graph.
2) The spring constant ( ) = the gradient of the linear region
Note: = /
The independent variable and the dependent variable
The independent variable: Load/weight
The dependent variable: extension
Explain how the data collected will be used to find the spring constant
1- , =
2- , =
3- Plot load (y -axis) against extension (x -axis)
4- Gradient of the linear region = spring constant,
Suggest two ways in which the accuracy of the measurements taken could be improved.
1) Avoid parallax error when the length or the extension of the wire is measured.
2) use set square to make sure that rule is vertical to the floor when length of spring is measured.
Identify the main sources of uncertainty and/or systematic error
1) Parallax error when the length or the extension of the wire is measured.
2) rule is NOT vertical to the floor when length of spring is measured.
Comment on safety
1) Risk from falling weights so use foot protection,
2) Risk from breaking spring so use goggles]
3) It needs to be able to take a large force without breaking so it is suggested that it is wrapped
round several times to prevent accidental breaking. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 59 of 161
Points to note:
Common support means that any sagging of the support
is irrelevant.
The reference wire P and the test wire Q are made from
the same material and have the same length to en sure
expansion is not a problem and to reduce the effect of
temperature and the effect of sagging
Small fixed weight to keep P taut.
Wires are long and thin too ensure extension is as large as possible.
Making measurements & Explain your choice of measuring instruments
Vernier arrangement measures the extension ( ). (Vernier arrangement can measure to
0. 1 mm to reduce percentage uncertainty ).
Digital balance to measure mass m (Resolution of digital balance is 0.1 g/0.0 1 g to reduce
percentage uncertainty ).
Measure the diameter d of the piece of wire with a micrometer screw gauge. ( Resolution of
micrometer screw gauge is 0.01 mm to reduce percentage uncertainty )
Add a mass m the end of the wire and then measure the extension x of the section of wire.
Continue to increase the load on the w ire by inc reasing the mass m. For each load, record
the extension x.
Rec ording and presenting your data
1) Calculate the cross -sectional area A of the wire using the equation
= =
2) Record your values for m and x in a table of results. Include columns for F and for the stress
and the strain, where
Mass ( ) = =
=
Experiment 25 : Experimental Determination of the Young Modulus (Method 1) Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 60 of 161
3) Plot a graph of stress ( y-axis) against strain ( x-axis).
Analyzing your data
1) Calculate the gradient (slope) of your graph.
2) The Young modulus E = the slope of the linear region
The independent variable and the dependent variable
The independent variable: st ress
The dependent variable: str ain
Explain how the data collected will be used to find the young modulus
1- , =
2- , =
3- measure diameter (d) of the wire using micrometer
4- radius (r) = diameter/2
5- Calculate the cross -sectional area = =
6- =
>
7- =
>
3- Plot stress against strain
4- Gradient of linear region = Young modulus
Suggest two ways in which the accuracy of the measurements taken could be improved.
1) A long wire should be used. Explain why.
Answer: If you make the wire long, you get a v alue for the extension which is bigger and easier
to measure to higher accuracy. The extension {the change in length} will be greater for a given
applied force .
2) Repeat measuring diameter this several times along the length of the wire, rotating the wire
each time and take the average .
3) The reference wire P is used . Explain why.
Answer: To reduce the effect of the change in temperature and the effect of sagging.
4) Avoid parallax error when the length or the extension of the wire is measured.
5) Check for zero error on digital balance
> Stress /Pa
> Strain
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 61 of 161
Identify the main sources of uncertainty and/or systematic error
1) A short wire should be used. Explain why.
Answer: If the wire is short , you get a v alue for the extension which is small and harder to
measure leading to lower accuracy.
2) Parallax error when the length or the extension of the wire is measured.
3) zero error on digital balance
Comment on safety
1) Risk from falling weights so use foot protection,
2) Risk from breaking spring so use goggles]
3) It needs to be able to take a large force without breaking so it is suggested that it is wrapped
round several times to prevent accidental breaking.
List any additional apparatus you might need
Set Square to make sure that the wire is level with the bench
Procedure
1) Clamp one end of the wire between two wooden b locks and to the bench with a G clamp.
2) Attach the pulley to the end of the bench or table and hang the end of the wire over it so
that it hangs just below the pulley and tie a loop of string.
3) Place a small piece of tape on the wire around 25 cm from t he pulley. This will act as the marker .
Making measurements & Explain your choice of measuring instruments
4) Measure the diameter d of the piece of wire with a micr ometer screw gauge. ( Resolution of
micrometer screw gauge is 0.0 1 mm to reduce percentage uncertainty )
5) Fix a meter rule (Resolution of rule is 1 mm to reduce percentage uncertainty ) to the table
directly below the w ire so that you can measure the length of the wire and its extension as
loads are added.
Experiment 26 : Experimental Determination of the Young Modulus (Method 2) Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 62 of 161
6) Digital balance to measure mass m (Resolution of digital balance is 0.1 g/0.0 1 g to reduce
percentage uncertainty ).
7) Measure the unstretched length l of the section of wire from the end of the wi re clamped to
the bench to the tape marker. Repeat it 3 times and take the average.
8) Add a mass m to the end of the wire. A fter a brief while, measure new length.
9) Continue to increase the load on the wire by increasing the mass m. For each load, let the
wire settle and the record the extension x. Continue until you have six sets of readings for m
and x. Take care when adding the loads in case the wire snaps.
Rec ording and presenting your data
1) Calculate the cross -sectional area A of the wire using the equation
= =
2) Record your values for m and x in a table of results. Include columns for F and for the stress
and the strain, whe re
Mass ( ) = =
=
3) Plot a graph of stress ( y-axis) against strain ( x-axis). Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 63 of 161
Analyzing your data
1) Calculate the gradient (slope) of your graph.
2) The Young modulus E = the slope of the linear region
Note: = /
The independent variable and the dependent variable
The independent variable: str ess
The dependent variable: str ain
Explain how the data collected will be used to find the young modulus
1- , =
2- , =
3- measure diameter (d) of the wire using micrometer
4- radius (r) = diameter/2
5- Calculate the cross -sectional area = =
6- =
>
7- =
>
3- Plot stress against strain
4- Gradient of linear region = Young modulus
Suggest two ways in which the accuracy of the measurements taken could be improved.
1) A long wire should be used. Explain why.
Answer: If you make the wire long, you get a v alue for the extension which is bigger and easier
to measure to higher accuracy. The extension {the change in length} will be greater for a given
applied force .
2) Repeat measuring diameter this several times along the length of the wire, rotating the wire
each time and take the average .
3) The reference wire P is used . Explain why.
Answer: To reduce the effect of the change in temperature and the effect of sagging.
4) Avoid parallax error when the length or the extension of the wire is measured.
5) Check for zero error on digital balance
Identify the main sources of uncertainty and/or systematic error
1) A short wire should be used. Explain why.
> Stress /P
> a
> Strain
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 64 of 161
Answer: If the wire is short , you get a v alue for the extension which is small and harder to
measure leading to lower accuracy.
2) Parallax error when the length or the extension of the wire is measured.
3) zero error on digital balance
Comment on safety
1) Risk from falling weights so use foot protection,
2) Risk from breaking wire so use goggles
3) It needs to be able to take a large force without breaking so it is suggested that it is wrapped
round several times to prevent accidental breaking.
Apparatus Hazard Risk Precaution
Support topples over hits
experimenter
secure
support to
bench with
G-clamp
Wire snaps/breaks specific risk e.g. injury to eye wear goggles
Hanging weights falls specific risk
e.g. injury to feet
keep feet away from
apparatus
Or place box of sand
under
apparatus
Or wear Protective shoes Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 65 of 161
Theory:
- Stokes Law can be verified, by dropping ball bearings into a thick liquid, like glycerol, and
measuring the rate of sinking once terminal velocity has been reached.
- Stokes Law gave us this equation for terminal velocity:
= 2 2 ( )
9
Apparatus
- Glycerol, measuring cylinder, ball bearing, light gates, rule, timing device, & micrometer.
The quantities to be measured
Diameter, distance, time
Or diameter, velocity
quantity Device used advantage
Diameter Micrometer Reolution is 0.01 mm or 0.001 mm
Distance (d) metre rule Resolution is 1 mm
Time (t) stopwatch Resolution is 0.01 s
Experiment 27 : Stokes Law in action
# ddDr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 66 of 161
Calculations
The radius ( ) of several steal ball bearings is to be calculated ( =
> 2
) .
- Calculate the terminal velocity of each ball bearing ( =
>
)
- calculate the square of the radius ( 2)
- Plot a graph of terminal velocity (y -axis) against 2 (x -axis).
- = 2 ( )
> 9
The independent and dependent variables :
1- The independent is the radius (r 2 along x -axis)
2- The dependent variable is the terminal velocity (along y -axis).
Identify the main source of uncertainty and systematic error:
1- Terminal velocity not reached
2- Reaction time
3- Temperature not constant
4- Micrometer zero error when the diameter is measured.
5- Parallax error when the distance fallen is measured
Safety Precautions
1- Mop up spills
2- Wear goggles to avoid splashes in eye
3- Use gloves if allergic to oil
4- Normal laboratory rules should be followed
5- Low risk experiment
NOTES:
1- If a student needs to time how long the ball bearing takes to fall a set distance using a
stopwatch (if the light gates are not available).
The following would improve the accuracy of the measurements:
a- Repeating the readings and calculating an average
b- Ensuring that readings are taken at eye level
2- Advantages of filming to obtain the data
a- The human response time is neglected when measuring the time taken.
b- Can be watched multiple times as much as requires to.
c- Video can be slowed down and stopped to gather accurate readings.
> Terminal velocity
> r2(cm 2)
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 67 of 161
A student wants to find the density of a metal block by using a top pan balance to measure the
upthrust acting on the key when it is suspended in water.
1- Measure the mass of an empty measuring cylinder as 1 using balance .
2- Put water of Volume V into the cylinder
3- Measure the mass of water and cylinder as 2 using balane.
4- The mass of water in the cylinder = 2 1
5- Calculate the density of water ( =
>
)
6- When an object is submerged in a fluid it experiences an upthrust equal to the weight of fluid
displaced. The balan ce reading will increase ( = ).
7- Suspend the metal key in the w ater and notes that the balance reading increases to 3.
8- Calculate the upthrust = ( 3 2)
9- The upthrust U on the key is given by = where V is the volume of the metal block
and is the density of the water.
10 - Calculate the volume of the key.
11 - Measure the mass of the metal key using balance
12 - Calculate the density of the metal key =
>
Identify the main sources of uncertainty and/o r systematic error:
1- Systematic/zero error on balance
2- The line of eye's sight should be perpendicular to the bottom of the meniscus scale to avoid
parallax errors.
Experiment 28 : Using Upthrust force to measure density of a metal key Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 68 of 161
Procedure Part 1: Force Measurement method - Weight
1. attach the cylinder to a spring balance and measure its weight in air W1.
2. Partially fill a beaker with water, then completely submerge the cylinder.
3. Measure the weight of the cylinder while it is submerged. Calculate its weight in water, W2.
4. The difference between the objects weight in air and its weight in water is the buoyant force
on the object
Upthrust force (buoyant force) = W 2-W1
Making measurements & Explain your choice of measuring instruments
Spring balance to measure weight W (Resolution of spring balance is 0.1 N to reduce percentage
uncertainty ).
Precautions
1- Zero error on the balance
2- Do not allow the cylinder to touch the sides of the container .
3- Eye level is perpendicular to the reading of the balance.
Safety Precautions
Wear safety shoes to avoid the risk of the falling masses
Experiment 29 : measure the buoyant force (Upthrust force) on a metal cylinder .Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 69 of 161
Part 2: The Displacement Method -displacement can
1. Fill the displacement can with water such that its water level touches the spout of the
displacement can.
2. Keep a beaker whose mass m1 is recorded, at the mouth of the spout of the displacement
can.
3. Gently immerse the metal cylinder into the water .
4. Collect the water that has overflown in a beaker till the last drop that comes out of the
spout.
5. Measure mass of the beaker with water ( m2).
6. Calculate the weight of the displaced water = (m2- m1) .
7. Upthrust force = the weight of the displaced water
Making measurements & Explain your choice of measuring instruments
digital balance to measure mass m (Resolution of the balance is 0.1 g to reduce percentage
uncertainty ).
Precautions
1- zero error on the balance
2- water level touches the spout of the displacement can.
Safety Precautions
Wear safety shoes to avoid the risk of the falling masses Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 70 of 161
Part 3: The Displacement Method -Measuring cylinder
1. Partially fill the graduated cylinder with water.
2. Record this initial volume, 1.
3. Gently immerse one of the metal cylinders into the water. Record the new water level, 2;
4. calculate the change in volume. This is the volume of displaced water = 2 1
5. Calculate the weight of displaced water =
6. Upthrust force = the weight of the displaced water
Making measurements & Explain your choice of measuring instruments
Measuring cylinder to measure volume (Resolution of the measuring cylinder is 1 cm 3 to reduce
percentage uncertainty ).
Precautions
Eye level is perpendicular to the bottom of meniscus.
Safety Precautions
Wear safety shoes to avoid the risk of the falling masses Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 71 of 161
Method
1- Place the Perspex (glass) block on the white paper and
trace the outline.
2- Use a ray box to shine a single incident ray onto the
block in a dark room, mark its path and draw the incident
ray with a ruler.
3- Mark the path of the ray emerging from the b lock,
draw ray with a ruler.
4- Remove the block, connect the entry and exit points to
show the path of light inside the block.
5- Draw a normal at the entry point, measure the angle
of incidence (i) and angle of refraction (r ) using a
protractor .
6- Repeat for a range of angles of incidence
Sources of errors
1- the resolution of the Protractors is 1 o or 0.5 o so use a
large angle to reduce percentage uncertainty.
2- Ray is thick, line drawn might not be accurate so use
a laser with a narrower beam.
3- Pencil line is thick so use a very sharp pencil.
4- Parallax error when the angle is measured using the
protractor so use a range of angles and determine mean
value of refractive index by drawing a graph.
5- Too few values
6- Limited range of values
Precautions
1- Use a large angle to reduce percentage uncertainty.
2- Use a laser with a narrower beam.
3- Use very sharp pencil.
4- Use a wide range of angles and determine mean value of refractive index by drawing a graph.
5- Repeat measurements at second face.
6- Seven or eight trails are needed to plot a graph.
Experiment 30 : Investigate and explain how to measure refractive
index: Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 72 of 161
Calculations
Plot a graph along -axis against along -axis
Calculate the gradient of the graph to get the refractive index of the transparent material.
Gradient = n = rise
> run
= sin i
> sin r
Calculating that the uncertainty in the value of n
The resolution of the protractor is 1
Use of their uncertainty in equation with max and min values
Example of calculation
If = 60 and = 34
nmax = sin (60 + 1)/sin (34 1) = 1. 61
nmin = sin (60 1)/sin (34 + 1) = 1. 49
= +
2 = 1.61 + 1.49
2 = 1.55
=
2 = 1.61 1.49
2 = 0.06
% 0.06
1.55 100 = 3.87%
Question
The relationship between the refractive index n, the angle of incidence and the angle of
refraction is given by the equation
=
Describe a graphical method she could use to determine n.
Answer:
1- Measure for different
2- Measure at least 5 pairs of angles
3- Plot graph of against
4- Refractive index is the gradient of the line Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 73 of 161
Note
- The photograph shows a spectrometer. This can be used to measure angles when
investigating refraction.
- Light emerges from the collimator in a narrow parallel beam.
- A Vernier scale allows the angles to be measured with a resolution of 0.1
Question:
Using a spectrometer reduces the uncertainty in the value of the refractive index of the
material compared with using a ray box and a protractor. Justify this statement.
Answer:
- The resolution of the spectrometer is 0.1 o compared to the resolution of the protractor which is 1 o
- Beam from the collimator is narrow er than the ray from a ray box
- So, uncertainty in angle of refraction is smaller Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 74 of 161
Apparatus
_ 60 Hz string vibrator
_ String
_ Mass set and hanger
_ Metric tape
_ Table support rod
_ Rod clamp
_ Table pulley
_ Digital balance
Procedure
1- Attach the string to the vibrator, run it over the table p ulley, and attach the other end to a
mass hanger.
2- Adjust the vibrator so that the wave amplitudes will be hori zontal to the top of the table.
3- Record the frequency of the vibrator.
4- Carefully measure and record the vibrating length (L) of the string from the end of the
vibrator to the top of the pulley. This distance is the length of string into which the standing
wave segments must fit.
5- Add weights to increase the tension in the string until resonance occurs and it is vibrating in
the fundamental tone or first harmonic (two nodes, one at each end of the string, with one
antinode in the middle).
6- Now reduce the mass on the weight hanger and determine the values of the mass that
produce standing wave patterns with increasing numbers of nodes. For each of 5 more standing
wave patterns (second and higher harmonics)
Note
Using a stroboscope, the motion of the cord can be frozen or slowed down
The frequency of the stroboscope can be adjusted by matching the frequency of a stroboscope
to that of the signal generator until the string is seen to be moving slowly or even frozen.
Experiment 31: Investigation of the variation of wave speed in a stretched string with tension Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 75 of 161
Making Measurements & Explain your choice of measuring instruments
1- Measure and record the length of the vibrating string using a meter rule . (resolution of rule
is 1 mm to reduce percentage uncertainty )
2- Measure the total mass of the hanger and the masses on a digital balance . (resolution of
digital balance is 1 m g to reduce percentage uncertainty )
Independent and dependent variables
Tension (T) is independent variable (along x -axis) and 2 is dependent variable (along y -axis)
Explain how the data collected will be used and sketch the expected graph.
1- For each experimental standing wave pattern, determine
the wavelength of the waves
2- Then use the frequency/wavelength formula to determine
wave speed .
3- Then use the hanging mass for each case to determine the
tension in the string. The tension will be mg , where m is the
total mass (including hanger) and g is the acceleration due to
gravity, 9.81 ms -2.)
4- Plot the square of the speed ( 2) vs. tension ( ) and draw a best -fit line passing through the
origin.
5- The gradient = 1
>
= 1
>
# of vibrating
segments
Wave length (m)
= 2
Wave speed (ms -1)
=
Tension (N)
=
2 ( 2 2)
( )Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 76 of 161
Identify the main sources of uncertainty and/or systematic error
1. Zero error on digital balance.
2. Parallax error when the length of the string is measured.
Comment on safety
1) Risk from falling weights so use foot protection
2) Risk from breaking wire or so use goggles.
Uncertainty calculations
1- Measure and record the total length of the string (resolution of rule is 1 mm to reduce
percentage uncertainty ).
2-Measure the mass (m) of the string on a digital balance (resolution of digital balance is 1 m g
to reduce percentage uncertainty ).
3- Use the measured mass and length to determine the linear mass density ( =
>
)of the
string in units of kg. m-1.
4- The experimental value of linear mass density is the one calculated using the gradient of your
graph ( )
% = | |
100
1- Attach the string to the vibrator, run it over the table pulley, and attach the other end to a
mass hanger.
2- Adjust the vibrator so that the wave amplitudes will be horizontal to the top of the table.
3- Add weights to the mass hanger to apply a tension (T) on the string.
4- Carefully measure and record the vibrating length ( L) of the string from the end of the
vibrator to the top of the pulley.
4- Slowly increase the frequency on the signal generator until resonance occurs and the string is
vibrating at its fundamental frequency.
5- Record the length of the vibrating string ( L) and its fundamental frequency ( f).
6- Repeat this procedure for different lengths of the stretched string using the same Tension (T).
Experiment 32: Investigation of the variation of the fundamental frequency of a stretched
string with length Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 77 of 161
Making Measurements & Explain your choice of measuring instruments
1- Measure and record the length of the vibrating string using a meter rule . (resolution of rule
is 1 mm to reduce percentage uncertainty )
2- Measure the total mass of the hanger and the masses on a digital balance . (resolution of
digital balance is 1 m g to reduce percentage uncertainty )
3- Measure the frequency of the vibrator (signal generator).
Independent and dependent variables
(1
>
) is independent variable (along x -axis) and ( ) is dependent variable (along y -axis)
Explain how the data collected will be used and sketch the expected graph.
1- Record the length of the vibrating string ( l) and its fundamental
frequency ( f).
6- Repeat this procedure 6 more times for different lengths of the
stretched string using the same Tension (T).
4- Plot the frequency ( f) vs. ( 1
>
) and draw a best -fit line passing
through the origin.
5- The gradient = 1
> 2
>
1- Attach the string to the vibrator, run it over the table pulley, and attach the other end to a
mass hanger.
2- Adjust the vibrator so that the wave amplitudes will be horizontal to the top of the table.
4- Carefully measure and record the vibrating length ( L) of the string from the end of the
vibrator to the top of the pulley.
4- Add weights to increase the tension in the string.
5- Slowly increase the frequency on the signal generator until resonance occurs and the string is
vibrating at its fundamental frequency.
5- Record the tension ( T) in the vibrating string (L) and its fundamental frequency ( f).
6- Repeat this procedure for different tensions using the same length of the stretched string.
7- every time, Increase the tension in the string and and slowly increase the freque ncy on the signal
generator until resonance occurs and the string is vibrating at its fundamental frequency again.
> f/ Hz 100 150 200 250 300 350 400
> 1/l ( m1)
( )
1
( 1)
Experiment 33: Investigation of the variation of the fundamental frequency of a stretched
string with tension Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 78 of 161
Making Measurements & Explain your choice of measuring instruments
1- Measure and record the length of the vibrating string using a meter rule . (resolution of rule is
1 mm to reduce percentage uncertainty ).
2-Measure the mass (m) of the string on a digital balance (resolution of digital balance is 1 m g
to reduce percentage uncertainty ).
3-Measure the total mass of the hanger and the masses on a digital balance . (resolution of
digital balance is 1 m g to reduce percentage uncertainty ).
4- Measure the frequency of the vibrator (signal generator) .
Independent and dependent variables
( ) is independent variable (along x -axis) and ( 2) is dependent variable (along y -axis)
Explain how the data collected will be used and sketch the expected graph.
1- Record the tension in the string ( ) and its fundamental
frequency ( ).
2- Repeat this procedure 6 more times for different tensions
using the same length of the stretched string.
3- Plot the frequency ( 2) vs. ( ) and draw a best -fit line passing
through the origin.
4- The gradient = 1
> 4 2
> f/Hz
> T/N 2. 03. 04.0 5.0 6.0 7.0 8.0
2 ( 2)
( )Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 79 of 161
In Experiments 3 1-33
Identify two significant sources of uncertainty in the students measurements.
For each of these sources of uncertainty, describe an experimental technique the
student could have used to obtain an accurate measurement.
sources of uncertainty An experimental technique to obtain an
accurate measurement
Frequency
Uncertainty in identifying when nodes form
Uncertainty in identifying maximum
amplitude
Frequency
Repeat and calculate the mean frequency
Vary frequency from above and below
resonance to find two values for the
frequency when the standing wave forms
Length
Parallax error when measuring length
Uncertainty in measuring length to top of
pulley
Or uncertainty in measuring length as string
is not straight
Length
Use a set square to reduce parallax error in
length
Or hold ruler in contact with the wire to
reduce parallax error in length
Or ensure ruler and string are at eye -level
Switch off vibrator
Or ensure string is straight
Mass
Zero error on mass balance
Mass
Zero balance before each measurement
to remove systematic error . I dea that this
error is not reduced by repeating Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 80 of 161
Draw a labelled diagram of the apparatus you used.
Name a source of monochromatic light.
Laser or sodium lamp.
Making Measurements
1- Distance from grating to screen (D)
2- Distance between bright spots on the screen (x)
How did you determine the wavelength of the light?
Using the formula n = a sin ,
Where a is the distance between two slits, n is the order of the bright spots on either side and
is the average of the angle between the central bright spot and each order bright spot to the
left and right of the central one.
n 0 1 2 3 4 5 6 7
Sin 0
Independent and dependent variables
( ) is independent variable (along x -axis) and ( ) is dependent variable (along y -axis)
Experiment 34: Measuring the wavelength of monochromatic light using diffraction grating Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 81 of 161
Explain how the data collected will be used and sketch the expected graph.
1- Measure sin =
>
for the first 7 bright spots.
2- Plot the frequency ( ) vs. ( ) and draw a best -fit line
passing through the origin.
3- Using the formula n = a sin to calculate the gradient.
The gradient =
>
Identify the main sources of uncertainty and/or systematic
error
1- Parallax error when the distance is measured using the meter rule.
2- Ensure that the diffraction grating is perpendicular to the (monochromatic) light,
3- Use a grating with a large number of slits.
4- Ensure D is large to reduce uncertainty
Comment on safety
Risk from using the laser. Do not point a laser light at anyone specially the eyes and you should
not look directly into a laser beam
Name the two phenomena that occur when the light passes through the pair of narrow slits.
Diffraction and Interference
A pattern is formed on the screen. Explain how the pattern is formed.
- LASER produces a single high intensity monochromatic (one wavelength) beam where all the
waves are in phase.
- A single slit S is a narrow slit is used as a point source.
- Diffraction occurs at the double slits
- The waves superpose
Experiment 35 : To demonstrate that light is a wave motion. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 82 of 161
- The double slit screen act as sources of two coherent waves which overlap to give areas of
Constructive interference (Bright bands) where they are in phase, i.e. path difference is
a whole number of wavelengths .
Destructive interference (Dark bands) Where they are antiphase , i.e. path difference is
an odd number of half wavele ngths.
What is the effect on the pattern when the wavelength of the light is increased?
The pattern becomes more spread out.
What is the effect on the pattern when the distance between the slits is increased?
The pattern becomes less spread out.
- The diagram below shows a loudspeaker which sends a note of constant frequency towards a
vertical metal sheet.
- As the microphone is moved between the loudspeaker and the metal sheet the amplitude of
the vertical trace on the oscilloscope continually changes several times between maximum and
minimum values.
- This shows that a stationary wave has been set up in the space between the loudspeaker and
the metal sheet.
- By superposition/interference. W hen two incident and reflected waves with the same speed
and wavelength propagating in opposite direction meet, they interfere and can form a
stationary wave.
- Producing points of maximum displacement (antinodes) where the waves are in phase and
points of zero displacement (nodes) where the waves are in antiphase.
Experiment 36 : To demonstrate the stationary waves using sound waves Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 83 of 161
1- (Microwave) transmitter, 2 slit metal barrier and receiver.
2- Move the receiver in plane perpendicular to slits along a line parallel to the plane of the slits, or round
an arc centered between them].
3- Series maxima and minima are detected.
Explain how you would find a value for the wavelength of the microwaves from your experiment.
(1) Using the receiver, locate the central maxima (O).
(2) Move the receiver away from central maxima until the first order maxima is received (X).
(3) Measure distance from each slit to X
Path difference =
Experiment 37 : Describe an experiment using microwaves to produce and detect a two slit
interference pattern. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 84 of 161
Note
If microwaves are used, the wave length is around 3 cm
- Moving the detector (the microphone in the case of sound wave, the probe receiver in the
case of microwaves) along the line between the wave source and the reflector.
- Alternating points of high and low signal strength can be found.
- These are antinodes and nodes of the stationary wave.
- The distance between successive nodes or antinodes can be measured, and is equal to
- If the frequency of source is known or can be measured, the speed of sound in air can be
calculated from the relation ship =
Note
The speed of microwaves (electromagnetic waves) in air can NOT be calculated using this
method.
Experiment 38 : To demonstrate the stationary waves using sound and microwaves
Experiment 38 : To calculate the speed of sound in air Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 85 of 161
Apparatus
Microwave oven
Microwave safe dish at least 30 cm in diameter (bigger is better)
One of the following: mini -marshmallows, chocolate chips, pappadums, grated cheese or any
food that melts (choc chips work the best)
Ruler
Calculator
Method
1. Cover the bottom of your oven proof dish with a thin layer of the food.
2. Remove the entire turntable from the microwave.
3. Put the oven proof dish in the microwave.
4. Cook on low heat for 30 seconds. The microwave will not heat evenly after the removal of
the turntable and the food will begin to melt or cook at the hottest spots in the microwave.
5. Open the door and without removing the dish check that melted spots have appeared. If they
have not, close the door and heat again for short busts until they do appear.
6. Carefully remove the dish and measure the distance between the melted spots.
7. The distance between two consecutive hot (melted) spots will correspond to half the
wavelength of the microwaves used in your oven.
8. Record this information in your data table.
9. Now, turn the microwave around and look for a small plate or sign. This will inform you of
the microwaves frequency ( ).
Calculations
Distance between consecutive melted spots =
Wave length = 2 distance between consecutive melted spots
Speed of the electromagnetic waves =
Experiment 39 : Measuring the speed of electromagnetic waves in air using the microwave
oven Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 86 of 161
Answer:
1- The vernier calipers have a smaller resolution Or the vernier calipers have a lower uncertainty .
2- The vernier calipers can measure without parallax error .
3- Tips of vernier calipers are easier to align with the rings as surface is curved .
Experiment 48 : Diffraction of Electrons Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 87 of 161
Procedure:
1) Measure and record the inside diameter of the
resonance tube.
2) Add water to your large cylinder until it is within a few
cm of the top.
3) Hold the resonance tube vertically in the larger
cylinder with the lower end in the water. The water will
seal one end of the tube. By raising and lowering the
tube, you can vary the length of the air column in the
tube.
4) Place a large rubber stopper on a table, and strike a
tuning fork on it.
5) Hold the vibrating tuning fork horizontally just above
the open end of the tube.
6) Move the tube up and down until the sound is best reinforced (loudest sound). Be sure you
have the shortest length that will resonate.
7) Record the frequency of the tuning fork (which is stamped on the fork) and the length of the
air column.
8) Repeat this process for tuning forks of 4 different frequencies.
Calculations
Way 1
Frequency of
Tuning Fork
Measured
Length of
Tube at 1 st
resonance
Calculated
Wavelength
for that note
Calculated
speed of
sound
V1=
V2=
= 1 + 2 + 3 + 4 + 5
5
Experiment 40 : Resonance of Sound Waves in Open -Closed Tubes to determine the Speed
of sound: Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 88 of 161
Apparatus
Length of metal wire , power supply, voltmeter , ammeter, & variable resistor .
Procedure
1. Set up the basic circuit. Connect up the ammeter
and the carbon resistor in series with the variable
power supply.
2. Connect a voltmeter in parallel with the resistor.
You will need to select a range that will measure
up to 5 V.
3. Use the variable resistor to vary the current in the metal wire . Record pairs of potential
difference and current values in the table.
4. Switch off for a few seconds, to prevent the resistor heating up and then switch on
again to take repeat readings.
5. Aim to get at least 7 readings
6. Plot a graph 'potential difference / V' versus 'current / A'
7. Draw a best -fit line passing through the origin.
8. The gradient =
>
=resistance of the resistor
9. A straight line graph through the origin shows that the
current is proportional to the potential difference. This result
is known as Ohm's law, which applies to metal or metal -like
wires as long as their temperature remains constant.
Precautions:
1. Place a heatproof mat under the resistor (you should not let the resistor get too hot ).
2. Switch off the supply or disconnect the resistor when not making a measurement! To
prevent the resistor heating up .
3. All connections must be neat and tight.
4. Use a low voltage so that the current flowing will be small enough that the temperature
of the wire stays constant.
5. Use a sensitive milli -ammeter and voltmeter to get accurate readings.
> A
> V
Experiment 41 : Investigate Ohms Law (Metal wires & Metal -like wires e.g. Carbon film )Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 89 of 161
6. Take care to connect the ammeter and voltmeter with their correct polarity. (+ve to +ve
and -ve to -ve) Determine the zero error and least count of the ammeter and voltmeter
and record them.
Quantities to be measured & explain your choice of measuring instrument
Current, voltage
quantity Device used advantage
Current - ammeter (0 2 A)
- Or Multimeter on A scale
- (0 2A)
- Or measure up to 2 A
Voltage - Voltmeter
- Or Multimeter on voltage
scale
- (0 5 V)
- measures up to 20 V because 5 V
power supply
Independent and dependent variables
Current is independent variable (along x -axis) and V is dependent variable (along y -axis)
Explain how the data collected will be use d and sketch the expected graph.
Obtain at least eight values for V and I using the voltmeter and the ammeter.
Plot a graph of V against I and draw a best -fit line passing through the origin.
Identify the main sources of uncertainty and/or systematic error
1. Zero error on meter if an analogue meter
2. Fluctuating reading on digital meter
3. Difficulty of taking simultaneous measurements
Safety Precautions
1. Low volt age/current/power so no risk of electrocution (electric shocks).
Note s:
1- If plotting V on the y -axis and I on the x -axis then the slope of t he graph corresponds to
the resistance of the metallic conductor.
2- If plotting V on the x -axis and I on the y -axis then the slope of t he graph corresponds to
the inverse of the resistance of the metallic conductor.
3- To calculate percentage error
% = | |
100
Question:
Explain how plotting a graph should improve the result.
Answer Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 90 of 161
1- Graph enables an average result to be taken
2- Graph will show anomalies
3- Graph will show if resistance changes
Further Experiments
Question: Will it make any difference to the conduction properties of the resistor if the current
is reversed?
Steps: disconnect the resistor and reverse i t in the circuit and repeat the experiment and repeat
steps 4 -6
Answer: The resistance is the same in both directions.
(Note: When the resistor is reversed you can show that the potential difference and the current
are reversed by using a negative sign ).
The resistance can be measured directly, using a digital multimeter, or indirectly, using an
analogue ammeter with an analogue voltmeter. Discuss the advantages and disadvantages of
each method of measuring the resistance.
Idea Analog Digital
Equipment/cost two meters needed /
may be more expensive
only one meter needed /
may be cheaper option
Ease of reading two readings must be taken one reading only / fluctuates
Parallax error Parallax error needs to be
considered
no parallax error
Systematic errors zero error s contact resistance
Scales fixed/
requires interpolation
variable/can be changed /
numeric reading
Sensitivity Sensitivity limited by size of
scale divisions
Or two decimal places 1
Setting up requires both series and
parallel connections/
additional apparatus
only requires series
connection / must set
function switch / use correct
terminals
Heating effect of current heating may change
resistance of wire
unlikely to be much heating
effect
Power supply meters Pow er supply meters do not
require individual batteries /
but circuit needs a power
supply
internal battery required
Uncertainties greater since two smaller since only one Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 91 of 161
readings reading
Data needs calculation from
two readings
no calculation
required/direct reading
Graphical
method
possible less simple for a fixed wire
Apparatus
Length of wire (nichrome) micrometer, power source, voltmeter , ammeter, variable resistor, &
meter stick.
Procedure:
1. Set up the basic circuit. Connect up the ammeter and the nichrome wire (resistor wire)
in series with the variable power supply.
2. Connect a voltmeter in parallel with the nichrome wire.
3. Use the variable resistor to vary the current in the nichrome wire . Record pairs of
potential difference and current values in the table.
4. Switch off for a few seconds, to prevent the nichrome resistor heating up and then
switch on again to take repeat readings (at least 6 times).
5. Plot a graph 'potential difference / V' versus
'current / A'
6. Draw a best -fit line passing through the origin.
7. The gradient =
>
=resistance of the resistor.
8. Measure the length l of the wire using a ruler and
avoid parallax error.
9. Check zero error on the micrometer.
10. Use the micrometer to find the diameter o f the
wire at different points. Find the average value of
the diameter d and then calculate the radius (r) .
11. Calculate the cross sectional area of the wire (A= 2)
Experiment 42 : To measure the resistivity of a metal wire using Ohms Circuit Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 92 of 161
12. Calculate the resistivity =
>
Quantities to be measured & explain your choice of measuring instrument
Current, voltage
quantity Device used advantage
Current - ammeter (0 2 A)
- Or Multimeter on A scale
- (0 2A)
- Or measure up to 2 A
Voltage - Voltmeter
- Or Multimeter on voltage scale
- (0 12 V)
- measures up to 20 V because 12 V power
supply
Independent and dependent variables
Current is independent variable (along x -axis) and V is dependent variable (along y -axis)
Explain how the data collected will be use d and sketch the expected graph.
Obtain at least eight values for V and I using the voltmeter and the ammeter.
Plot a graph of V against I and draw a best -fit line passing through the origin.
Identify the main sources of uncertainty and/or systematic error
1. Zero error on meter if an analogue meter
2. Fluctuating reading on digital meter
3. Difficulty of taking simultaneous measurements
Safety Precautions
1. Low volt age/current/power so no risk of electrocution (electric shocks).
Note:
To calculate percentage error
% = | |
100 Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 93 of 161
Apparatus
Filament lamp 12 V, 24 W
Power supply, 0 to 12 V, DC to supply up to 4 A
Leads, 4 mm
Multimeters, 2, or 1 ammeter and 1 voltmeter of suitable ranges
Rheostat, e.g. 8 ohm rated at 5 A
Procedure
1. Set up the basic circuit. Connect up the milli ammeter and the filament lamp in series
with a DC power supply and a variable resistor .
2. Connect a voltmeter in parallel with the filament lamp .
3. Use the variable resistor to vary the potential difference across the lamp. Record
pairs of potential difference and current values in the table.
4. Obtain at least eight values for V and I using the voltmeter and the ammeter.
5. Plot a graph of I against V and join the points in a smooth, continuous curve.
OR ( Plot a graph of V against I and join the points in a smooth, continuous curve ).
Experiment 43 : To investigate the variation of current with potential difference for
a filament lamp (12 V -24 W) using ohms circuit .Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 94 of 161
Plot a graph
1. Plot a graph of current/A (y -axis) against potential difference/V (x -axis). Join the
points in a smooth, continuous curve. OR ( Plot a graph of potential difference/V (y -
axis against current/A (x -axis) and join the points in a smooth, continuous curve ).
2. The resistance of the filament increases with temperature.
3. The shape of the curve shows that Ohm's law is not obeyed as the temperature of
the filament changed with changing current.
Quantities to be measured & explain your choice of measuring instrument
Current, voltage,
quantity Device used advantage
Current - ammeter (0 2 A)
- Or Multimeter on A scale
- (0 2A)
- Or measure up to 2 A
Voltage - Voltmeter
- Or Multimeter on voltage scale
- (0 12 V)
- measures up to 20 V because 12 V power
supply
Note:
Comment on whether repeat readin gs are appropriate in this case.
No, as the bulb gets hot
Or yes, repeat experiment after bulb cooled
Explain how the data collected will be use d and sketch the expected graph.
Obtain at least eight values for V and I using the voltmeter and the ammeter.
Plot a graph of I against V and join the points in a smooth, continuous curve.
OR (Plot a graph of V against I and join the points in a smooth, continuous curve). Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 95 of 161
Identify the main sources of uncertainty and/or systematic error
1. Zero error on meter if an analogue meter
2. Difficulty of taking simultaneous measurements
Comment on safety
1. Low voltage supply, so low risk
2. Hot bulb, do not touch (ignore reference to hot wire)
Procedure
1. Set up the basic circuit. Connect up the milli ammeter and the filament lamp in series
with a DC power supply and a variable resistor .
2. Connect a voltmeter in parallel with the
filament lamp .
3. Use the variable resistor to vary the potential difference across the lamp . Record pairs of
potential difference and current values in the table .
4. Obtain at least eight values for V and I using the voltmeter and the ammeter.
5. Calculate the resistance of the filament lamp in each trail. =
>
Plot a graph
1. Plot a graph of R against V and join the points in a smooth, continuous curve.
2. The resistance of the filament increases with temperature.
The shape of the curve shows that Ohm's law is not obeyed as the temperature of the
filament changed with changing current.
Voltage/V
Current/A
Resistance/
Experiment 44 : To investigate the variation of resistance with potential difference for a
filament lamp (12 V -24 W) using ohms circuit .Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 96 of 161
Quantities to be measured & explain your choice of measuring instrument
Current, voltage
quantity Device used advantage
Current - ammeter (0 2 A)
- Or Multimeter on A scale
- (0 2A)
- Or measure up to 2 A
Voltage - Voltmeter
- Or Multimeter on voltage scale
- (0 12 V)
- measures up to 20 V because 12 V power
supply
Explain how the data collected will be use d and sketch the expected graph.
1. Obtain at least eight values for V and I using the voltmeter and the ammeter.
2. Calculate the resistance of the filament lamp in each trail. =
>
> 3.
Plot a graph of R against V and join the points in a smooth, continuous curve.
Identify the main sources of uncertainty and/or systematic error
1. Zero error on meter if an analogue meter
2. Difficulty of taking simultaneous measurements
Comment on safety
1. Low voltage supply, so low risk
2. Hot bulb, do not touch Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 97 of 161
Apparatus:
Coil of wire, glycerol, beaker, heat source, thermometer, ohmmeter, boiling tube
Procedure:
1. Set up the apparatus as shown in the diagram.
2. Use the thermometer to note the temperature of the glycerol, which we assume to be the
same as the temperature of the coil.
3. Record the resistance of the coil of wire using the ohmmeter.
4. Heat the beaker and for each 10 C rise in temperature record the resistance and
temperature using the ohmmeter and the thermometer.
5. Plot a graph of resistance against temperature.
Experiment 45 : To investigate the variation of the resistance of a metallic conductor with
temperature Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 98 of 161
RESULTS:
R ( )
( 0C)
Precautions/Sources of Errors :
1. Check for the resistance of the connecting leads and contacts on the ohmmeter. Subtract
from each reading.
2. Heat very slowly to try to maintain thermal equilibrium between the water and glycerol and
coil. When the Bunsen burner is removed wait until the temperature is steady before taking
the resistance readings.
3. Use glycerol/oil in the test tube as it is a better heat conductor than water.
Material needed :
Thermistor, glycerol, beaker, heat source, thermometer, ohmmeter, boiling tube
Procedure :
Experiment 46 : investigate the variation of the resistance of a negative temperature
coefficient thermistor with temperature. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 99 of 161
1- Set up the apparatus as shown in the diagram.
2- Use the thermometer to note the temperature of the glycerol, which we assume to be the
same as the temperature of the thermistor.
3- Record the resistance of the thermistor using the ohmmeter.
4- Heat the beaker and for each 10 C rise in temperature record the resistance and
temperature using the ohmmeter and the thermometer.
5- Plot a graph of resistance against temperature.
Results :
R ( )
( 0C)
Quantities to be measured & explain your choice of measuring instrument
Resistance & Temperature
quantity Device used advantage
Resistance Ohmmeter Provides direct reading of the resistance
so more precision
Temperature Digital thermometer Avoid parallax error reducing uncertainty
Independent and dependent variables
Temperature is independent variable (along x -axis) and resistance is dependent variable(along y -axis)
PRECAUTIONS / SOURCES OF ERROR:
1. Check for the resistance of the connecting leads and contacts on the ohmmeter. Subtract
from each reading .
2. Heat very slowly to try to maintain thermal equilibrium between the water and gl ycerol and
thermistor. When Bunsen burner is removed wait until the temperature is steady before taking
the resistance readings.
3. Use glycerol/oil as it is a better heat conductor than water.
Identify the source s of uncertainty and systematic error :
1- Simultan eous reading of two variables
2- Systematic error on t hermometer Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 100 of 161
3- Par allax error of scale readings (liquid in glass thermometer, ammeter, voltmeter)
5- Uneven temperature in liquid .
Appropriate comment on safety and how to deal with
1- Risk of scalding from hot water so c lamp beaker
2- Risk of burns from hot apparatus so w ear gloves
3- Risk of electric shock from power supply so u se a low voltage power supply
You are to plan an Experiment to determine how the resistance of a thermistor changes as
temperature is increased fro 0 oC to 100 oC
Apparatus - Material needed
Thermistor connected to leads that have been insulated with silicone jelly
Variable voltage d.c power supply (0 -12V),
Milliammeter (0 - 100mA),
Voltmeter (0 -12v), a beaker of water,
Thermometer (0 -100 o C),
Bunsen burner tripod, gauze and mat.
How you would obtain the temperature range
Use ice (to get to 0 C) and heat( to get to 100 C)
Procedure
1- Set up the circuit shown in the diagram and place the thermistor in the beaker of water at
room temperature.
2- Measure the temperature of the water, the current shown by the milliammeter and the
voltage shown by the voltmeter.
3- Heat the water and record readings of temperatu re, current and voltage every 10 o C.
> mA
# V
> thermistor
Experiment 47 : investigate the variation of the resistance of a of a negative temperature
coefficient thermistor with temperature. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 101 of 161
4- Cool the thermistor by adding cold water or ice . You should obtain ten readings as
temperature is taken down near 0 o C.
Calculations
Work out the resistance of the thermistor for each temperature value (V/ I) and record them in
your table.
Plot a graph of resistance (Y axi s) against temperature (X axis)
R ( )
( 0C)
Quantities to be measured & explain your choice of measuring instrument
Current, voltage, temperature
quantity Device used advantage
Current - Milliammeter (0 - 100mA)
- Or Multimeter on mA scale
- (0 100 mA)
- Or measure up to 200 mA
Voltage - Voltmeter
- Or Multimeter on voltage scale
- (0 12 V)
- measures up to 20 V because 12 V power
supply
Temperature liquid in glass thermometer adequate range and precision
Independent and dependent variables Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 102 of 161
Temperature is independent variable (along x -axis) and (V/I= resistance ) is dependent variable
(along y -axis)
Identify the ma in source of uncertainty and systematic error :
1- Simultan eous reading of two variables
2- Systematic error on t hermometer
3- Par allax error of scale readings (liquid in glass thermometer, ammeter, voltmeter)
4- Voltmeter/ammeter zero error
5- Uneven temperature in liquid
The precautions you would take to ensure accurate measurements
1- such as removing heat source when reading temperature
2- Stirring for even temperature in liquid
3- Keep thermometer close to thermistor ( Or Keep thermometer away from sides/bottom)
(Do not allow repeat readings)
The student removes the heat source before taking each reading. Explain why this will
improve the accuracy of the experiment.
Answer:
Ensures the temperature measured is the temperature of the diode
Or
So the temperature remains steady (during reading)
Or
Thermal equilibrium between diode and water
Appropriate comment on safety and how to deal with
1- Risk of scalding from hot water so c lamp beaker
2- Risk of burns from hot apparatus so w ear gloves
3- Risk of electric shock from power supply so u se a low voltage power supply
Explain why the resistance of a thermistor decreases as its temperature increases
An increase in the number of conduction electrons
Linked to the increase in energy of these electrons Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 103 of 161
This experiment, for advanced level students, shows that the current through a thermistor
increases with temperature, as more charge carriers become available
Apparatus -Material needed
Timer or clock
Leads, 4 mm
Crocodile clip holder
Thermometer -10C to 110C
Thermistor - negative temperature, coefficient,
e.g. 100 ohm at 25C (available from Rapid
Electronics).
Power supply, 5 V, DC or four 1.5 V cells
Beaker, 250 ml
Kettle to provide hot water
Digital multimeter, used as a milliammeter
4. Heat -resistant mat
Power supply, low voltage, DC, continuously variable or stepped supply with rheostat (>1 A)
Procedure
1- Set up the circuit as shown above.
Experiment 48 : the effect of temperature on the current through a thermistor Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 104 of 161
2- Pour boiling water into the beaker and take readings of the current through the thermistor
as the temperature falls. Record the results.
Analysis
3- Plot a graph of current/ mA (y -axis) against temperature/ C (x -axis).
4- Assuming that the voltage is constant, describe how the conductance
or resistance varies with temperature.
> Precautions
Avoid touching thermocouple connections, as this will cause temperature errors in calibration.
(The thermistor connected to leads should be insulated with silicone jelly )
Experiment 49 : To investigate how changing the temperature of a diode will change
the potential difference at which the diode starts to conduct Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 105 of 161
How you would obtain the temperature range
Use ice (to get to 0 C) and heat( to get to 100 C)
Procedure
1- Measure the temperature of the water by thermometer , the current shown by the
milliammeter /ammeter and the voltage shown by the voltmeter.
2- Heat the water and use the variable resistor to change the potential difference at which the
diode starts to conduct every 10 o C.
Quantities to be measured & explain your choice of measuring instrument
Current, voltage, temperature
quantity Device used advantage
Current - Milliammeter
- Or Multimeter on mA scale
- (0 100 mA)
- Or measure up to 200 mA
Voltage - Voltmeter
- Or Multimeter on voltage scale
- (0 12 V)
- measures up to 20 V because 12 V power
supply
Temperature Digital thermometer The resolution is 0.1 oC
Identify the sources of uncertainty and systematic error :
1- Simultan eous reading of two variables
2- Systematic error on t hermometer
3- Par allax error of scale readings (liquid in glass thermometer, ammeter, voltmeter)
4- Voltmeter/ammeter zero error
5- Uneven temperature in liquid
The precautions you would take to ensure accurate measurements
1- such as removing heat source when reading temperature
2- Stirring for even temperature in liquid
3- Keep thermometer very close to or in contact with diode
Appropriate comment on safety and how to deal with
1- Risk of scalding from hot water so c lamp beaker
2- Risk of burns from hot apparatus so w ear gloves Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 106 of 161
3- Risk of electric shock from power supply so u se a low voltage power supply Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 107 of 161
Apparatus
Low voltage power supply, variable power supply , voltmeter, milliammeter, resistor, diode
Procedure - Forward Bias
1. Set up the circuit with the semiconductor diode in forward bias as shown and set the voltage
supply to 9 V.
2. Adjust the power supply to obtain different values for the voltage V across the diode and
hence for the current I.
3. Obtain at least ten values for V and for I using the voltmeter and the milliammeter.
4. Plot a graph of I against V and join the points in a smooth, continuous curve.
5. You can clearly see the non -Ohmic behavior of the diode.
Note
The load resistor is used to limit the total amount of current flowing through the diode so that
it does not burn up.
Procedure Reverse Bias (when the diode just begins to conduct)
1. Set up the circuit as above and set the voltage supply at 20 V.
Experiment 50 : To investigate the variation of current with potential difference for a
semiconductor diode. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 108 of 161
2. The microammeter is used in this part of the experiment, as current values will be very low
when a diode is in reverse bias.
3. Adjust the power supply to obtain different values for the voltage V across the diode and
hence for the current I.
4. Obtain at least six values for V (0 -20 V) and for I using the voltmeter and the microammeter.
Higher voltage values are required for conduction in reverse bias.
5. Plot a graph of I against V and join the points in a smooth, continuous curve.
Notes
1. The position of the voltmeter has changed since a reverse biased diode has a very large
resistance that is greater than the resistance of most voltmeters.
2. It is essential that the microammeter reads only the current flowing through the reverse
biased diode as the sum of the currents flowing through the voltmeter and reverse biased
diode may be much larger.
3. Since the resistance of the microammeter is negligible compared with the resistance of the
reverse biased diode the potential difference across the microammeter and diode is almost the
same as the potential difference across the diode alone.
4. The conduction of diodes increases rapidly when the diode is heated (by hand). Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 109 of 161
1.
Apparatus
1.5 V battery
variable resistor
ammeter
voltmeter
switch
protective resistor (10 )
Procedure
1- Use the adjacent circuit to measure the potential difference across the terminals of
the cell when a measured current passes through the cell .
2- The protective resistor R is included in the circuit to limit the current in the cell.
3- Use the variable resistor to alter the current and repeat the procedure for different
measured currents.
4- Plot a graph of the pd across the cell terminals (on the y-axis) against current (on the x -axis) .
Quantities to be measured
Potential difference across the battery , current
quantity Device used reason
Current Ammeter
or multimeter on current
scale
0.1 A interval or better because 1.5 V cell
Or measures up to 2 A because 1.5 V cell
Voltage Voltmeter
or multimeter on voltage
scale
0.1 V interval or better because 1.5 V cell
Or measures up to 2 V because 1.5 V cell
Experiment 51 : Measurement of the emf and the internal resistance of a 1.5 cell Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 110 of 161
Results
The graph should be a straight line with a negative
gradient equal to r and y-intercept equal to emf .
Identify the main sources of uncertainty and/or
systematic error:
1- Systematic/zero error on meter
2- Parallax errors if analogue meter
3- Accuracy of meters
4- Fluctuating reading on digital meter
Safety Precautions
1- Avoid touching hot wires .
2- Low volt age so no risk of electrocution (electric shocks).
3- Ensure cell is not short -circuited otherwise cell will get hot (the protective resistor is
included to limit the current when the resistance of the variable resistance is reduced,
otherwise the battery could be damaged ).
4- The voltmeter must be connected directly across the battery terminals.
Apparatus
5 V -100 mA solar cell
variable resistor
ammeter
voltmeter
switch
Protective resistor (10 )
Procedure
1- Use the adjacent circuit to measure the potential difference across the terminals of
the solar cell when a measured current passes through the solar cell .
2- The protective resistor R is included in the circuit to limit the current in the solar cell.
3- Use the variable resistor to alter the current and repeat the procedure for different
measured currents.
4- Plot a graph of the pd across the solar cell terminals (on the y-axis) against current (on the x -axis) .
Experiment 71 : Measurement of the emf and the internal resistance of 5 V and 100 mA
solar cell Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 111 of 161
Quantities to be measured
Potential difference across the solar cell , current
quantity Device used reason
Current Ammeter
or multimeter on current
scale
scale stated suitable for reading up to 100
mA
(e.g. 100 mA,150 mA, 200 mA, 300 mA, 500
mA, 600 mA, 1 A)
Or measures up to (e.g. 100 mA,150 mA, 200
mA, 300 mA, 500 mA, 600 mA, 1 A)
Voltage Voltmeter
or multimeter on voltage
scale
scale suitable for reading 5 V
(e.g. 5 V, 6 V, 10 V, 12 V, 15 V, 20 V, 30 V)
Or measures up to (e.g. 5 V, 6 V , 10 V, 12 V,
15 V, 20 V, 30 V)
Results
The graph should be a straight line with a negative gradient equal to r and y-intercept equal to emf .
Identify and state how to control all other relevant variables to make it a fair test
1- Keep light level constant.
Light meter is used to make sure that the light level
is constant
Avoiding shadows
Using a lamp at a fixed distance or do outside on a
clear day.
2- Keep temperature of solar cell constant
Do the experiment quickly or
open switch between readings
Identify the main sources of uncertainty and/or systematic error:
1- Main source of uncertainty is keeping light level constant
2- Systematic/zero error on meter
2- Parallax errors if analogue meter
3- Accuracy of meters
4- Fluctuating reading on digital meter
Safety Precautions
1- Avoid touching solar cell / variable resistor / protective resistor .
2- Low volt age/current/power so no risk of electrocution (electric shocks). Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 112 of 161
Question (WPH13/01 - May 2023)
A student investigated the properties of a solar cell.
He illuminated the solar cell using the apparatus shown.
(a) (i) Explain the purpose of the convex lens.
Answer:
1- using the lens concentrates the light on the solar cell
2- to increase the intensity of light Or to ensure the intensity of light is even
(ii) The power supply provided a constant 12 V potential difference across the filament bulb.
Describe two more ways to keep the light intensity incident on the solar cell constant.
Answer:
1- Control background light [Accept any method to control background light]
2- Keep the solar cell at the same distance from the filament bulb
3- Keep the solar cell at the same angle to the filament bulb Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 113 of 161
(b) The student investigated how the potential difference across the solar cell varied with the
current through the solar cell.
The light intensity incident on the solar cell was kept constant during the investigation.
The circuit diagram for the investigation is shown below.
(ii) Suggest a reason for using a fixed resistor in the circuit.
Answer:
To limit the (maximum) current in the solar cell
Or to avoid short -circuiting the solar cell
(c) Solar cells are used as a source of electrical power.
Describe two advantages of using solar cells to power a small water pump in a garden.
Answer:
1- Solar cells do not emit CO 2 gas /greenhouse gases
Or using solar cells does not contribute to global warming Or using solar cells would not cause
acid rain Or using solar cells would reduce the need for fossil fuels
2- Solar cells use a renewable energy source
3- Sunlight/energy used is free
4- No need for mains wiring
Or can be used where there is no mains electricity Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 114 of 161
Experiment 5 2: To investigate law of conservation of momentum
Procedure
1. Measure the length of card ( ) using ruler.
2. Connect light gates to a data logger.
3. Measure the time taken ( 1) by the card to interrupt light gate 1.
4. Calculate speed of truck A ( 1) =
> 1
5. The two trucks collide and stick together.
6. Measure the time taken ( 2) by the card to interrupt light gate 2.
7. Calculate speed of both truck s A & B ( 2) =
> 2
8. Measure mass pf each trolley ( & ) using balance.
Calculation
1- = 1
2- = ( + ) 2
Conclusion
=
provided no external force
Light gate 1 Light gate 1 Light gate 2
Data logger Data logger Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 115 of 161
Note:
If =
1 = ( + ) 2
1 = 2 2
2 = 1
the final velocity should be the initial velocity
Question 1
State and explain what you would do to minimise the effects of friction on the motion of the
tr uck s.
Answer:
- Use air track
Or
- Use lubricating oil to reduce friction between track and trucks
Or
- Incline the plane until component of weight of truck that is parallel to the plane balance the
friction
Question:
Truck B carries no card and is placed so that it is stationary between the light gates. Truck A is set
off towards truck B. As the card passes through the first gate it records a time t1. Truck A then
collides with truck B. They stick together and move through the second gate which records the
time t2.
Both trucks have the same mass. Explain why t2 = 2 t1 if momentum is conserved.
Answer:
- Mass doubles So velocity halves
Since = ( )
>
so time is doubled
= 2
1
= 2
2
2 = 2 1Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 116 of 161
Experiment 53: Investigating conservation of momentum -Newtons Cradle
Materials:
Newtons Cradle apparatus
Two photo gates, with connection to computer or other data -logging device
Two ring stands, onto which to clamp the photo gates
Procedure
1. Use a ruler to measure height of bob centers above lab
bench and record this value when they are at equilibrium
position (h 1).
2. Initially pull one ball to the side as shown. Use a ruler to
measure height of bob center above lab bench (h 2).
3. Release bob, and let all bobs swing down to equilibrium.
4. Measure the velocity of the dropped bob using the photo
gate and its velocity ( u1) will be the first velocity reading
from a photo gate on its side.
5- It collides with the nearest stationary ball and stops ( 1 = 0).
6- The ball furthest to the right which is initially at rest ( 2 = 0) immediately moves away and
its velocity ( 2) will be the first velocity reading from a photo gate on its side.
7- The middle three balls remain stationary.
9. Repeat using different bob drop heights and number of dropped bobs.
Question 1:
What measurements the students would take and describe how he would use them to
investigate whether momentum had been conserved in this event.
Answer:
1- Measurement the change in height of the bob h = h 2-h1.
2- Measure the velocities before and after collision using photo gates
3- Measure the mass of the bobs using digital balance.
Trial
Number
#
Number
of
Dropped
Bobs
Bob Drop Height
( = 2 1), m
Velocity
before
collision ( 1)
m s -1
Velocity after
collision ( 2)
m s -1
1 1
2 1
3 2
4 2Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 117 of 161
Apply law of conservation of momentum
=
+ = +
+ = +
Apply law of conservation of energy
=
>
+
>
=
>
+
>
>
+ + = +
>
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 118 of 161
Question 2:
The student makes the following observations:
the ball on the right returns and collides with a similar result; this repeats itself a
number of times
after a while, the middle balls are also moving
shortly afterwards, the balls all come to rest.
Discuss these observations in terms of energy.
Answer:
- Collisions are inelastic
- KE is transferred in collisions to internal energy (heat) of balls / to KE of middle balls/to sound
- Eventually stops because all energy is transferred
To reduce uncertainty
1- The balls in Newton's Cradle should be the
same mass. Even a slight deviation will
change the derivation equations and result in
slightly different results.
2- The balls should be perfectly aligned. If
some balls were not on a straight line, the
transfer of momentum and energy would
also be misaligned, changing the results.
3- Spherical balls are used because their
contact is approximately a point. Other shaped objects could be used, but that increases the
chances for misalignment.
4- Hard metal balls such as made of hardened steel are used to minimize losses in energy
due to elastic distortions.
5- Balls are hung with a pair of strings or wires in order to keep them in alignment and to
minimize losses due to friction.
6- Place the Newtonian Demonstrator on a smooth level surface away from any magnetic fields.
Ideally, orient the balls parallel to the east -west plane to avoid interference from the Earths
magnetic field.
Safety Precautions:
Wear closed -toe shoes for this lab in case a bob falls off and onto the floor. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 119 of 161
Aim:
To prove that .
To determine acceleration due to gravity.
Procedure:
1- Tie the hook of the bob on one end of a thread (more than 1 meter).
2- Clamp the other end firmly between the gap of a split cork which is fixed to the clamp of the
retort stand as shown in the diagram.
3- Measure the length 'l' from the middle of the bob to the lower edge of the split cork.
4- Use timing marker at the center of oscillation
4- Pull the bob to one side (making an angle of 10 o with the vertical line) and allow it to oscillate
in one plane.
5- Using a stopwatch record the time (t) taken for 20 complete oscillations.
6- Repeat the experiment for different lengths (l) and record the corresponding time (t) in the
tabular form as shown below:
Observations :
No. of
Trails
Lengths 'l' of pendulum
(cm)
Time for 20 Oscillations
't' (s)
Time for one oscillation
'T' (s) T2(s)2
1 20
2 40
3 60
4 80
5 100
Experiment 54 : Using a pendulum to find acceleration due to
gravity Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 120 of 161
Calculations :
We know that the period is given by = 2
>
Squaring the above equation gives 2 = 4 2
>
Draw a graph of 2 along y -axis against along x -axis
Straight line passing through the origin is obtained
= 4 2
= 4 2
Question 1:
State the reason for using a small displacement
Answer:
- To ensure angle is small
- To ensure the conditions for SHM
- To ensure the time period is independent of amplitude
Question2:
Explain why Use timing marker at the center of oscillation would lead to a more accurate value for
Answer:
- Velocity is at a maximum
- So uncertainty in time is reduced
Or
- This is the only Fixed point in the oscillation
- Whereas amplitude may vary because of damping .Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 121 of 161
Objective
To verify T = m
2L.
Set -up
1 rubber band 1 nylon string (1.5 m) 1 glass tube (15 cm, wrapped by rubber tubing) screw
nuts 1 wire hook 1 small paper marker 1 electronic balance - 1 metre rule 1 stop watch.
Procedure
- Measure the mass of the rubber bung and the screw nuts.
- Construct a centripetal force apparatus.
- The weight of the screw nuts gives the measured value of the tension T in the string.
- Measure the length L of the nylon string between the rubber bung and glass tube to 1 m using
a paper marker.
- Whirl the rubber bung around so that the paper marker is just below the glass tube.
- Measure the time for 50 revolutions of the rubber bung (50t) and calculate the angular velocity
.
- Find the theoretical value of tension by T = m
2L.
- Repeat several times with different lengths L of the nylon.
- Record the results in the Data Table and calculate the mean value of the Tension in the nylon string.
Trail #1 Trail #2 Trail #3 Trail #4
Length of nylon string L/m
Time for 50 revolutions of the
rubber bung 50t/s
Angular velocity of the rubber
bung
= 2
>
/ rad s -1.
Tension in the nylon string T
= m
2L / N
T1 = T2 =.. T3 =.. T4 =..
>
Experiment 55 : Verifying the equation for centripetal force Dr. Desouky Unit 6: Practical Skills in Physics II
A2 Physics Practical Skills in Physics II Page 122 of 161
Mean Tension in the nylon string = 1+ 2+ 3+ 4
> 4
Results and conclusion
- The theoretical value of T is smaller than the measured value.
Question:
- How will the angle change if the rubber bung is whirled with a higher angular velocity?
Explain your answer.
Answer:
by T = m
2L, the tension T will increase if the rubber bung i s whirled with a higher angular
velocity
. The vertical component of the tension, , shoul d be unchanged because it still
balances the weight of the rubber bung. Therefore, will become smaller and will
become larger.
Uncertainty
Sources of error
- There is a friction between the string and the glass tube.
- The paper marker is not kept just below the glass tube during the experiment.
=
100 Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 123 of 161
- You can investigate the force between
two charges using an electronic balance.
- You can charge a pair of insulated
metallised spheres using a Van der Graff
generator, or simply by induction using
a charged plastic rod.
- Clamping the spheres close to each
other will cause a force between them
which can be measured using an
electronic balance.
- By adjusting the distance of separation and measuring the force at each distance, this setup
can be used to confirm that coulombs law follows an inverse square law.
Two parallel plates (accept wires for plates)
Connected to a potential difference Or potential difference is applied
Examples
Seeds in tray of glycerol,
Charged foil on end of rule,
Charged pith ball on thread,
Beam of electrons (in teltron tube)
Charged oil drops
(do not credit charged object)
You can investigate the shapes of electric fields using an EHT power supply to provide a
potential difference, and castor oil with floating semolina becomes slightly charged.
The forces on the charged semolina grains cause the grains to line up, showing the lines of
action of the forces produced by the field. Try this investigation with different shaped
electrodes to see uniform and non -uniform fields.
Experiment 56 : Investigating Coulombs Law (measuring the force between two charges)
Experiment 57 : Describe how a uniform electric field can be demonstrated in a laboratory. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 124 of 161
You can investigate how electrical potential
varies across an electric field by measuring the
voltage between zero and the point within the
field that you are interested in.
A simple experiment using conducting paper
and a battery pack to set up an electric field
allows you to map out equipotentials which
you can use to produce a picture of electric
field.
Try it with differently shaped electrodes to see how they change the field shape.
Experiment 59: Testing coulombs inverse square law for the force between two charges
1. Forces are going to be small, so a sensitive measuring device is needed to register changes in
force
2. Electric top pan registers changes to 0.01g
3. Two conducting spheres are charged (make
sure the charged spheres are insulated so they
dont lose charge during the experiment)
4. The reading m is taken at different distances
between the spheres.
Experiment 58 : Describe how you can investigate equipotentials in a laboratory. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 125 of 161
Experiment 60 : Investigate and recall that applying a potential difference to two parallel
plates produces a uniform electric field in the central region between them, and recognise
and use the expression E = V/d
1. In a uniform field, the size of the electric field strength can also be expressed as =
>
2. Two counducting plates are linned up parallel to each other in a shallow solution of copper
sulfate
3. A metal probe Is connected to the negative
terminal of a power supply via a digital voltmeter
4. When the switch is closed there is an uniform
electric field produced between the plates
5. When the probe is placed in the liquid it will
register a reading on the voltmeter
- A device which will measure the amount of charge directly is called a coulombmeter
- When a coulomb meter is connected to a charged capacitor, it will take all the charge from the
capacitor, measure it and display the result on a digital readout.
- By depressing the left hand key of the rocker switch, the capacitor is charged to a potential
difference V which can be adjusted by the variable d.c. supply.
- Upon depressing the right hand key, the stored charge Q is measured and displayed by the
coulombmeter.
- By charging a capacitor to various different voltages, and discharging through the
coulombmeter each time, you can verify the basic capacitor equation that C=
>
.
Experiment 61 : To investigate the charge stored on a capacitor using coulombmeter Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 126 of 161
- Table 1 shows the charge stored for various potential differences up to 10 volts.
The results are displayed in the graph below. You can see that there is a linear relationship.
- A graph of charge (on the y -axis) against pd (on the x -
axis) should produce a straight line through the origin.
The gradient of this line will equal the capacitance.
- The ratio
>
is the gradient of the straight line, so
>
=
constant
- This constant is called the capacitance of the capacitor.
= =
Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 127 of 161
A designer needs a circuit that will cause a delay in switching off the interior light in a car after
the door is shut.
She uses a circuit with a resistor and a capacitor. She knows that the time constant T is given by
T = RC where R is the resistance in ohms and C is the capacitance in farads
With the switch in position X the capacitor is charged to 12 V. When the switch is moved to
position Y the capacitor discharges through the resistor and the potential difference (p.d.)
across the resistor falls steadily from 12 V.
Question 1:
What is the significance of the time constant for such a discharge?
Answer:
Time taken for the p.d/charge/current to change by 63%
or to fall to 1/e of its original value or to fall to (12/e)V or to fall to 4.41 V
Question 2:
State why the voltmeter needs to have a high resistance.
Answer:
To reduce/prevent charge/current through the voltmeter.
Question 3:
State why a stopwatch with a precision of 0.01 s is suitable for measuring the time constant.
Answer:
Precision of stopwatch much less than reaction time
Experiment 62 : To determine the time constant of RC Circuit Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 128 of 161
Question 4:
State what she should do to make her value for T as reliable as possible.
Answer:
Repeat experiment and calculate mean value or use graphical method
Question 5:
For a capacitor discharging through a resistor, the potential difference V across the resistor at
time t is given by V = V0 et/RC . Explain why a graph of ln V against t should be a straight line.
Answer:
Correct expansion ln V = - t/RC + ln V 0
Compare with y = mx + c
Notice the following:
>
= 1
>
( ) = 1 / ;
>
The y-axis intercept is ln Vo (the starting voltage);
>
The x-axis intercept is the time taken for the voltage to fall to 1 volt (as ln 1 = 0). Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 129 of 161
Aim
A student writes a plan for an experiment to mea sure the current as a capacitor discharges
through a resistor. His aim is to find a valu e for the time constant for the exponential decay of
the current. His outline plan, which includes a circuit diagram, is shown below.
Procedure:
1- Set up the circuit shown using a multimeter as the ammeter and use a stopwatch with a
precision of 0.01 s.
2- Set the switch to position X.
3- Move the switch to position Y and record the current at regular time intervals.
4- The current decays according to the equation
=
> /
C where RC is the time constant
5- Plot the measurements on a graph to find a value for the time constant.
Q1: What is the initial current and the range he should set on the multimeter?
Answer: Meter range should be greater than their value (e.g. 200 A)
Q2: What is the expected value for the time constant and for how long he should take
readings of the current?
Answer: = 47 470 = 22
Q3 : What is the expected value for the time constant and for how long he should take
readings of the current?
Record for a time equivalent to 3 - 5 times their time constant
= 5 = 110
Q4: a stopwatch with a precision of 0.01 s. Explain why this stopwatch is suitable.
Answer: Precision (0.01 s) < human reaction time (0.1 s)
Experiment 63 : To determine the time constant of RC Circuit Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 130 of 161
Q5: State a technique that should be used to ensure his readi ngs are as accurate as possible.
Answer:
1- Meter and stopwatch close together Or both in eye line at same time
Or use of lap -timer
(DO NOT accept repeat and average)
Q6: State a safety pre caution that might be necessary.
Answer:
1- Capaci tor connected correct way round.
2- Capacitor rated voltage at least 6V (NOT less than 6 V)
Q7: A teacher suggests that with this circuit it would be necessary to wait for some time
before switching from position X to position Y.
Answer:
Capacitor will require time to charge up (through the resistor)
> Or capacitor will not be fully charged
Remark:
A different arrangement of the circuit components so that there is no time delay. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 131 of 161
Draw an appropriate circuit diagram for this investigation.
Describe how the student should determine an accurate value for the total capacitance
of the capacitors. The student had a stopwatch.
1. Choose the value of resistor to give a reasonable discharge time
2. Use a multimeter to m easure the resistance of the resistor
3. Charge the capacitors to the initial p.d.
4. Ensure that the ammeter and stopwatch are close together
5. Start the stopwatch at the same time as changing the switch
6. Record the current from the ammeter at times determined using the stopwatch
7. Take many measurements over at least one time constant
8. Plot a graph of ln I along y -axis against t along x -axis
9. Gradient = 1
>
So = 1
>
The student repeated the investigation but used a data logger instead of a stopwatch
and an ammeter.
Suggest why using a data logger would improve this investigation.
Readings (of current and time) can be taken simultaneously
Many readings can be taken in a short tim e
State one safety precaution the student should take
Ensure the working p.d. is not exceeded
Or Ensure the capacitors are fully discharged after the experiment
Experiment 64 : Two identical capacitors were connected in series and charged. They were then
discharged through a resistor and ammeter.
A student investigated how the current in the resistor varied as the capacitors discharged. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 132 of 161
Materials:
>
two mild steel yokes
>
four slab magnets
>
rheostat
>
digital multimeter, used as an ammeter
>
power supply, 0 12 V
>
mass balance, electronic, 0 2 kg, 0.01 g
>
two retort stands with boss and clamp
>
metre rule (used for holding wire in position)
>
shaped bare wires, six wires of different length
Procedure:
For theta as variable
1- The balance will probably be calibrated in grams and milligrams. One gram is equivalent to a
weight of 0.0098 N (0.01 N will probably be accurate enough). 2. Turn the electronic balance on
with everything in place WITHOUT current running through the circuit. This is most easily done
by turning off the power supply.
3. Once the balance has a measurement for mass of t he magnet and setup, set the balance to
zero. There should be a set/zero button to the left of the screen on the balance.
4. A single straight wire, connected to a dc power supply in series with an ammeter and a
rheostat, is taped to a metre rule and clamp ed horizontally at right angles to the field.
5. After setting the balance to zero, connect the circuit by turning on the power supply. A single
straight wire, connected to a dc power supply in series with an amme ter and a rheostat.
NOTE: With the magnets and wires at an angle of 0 the balance should still read a mass of 0, as
there should be no magnetic force.
6. In small intervals, we chose 15, measure the magnetic force downward on the balance with
each respective angle. Record the data to create a graph and measure from 0 up to 90.
7. Plot a graph magnetic force versus to find the relationship between sin and the magnetic
force. l
> I
> 0.003N
Experiment 65: The magnitude of the electromagnetic force may be investigated using current
balance apparatus.
Current Balance Lab - investigate how a magnetic field affects a wire carrying an electric current.
> Magnetic force (N)
This graph turned out to be a linear fit
which gives evidence for the direct
relationship theory. In this graph, y is
equal to the force while x is equal to
sin .
= Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 133 of 161
For current as variable
1. Set up the main circuit seen here.
2. Repeat steps 1 -5 as seen above.3. Clamp the wire horizontally at right angles to the field =
90 , 90 = 1.
3. Vary the current using the power source and record the weight on the balance.
4. Plot a graph magnetic force versus current to find the relationship between the current and
the magnetic force.
For length as variable
1. Set up the main circuit as seen above.
2. Repeat steps 1 -5 as seen above.3. Clamp the wire horizontally at right angles to the field =
90 , 90 = 1.
4. Vary the length of the wire using wires of different lengths and record the weight on the
balance.
5. Plot a graph magnetic force versus length of wire to find the relationship between the length
of the wire and the magnetic force. This graph turned out to be a linear fit
which gives evidence for the direct
relationship theory. In this graph, y is
equal to the force while x is equal to
the length .
=
> Current (A)
> Magnetic f orce (N)
> length (m)
> Magnetic force (N)
This graph turned out to be a linear fit
which gives evidence for the direct
relationship theory. In this graph, y is
equal to the force while x is equal to
the current .
= Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 134 of 161
Procedure:
1. Measure the mass of the block (m).
2. Insert a thermometer and an immersion heater of known electric power, P, into their
respective holes in the block.
3. Allow the thermometer to reach thermal equilibrium and then write down the initial
temperature of the block ( 1).
4. Turn on the current, to provide electrical energy to the heating coil.
5. Allow the block to heat up by about 10 o C, then turn off the current and measure the final
temperature of the block ( 2).
6. Use stopclock to measure time of current flow (to r ecord the time of heating (t) from the
turn -on time to the turn -of time.
7. Electric energy supplied by the heater = P.t
8. Heat energy gained by the block= m c
Calculations:
Electric energy supplied by the heater = Heat energy gained by the block
To find specific heat capacity of a
metal block by electrical method
To find specific heat capacity of
a liquid by electrical method
Experiment 66: Measuring Specific Heat Capacity (by an Electric Method)
. = = Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 135 of 161
Precautions to reduce uncertainty :
>
Start below and finish above room temperature .
>
Ensure that the heater is inserted totally in the metal block or immersed totally in liquid to
avoid any loss of heat energy.
>
Measure the p.d. (voltage) and current at start and at the end and average
>
Ensure that the metal block or the liquid container is well insulated to avoid loss of heat
energy by conduction .
>
Stir the liquid throughout the experiment to ensure that the thermometer reading reflects
the heat supplied.
>
Use a sensitive thermometer graduated to 0.1 oC or 0.2 OC. An error of 1 oC in small (e.g.
=10 oC) is a large relative error.
>
You may wish to drop a small amount of oil (glycerin) into the thermometer hole to improve
the thermal contact between heater, thermometer , and block.
Measurement s to be taken :
1- Measure the mass of the block (m).
2- Allow the thermometer to reach thermal equilibrium and then write down the initial
temperature of the block ( 1).
3- Use stopclock to measure time of current flow
4- Switch off current and measure highest temperature reached
5- Calculate (energy) by multiplying voltage by current by time (E=VIt)
6- Repeat the previous steps to vary the electrical energy supplied several times and each time find
Calculations
5- Calculate (energy) by multiplying voltage by current by time (E=VIt)
7- Draws Electric energy versus graph.
8- = so =
>
Notes:
1- If the power of the heater is not known, you can use an ammeter and a voltmeter to
record its power (P =VI).
2-The liquid should also be stirred to allow an even distribution of the heat energy
throughout its volume. This is necessary since liquids are such poor thermal conductors. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 136 of 161
N.B.
What assumption would you make in calculating the specif ic heat capacity of the metal?
Assumption s:
- No energy is transferred to the surroundings
- OR all energy transferred from washers to water
- OR energy required to raise temperature of container is negligible
- OR no water evaporates
- OR all energy supplied results in a rise in temperature
In reality, t he calculated value for the specific heat c apacity has been overestimated because
energy is t ransferred to the surroundings by heating so the energy gained by the metal has been
overestimated .
In reality, the calculated value for the specific heat capacity has been underestimated because
the mass of the block was overestimated. Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 137 of 161
Apparatus
Joule meter, block of metal, heating coil to match, beaker, lagging, thermometer accurate to
0.1 C, glycerol, electronic balance and a low voltage a.c. supply.
Procedure
1. Measure the mass of the metal block m.
2. Set up the apparatus as shown in the diagram.
3. Record the initial temperature 1 of the metal block.
4. Plug in the joule meter and switch it on.
5. Zero the joule meter and allow current to flow until there is a temperature rise of 10C.
6. Switch off the power supply, allow time for the heat energy to spread throughout the metal
block and record the highest temperature 2.
7. The rise in temperature = 2 1
8. Record the final joule meter reading Q.
Calculations
The specific heat capacity of the metal c can be calculated from the following equation:
Energy supplied electrically = energy gained by the metal block
=
Experiment 67: Measurement of the specific heat capacity of a metal by an electrical method Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 138 of 161
Apparatus
Joule meter, calorimeter, heating coil, beaker, lagging, thermometer reading to 0.1 C,
electronic balance and a low voltage a.c. supply.
Procedure
1. Find the mass of the calorimeter
mcal .
2. Find the mass of the calorimeter
plus the water m 1. Hence the
mass of the water m w (m w = m 1
mcal ).
3. Set up the apparatus as shown.
Record the initial temperature 1.
4. Plug in the joule meter, switch it
on and zero it.
5. Switch on the power supply and
allow current to flow until a
temperature rise of 10 C has
been achieved.
6. Switch off the power supply, stir
the water well and record the
highest temperature 2. Hence
the rise in temperature = 2 1.
7. Record the final joule meter reading Q.
Calculations
Given that the specific heat capacity of the calorimeter is known, the specific heat capacity
of water can be calculated from the following equation:
Energy supplied = energy gained by water + energy gained by calorimeter
= +
Experiment 68: Measurement of the specific heat capacity of water by an electrical method Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 139 of 161
Apparatus
Copper calorimeter, copper rivets,
beaker, boiling tube, lagging,
thermometer accurate to 0.1C, heating
source and electronic balance.
Procedure
1. Place some copper rivets in a boiling
tube. Fill a beaker with water and place
the boiling tube in it.
2. Heat the beaker until the water boils.
Continue boiling for a further five
minutes to ensure that the copper
pieces are 100 C.
3. Find the mass of the copper
calorimeter m cal .
4. Add cold water to the calorimeter until
it is quarter full. Find the combined mass of the calorimeter and water m 1. Hence the mass of
the water m w (m w = m 1 mcal ).
5. Record the initial temperature of the calorimeter plus water 1.
6. Quickly add the hot copper rivets to the calorimeter, without splashing.
7. Stir the water and record the highest temperature 2. The fall in temperature of the
copper rivets is 100 C 2. The rise in temperature of the calorimeter plus water is 2.
1.
8. Find the mass of the calorimeter plus water plus copper rivets m 2 and hence find the mass of
the rivets m m.
Calculations
Energy lost by copper rivets = the energy gained by the water + energy gained by copper
calorimeter
= +
If cw is known, then cm can be calculated or alternatively if cm is known, cw can be found.
Experiment 6 9: Measurement of the specific heat capacity of water by a mechanical method Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 140 of 161
1- One method to find the temperature of a Bunsen burner flame involves heating a metal screw.
2- The screw is held in the flame and then cooled in a test tube of water.
3- The thermal energy lost by the screw raises the temperature of the water so that
energy lost by screw in cooling down = energy gained by water in heating up
=
Question 1: State the measurements to be made
1- Mass of screw
2- Mass of water
3- Original temp (of water) and final temp (of water) [D o NOT allow temperature change]
Question 2: state one technique to improve accuracy
1- Avoid spilling water from test tube
2- Transfer screw quickly
3- Stir water and take highest temperature
4- Read thermometer with eye close to and level with scale
5- Repeat and average
Question 3: Give two sources of error in your experiment
1- Thermal energy transferred from screw to surroundings
2- Energy gained by tube
3- Screw not at hottest temp of flame
Steam produced when screw dropped in
[Do not allow thermal energy lost by water nor energy lost to surroundings]
Question 4: Explain which measurement is likely to give the greatest percentage uncertainty
Rise in temp of water () since U is approximately 1 and rise will be small
Question 5: Comment on safety.
Hazard and precaution
1- Goggles to avoid splashes
2- Dont touch screw, danger of burns ( take care with naked flames, danger of burns )
Experiment 70 : To find the temperature of a Bunsen burner flame Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 141 of 161
Procedure:
1) Set up apparatus as shown in the diagram.
2) Connect air pump to the inlet of the oil reservoir.
3) Open air tap and pump in air until pressure gauge reads its max value.
4) Quickly close tap.
5) Leave for a minute or two, after changing the pressure of the trapped
air wait a minute or two before reading the pressure or volume, to
allow the air to reach room temperature. (This is necessary because
when the air is compressed or expanded there may be slight changes in
temperat ure).
6) Then measure the gas pressure by reading it off the Bourdon gauge.
Record these volumes.
7) Read the volume of gas off the scale next to the glass tube.
8) Gently open then quickly close tap to release some air.
9) Leave for a minute or two, then measure the gas pressure by reading it
off the Bourdon gauge.
10) Record these values.
11) Repeat steps 7 -9 at least six times until the pressure of the gas is back
to atmospheric pressure
12) Plot a graph of pressure (p) against the inverse of volume (1/V)
Results:
The graph is a straight line through the origin, verifying that the pressure (p) is
proportional to (1/V), verifying Boyles Law. Also all values of pV are the same.
1
= (1/ ) =
Precautions:
1) After changing the pressure of the trapped air wait a minute or two before reading the
pressure or volume, to allow the air to reach room temperature. This is necessary because
when the air is compressed or expanded there may be slight changes in temperatu re which
will affect t the volume of gas )due to expansion or contraction)
2) When reading volume make sure your eye is level with the Mercury Meniscus.
3) Make sure air is connec ted tightly to oil reservoir in let.
Experiment 71: To i nvestigate Boyles Law Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 142 of 161
Procedure:
1- A flask containing air.
2- A pressure gauge is screwed into the top of the flask.
3- Water/oil bath surrounding flask
4-The water is heated and the pressure of the air in the sealed
glass beaker is measured with the pressure gauge. (The volume of
the air is effectively constant).
5- To change the temperature over the range 0 C to 100 C.
add ice to get to 0 oC and heat to reach 100 oC
Question 1
Suggest how you would determine the temperature of the air in
the flask.
Answer:
Temperature sensor/thermometer submerged in the water/oil bath
Question 2
Describe what you would do to make your readings as accurate as possible.
Answer:
1- Flask full immersed in the water/oil bath
2- Remove heat source whilst taking reading or adjust thermostat on water bath
3- Allow time for apparatus to come to thermal equilibrium
4- Temperature sensor close to flask or temperature sensor not touching sides or bottom
5- A short length of rubber tubing should be used to connect the flask to reduce the volume of
air inside the rubber tubing .
Question 3
Sketch the graph with pressure as y -axis and temperature as x -axis. What does the x-intercept
represent?
Answer:
The horizontal intercept is approximately -273C.
Question 4
What i s the relationship betwee n the pressure and temperature Kelvin sca le of a fixed mass of
gas at constant volume ?
Answer:
They are directly proportional to each other.
Experiment 72: To Investigate Pressure Law Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 143 of 161
1 - The capillary tube has a small plug of concentrated sulphuric
acid placed in it and it is then sealed at the other end.
2- The water in the beaker is heated and the length of the trapped
air column and the temperature are both recorded.
3- We will assume that the temperature of the gas is the same as
the temperature of the water.
4- The volume of the gas will not be measured but we will assume
that the tube is of uniform cross -sectional area. This means that
the change in volume is directly proportional to the change in the
length of the air column.
Question 1:
Explain how a graph provides an evidence for an absolute zero
of temperature
Answer:
Extending/extrapolating the line backwards
The volume occupied by a gas will be zero at a particular temperature
By plotting the volume against the temperature a straight line is obtained.
#
=
=
Experiment 73: To Investigate Charles Law Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 144 of 161
Question 2:
Describe what you would do to make your readings as accurate as possible.
Answer:
1- Remove heat source whilst taking reading or adjust thermostat on water bath
2- Allow time for apparatus to come to thermal equilibrium
3- Temperature sensor close to capillary tube or temperature sensor not touching sides or bottom
4- The capillary tube has very small cross -sectional area so the change in its cross -sectional area
due the change in temperature can be neglected.
1- Determine the background count rate
2- Determine the total count rate
3- Put a paper as an absorber of radiation between the radioactive sample and GM tube
-If count rate decreases to the background so alpha is present
-if count rate is more than the background so beta or gamma
4- Use an Al foil sheet (5mm thick) as an absorber
-If count rate decreases to the background so beta is present
-if count rate is more than the background so gamma is present
5- Use Pb (lead) sheet (5cm thick) as an absorber
-if count rate decreases to background radiation so gamma is present
To reduce uncertainty:
1- Source and counter in line
2- Zero counter and start clock
3- Measure count for the same time each time
4- Repeat readings and find mean
5- Measure thickness with micrometer screw gauge/Vernier callipers
Safety precaution
Keep hands/people away from the source/use tongs to handle source
Experiment 74: Identifying alpha, beta, gamma using absorber and GM tube
Note:
This investigation is
often simulated using
computer software.
This removes all risk of
exposure to radiological
hazards Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 145 of 161
- This is demonstrated by placing a GM tube at various distances from a gamma point source
and measuring the count rates.
1- Measure the background count rate
1- Place the GM tube near the source.
2- Count for 1 minute, and divide by 60 to get the count rate per second.
3- Move away in steps of 5 cm and every time measure the new count rate.
4- The count rates are corrected for background radiation.
6- If we plot distance 2 (y-axis) against 1/Corrected Count Rate, we will get a straight -line graph:
Remark:
Notice that the line makes an intercept with the y -axis below the origin. This is because the
gamma source is deep within its container. It would clearly be most undesirable to have it
exposed immediately to the room. The intercept gives us the count rate right at the source.
Errors
Resulting in the graph not being exactly a straight line are due to
The source itself having a size in other words, not being a point source
Experiment 75: Investigating the relationship between the intensity of radiation and
distance from source of gamma radiation Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 146 of 161
The GM tubes dead time.
They arise in the use of the GM tube because after it has registered one count, there is a short
interval (known as the dead time ) before it can register another, so any decays occurring in this
period are not registered. This problem is most noticeable when the count rate is high.
The random nature of decay.
To reduce uncertainty:
Use increased time samples for low rates to get an accurate count rate per
Safety precaution
Keep hands/people away from the source/use tongs to handle source
1- Initially, the background count rate shou ld be measured.
2- Place a source, emitting a certain type of radiation, at very short distance (5 mm < d < 35 mm)
from a G.M. tube and record the count rate.
3- Move the source farther away from the tube slowly and notice the drop in the count rate.
4- The distance of the source from the tube at which the count rate drops sharply to the
background, gives the range of this radiation in air
a) The range of alpha in air is few centimeters
b) The range of beta in air is usually one meter or more
c) The rang e of gamma in air is very large
To reduce uncertainty:
1- Source and counter in line
2 Avoid parallax error when metre rule is used to measure d
3- Measure to the same points on the tube and source each time
4- Zero counter and start clock
5- Measure count for the same time each time
6- Repeat readings and find mean
7- Plot graph of count against d
Safety precaution
Keep hands/people away from the source/ use tongs to handle source
Experiment 76: The range of radiations in air Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 147 of 161
1- After the background count has been measured,
2- The source is placed close to the GM tube.
3- Count for 1 minute, and divide by 60 to get the count rate per second
4- Repeat step 3 every 10 min
5- A graph of ln corrected count rate (y -axis) against time (x -axis) can be produced.
- It is much easier to determine a quantity from a straight line graph instead of a curve, since
quantities such as the gradient and intercept can be measured accurately, and a best straight
line can be drawn reliably.
= 0
ln = ln ( 0 )
ln = ln 0
By comparing this to the straight line equation = + , we obtain:
In a graph of ln (y -axis) against (x -axis)
The gradient is
The y -intercept is ln 0
= 2
Safety precaution
Keep hands /people away from the source/ use tongs to handle source
To reduce uncertainty:
1- Source and counter in line
2- Measure to the same points on the tube and source each time
3- Zero counter and start clock
4- Measure count for the same time each time
5- Repeat readings and find mean
Experiment 77: Measuring half life Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 148 of 161
A variable you will control to make it a fair investigation
Keep distance between the source and detector constant
Procedure
1- Initially, the background count rate should be measured.
2- Place a source, emitting a certain type of radiation, at very short distance (5 mm < d < 35
mm) from a G.M. tube and record the count rate.
3- Record count rate for different thicknesses
4- Record count for a specified time
5- Subtract background count
6- Take several readings at each thickness
7- The thickness of lead/metal at which the count rate drops sharply to the background, gives
the thickness needed to stop the radiation
To reduce uncertainty:
1- Source and counter in line
2- Zero counter and start clock
3- Measure count for the same time each time
4- Repeat readings and find mean
5- Measure thickness with micrometer screw gauge/Vernier callipers
Safety precaution
Keep hands/people away from the source/ use tongs to handle source /use tongs to handle lead
sheets
Experiment 78: The thickness of lead needed to stop a radiation from a radioisotope
Question:
Explain why it is possible to correct for background count rate
Answer:
- Background count rate is a constant value
- Hence it is a systematic error which can be subtracted Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 149 of 161
A box containing a radioactive source has the following labels.
Beta emitter only
Range in air is 25 cm
You are to plan an experiment to check that these labels are correct.
The apparatus to be used
1- G-M tube and counter ( OR rate -meter)
2- metre rule/absorbers
The measurements to be taken
Range method
1- Background count recorded
2- Count rate recorded as dist ance between source and GM tube increased
3- In range 2cm (should be shorter than range for alpha) to longer than 25 cm
Or
Absorber method
1- Background count recorded
2- Varies distance to determine positi on at which count rate falls to background count
3- Count rate recorded using different absorbers
A technique to reduce the uncertainty in the measurements
Take them over a long time (minimum 60s) (1)
How these measurements should be used to check that the labels are correct
Range method
1- Count rate should not change at short range (5 cm)
2- Hence no alpha (dependent mark)
3- At around 25 cm count rate should fall to background
4- Hence no gamma (dependent mark)
Or
Absorber method
1- No drop in count rate with paper
2- Hence no alpha (dependent mark)
3- Count rate drops to background with lead
4- Hence no gamma (dependent mark)
(e) a comment on safety.
Use handling tools (1) 1
Experiment 79: Identifying type of a radiation in a box containing a radioactive source Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 150 of 161
Aim
To measure the half -life of protactinium -234.
Set -up
The set -up consists of a Geiger -Mller tube, a pulse counter and
a protactinium generator (Pa -234).
Note: The generator has no removable lid. So, do not try to open
up the generator in one way or another.
Measurements
1. Measure the intensity Ib of the background
radiation (in pulses per 10 s) three times, and record
your measurements in the table below.
Calculate the average intensity Ib,avr of the background radiation (in pulses per 10 s). Record the
result in the table below.
Ib (pulses/10 s) Ib,avr (pulses/10
s)
2. By thoroughly shaking the protactinium generator, mix the two liquids it contains.
3. Position the generator underneath the GM tube, and wait for about ten seconds. Then
simultaneously start the counter and the stopwatch. The counter will automatically stop after 10
s.
4. Let the stopwatch continue to keep track of the time elapsed. Record the measured radiation
intensity I (in pulses per 10 s) in the table below.
5. Restart the counter at t = 20 s on the stopwatch. The counter resets itself if you push the start
button.
6. In this way, complete your measurements as indicated in the table below.
7. And finally, correct for the background radiation: Icor = I Ib
t (s) 0-10 20 -30 40 -50 60 -70 80 -90 100 -110 120 -130
I (pulses/10 s)
Icor
Experiment 80 : Radioactive Decay of Protactinium -234 Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 151 of 161
Assignments
1 Plot your measurements (intensity Icor as a function of time t)
2 Determine the half -life t1/2 of Pa -234 by choosing a value of the intensity Icor and measuring in
the graph after how much time this value is halved. Do this three times for three different
values of the intensity.
Record your measurements below (A, B and C), and calculate the average value of the half -life
of Pa -234.
The intensity Icor decreased
A from to in s
B from to in s
C from to in s
Average half -life t1/2 of Pa -234:
t1/2 = s
Equipment
A set of wooden cubes with one marked face or dice (at least 50 per group or 300 per class)
Introduction
- To simulate radioactive decay, we need to know when a die has decayed. The easiest way is
to blacken one of the faces on a wooden cube to represent a decay. When the blackened
face comes up the die has decayed and is removed from the set.
- The probability of any dice decaying is always the same it is 1
> 6
. So the number of dice
decaying each time will be about is 1
> 6
of the dice that are left.
Procedure:
Throw the box of dice into the corner of the room.
Collect all the dice that have decayed, i.e. the blackened face is uppermost.
Count these decayed cubes, record the number and then put them aside.
Now throw the remaining cubes into the corner, and repeat this process until there are
no cubes left.
Experiment 81: The Half -Life of Dice Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 152 of 161
Results
Number of throws Group 1 Groups 2, 3, 4 Total
No of dice decayed No of dice left No of dice left
0 0 50 50, 50, 50 200
1 8 42 41, 44, 40 167
2 7 35 36, 37, 34 142
Questions
1. Plot the Number of cubes left (last column) on Y axis against the Number of throws (first
column) on X axis.
2. Draw a smooth curve of best fit through the points.
3. Use your graph and the table over the page to determine the half -life.
- The graph produced by the dice has a characteristic shape it is called an exponential decay
curve.
To increase the accuracy of the experiment
Record the data from the other groups from the board and determine the totals
Number of
cubes left
Value on throws
axis for this
number of
cubes
Half this
number of
cubes
Value on throws
axis for half this
number of
cubes
Difference
between the
values of throws
Example 180 1.4 180/2 = 90 4.9 3.5
Average = Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 153 of 161
Aim:
To prove that .
Procedure:
1- Tie the hook of the bob on one end of a thread (more than 1 meter).
2- Clamp the other end firmly between the gap of a split cork which is fixed to the clamp of the
retort stand as shown in the diagram.
3- Measure the length 'l' from the middle of the bob to the lower edge of the split cork.
4- Pull the bob to one side (making an angle of 10 o with the vertical line) and allow it to oscillate
in one plane.
5- Using a stopwatch record the time (t) taken for 20 complete oscillations.
6- Repeat the experiment for different lengths (l) and record the corresponding time (t) in the
tabular form as shown below:
Observations :
(i) Draw a graph of against 2
(ii) Draw a graph of l against
No. of
Trails
Lengths 'l' of
pendulum (cm)
Time for 20
Oscillations 't' (s)
Time for one
oscillation 'T' (s) T2(S) 2 T2/l (s 2
cm -1)
1 20
2 40
3 60
4 80
5 100
Experiment 82: Simple Pendulum Experiment Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 154 of 161
- The acceleration due to gravity can be found with a simple pendulum.
- We know that the period is given by = 2
>
- So, by measuring T and we should be able to figure out a value for g.
- Squaring the above equation gives 2 = 4 2
>
- In this equation, and are the variables whilst and g are constants.
- Re -writing the equation with the constants in a bracket: 2 = (4 2
>
)
- So, a graph of 2 against will be a straight line passing through the origin
- = 4 2
>
Describe how to use a metre rule to measure h
1- Rule shown vertical (stated or shown in diagram)
2- Use of Set Square to bottom/top of ball
3- Makes second reading and halves the difference
Describe what the student should do to make his value for T as accurate as possible.
1- Measure at least 10T
2- Repeat and average
3- Use timing marker in centre of swing Or allow swings to settle before timing
Or use small angle of swing
Experiment 83: Using a pendulum to find acceleration due to gravity Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 155 of 161
.
- The method is to keep the length of the dependent pendulum Q the same, while varying the
length of the driver pendulum P.
- Measurements taken:
1) Frequency of P
2) Frequency of Q
3) Amplitude of Q
4) Phase difference between P and Q
- Initial graphs are draw for the amplitude of Q and
the phase difference of Q against driver frequency.
- Various levels of damping are then applied to Q. Families of curves are produced.
- More damping is produced by decreasing the bob mass of Q and/or adding cardboard fins to
increase air resistance.
- It can be seen that the amplitude of
pendulum Q is maximum (maximum
resonance) just before the natural
frequency of the driver pendulum P.
- If Q is heavily damped the amplitude is
much less at its maximum and occurs at
a much lower frequency than the
natural driver frequency.
- Whether pendulum Q has light or heavy
damping, it always has a phase difference
of /2 radians (90 o) with P, at the natural
frequency with which P oscillates.
- For higher driver frequencies and light
damping the phase difference rises to a
maximum of (pi) radians (180 o).
- Higher driver frequencies and heavy
damping produce a phase difference only
slightly above /2 radians.
Experiment 84: Using Barton's pendulums to investigate forced vibrations Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 156 of 161
The graph shows how the amplitude of the oscillating mass varies over a range of forcing
frequencies.
Part 1:
Curve A shows the results of the investigation using the apparatus as shown in figure 1.
The system has a maximum amplitude at a particular frequency
This is an example of resonance
Resonance occurs when the forcing frequency is at (or near to) the natural frequency of
the system
At resonance there is an efficient/maximum transfer of energy (to the mass -spring
system)
Part 2:
The student repeats the investigation with the oscillating mass in a beaker of water as shown in
figure 2. Curve B shows these results.
Curve B has a smaller amplitude than Curve A (for a wide range of frequencies)
The modified system has (greater) damping
Energy is being removed from the system
The frequency at which resonance occurs is lower for the damped system
Experiment 85: Using Mass -spring system to investigate forced vibrations Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 157 of 161
Procedure:
1- A mass is hung on a spring as shown in the diagram.
2- When the mass is pulled down and released, it oscillates at the natural frequency of the system.
3- When the top of the spring is forced to move up and down at this natural frequency, resonance
occurs.
4- The system below is set up to observe what happens to the oscillations of the mass as the
frequency f of the vibration generator is varied.
5- As f is varied, the amplitude of oscillation A of the mass is recorded. The results are shown
on the graph.
Question 1: State what you would observe as f gets close to the resonant frequency.
Answer: The amplitude of the oscillation increases
Question 2: Describe how you would improve the experiment to obtain a more accurate
value for the resonant frequency.
Answer:
Take more readings of f near peak (Around the resonant frequency )
Question 3: Suggest why it would be better to use an ultrasound position sensor and data
logger to record the position of the mass.
Answer:
More accurate
Or Reduces random errors
Or Reduces (percentage) uncertainty
Experiment 86: Using Mass -spring system to investigate forced vibrations Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 158 of 161
Or Allows many more readings to be taken in a given time (1)
Objective:
The objective is to measure the speed of sound at Room Temperature .
Equipment:
A few tuning forks, a mallet, a resonance tube apparatus, and a calculator
Procedure:
1) Obtain a resonance tube apparatus . The apparatus is just a long piston -cylinder system that
allows a variable length closed pipe, as shown below .
2) Hit a tuning fork of a known frequency with the rubber end of the mallet to make it oscillate .
This should be done away from the pipes opening so that it doesn t hit the glass tube causing it
to break .
3) While one student is holding the oscillating fork at about 1 -2 cm from the pipes opening,
another student may pull the piston's handle out and increase the pipe length until resonance
is heard .
4) A few trials and adjustments may be needed to locate the best length of the pipe at which
the loudest possible sound can be heard (resonance) .
Experiment 87: Resonance of Sound Waves in Open -Open Tubes to determine the Speed of
Sound in air: Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 159 of 161
5) Read the pipes length from the meter -stick and record it as L 1. Note that L 1 = 1/4 .
6) In a similar manner locate the position of the 2nd resonance . As you know the second
resonance should occur at about L 2 = 3/4 .
7) Locate the position of the 3rd resonance , as well . The 3rd resonance should occur at about
L3 = 5/4 .
8) Calculate the wavelength () in two different ways as shown below :
L1= 1/4 ; L2 = 3/4.
L2 - L1 = 2/4 = / 2,
= 2(L 2 - L1).
Or
L1= 1/4 ; L3 = 5/4.
L3 - L1 = 4/4 = ,
= L 3 - L1
Once is determined, equation v = f may be used to find the measured value for v, the speed
of sound. Of course, f is the frequency of the tuning fork used .Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 160 of 161
Procedure:
1) Measure and record the inside diameter of the resonance
tube.
2) Add water to your large cylinder until it is within a few cm
of the top.
3) Hold the resonance tube vertically in the larger cylinder
with the lower end in the water. The water will seal one end
of the tube. By raising and lowering the tube, you can vary
the length of the air column in the tube.
4) Place a large rubber stopper on a table, and strike a tuning
fork on it.
5) Hold the vibrating tuning fork horizontally just above the
open end of the tube.
6) Move the tube up and down until the sound is best reinforced (loudest sound). Be sure you
have the shortest length that will resonate.
7) Record the frequency of the tuning fork (which is stamped on the fork) and the length of the
air column.
8) Repeat this process for tuning forks of 4 different frequencies.
Calculations
Way 1
Frequency of
Tuning Fork
Measured
Length of
Tube at 1 st
resonance
Calculated
Wavelength
for that note
Calculated
speed of
sound
V1=
V2=
= 1 + 2 + 3 + 4 + 5
5
Experiment 88: Resonance of Sound Waves in Open -Closed Tubes to determine the Speed
of sound: Dr. Desouky Unit 6: Practical Skills in Physics II
> A2 Physics Practical Skills in Physics II Page 161 of 161
Way 2
Plotting a graph wavelength versus 1/f
The gradient = speed of sound in air
Precautions:
- The antinode at the top is actually a little above the top of tube.
- The correction factor is 0.4 d, where d is the inside diameter of the tube.
Calculate the corrected length L of the tube according to the equation L = h + 0.4 d, where h is
the length of the air column you measured.
Explain why
The antinode at the top is actually a little above the top of tube.
Answer:
The length of the air column must be increased to correct for the small amount of air above the
top of the tube that also vibrates.
# END