[Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure electric charge object can be given one of two types of electric charge positive and negative when two charged objects are brought close together there will be a force between those objects like charges repel unlike charges attract electric charge is measured in units called kums where charges come from everything is made of tiny C atoms they have electric charges inside them a simple model of the atom is shown on the diagram there is a central nucleus made up of protons and neutrons protons have a positive charge neutrons have no charge much lighter electrons are orbiting around a nucleus electrons have a negative charge atoms have equal numbers of electrons and protons so the net charge is zero this is called the neutral atom when the atoms lost the electrons so the number of electrons less than the number of protons and the net charge is a positive this is called a positive ion atom when the atoms gain the electrons so the number of electrons more than the number of protons and the net charge is a negative this is called a negative ion atom electric fields electric field is the region around an electric charge where another charge experiences a force this can be shown by electric field lines Fields lines always point away from positive charges towards negative charges the direction of the field lines in an electric field is the direction of the force on a positive charge the strength of an electric field increases where the field lines are close together the strength of an electric field decreases where the field lines are far apart the field lines cannot cross together electric field patterns of two opposite charged points electric field patterns of two same charged points this is the neutral point which is the point that has no electric field due to charges uniform electric Fields patterns of two parallel opposite charges the field lines are parallel and same space between each line this shows that the strength of an electric field is constant electric field patterns of a changed conducting sphere the field lines around a charge conducting sphere are symmetrical as with a point charge this is because the charges on the surface of the sphere will be evenly distributed conductors and insulators a conductor is a material that allows charge usually electrons to flow through it easily this is because it has free moving electrons conductors tend to be Metals For example of the material that order from best conductor to the poorest conductor as shown on the diagram Metals conduct electricity very well because current is the rate of flow of charged particles an insulator is a material that does not allow the flow of charge through them very easily this is because it has no free moving electrons for example of insulators are rubber plastic glass and wood some non-metals such as graphite allow some charge to pass through them so the graphite is Conductor investigate how an insulator can be charged by friction when the uncharged plastic rod is rubbed with an uncharged cloth the friction causes the electrons transfer from the rod to the cloth this causes the cloth to become a negative charge and the plastic rod to become a positive charge charging a conductor by induction this is a neutral metal sphere this is an insulating stand that prevents the negative charges from transferring between the metal and the Earth place a negatively charged Rod nearby the top of metal the negative charges in the metal are repelled and move away to the bottom of the metal sphere connected an earth wire to the metal sphere negative charges from the metal travel to the Earth remove the Earth wire first then remove the negative the metal sphere is left with a positive charge if we change a negative Rod to a positive Rod place a positively charged Rod nearby the top of metal the negative charges in the metal are attracted and move towards the positive Rod connect an earth wire to the metal sphere negative charges from the Earth travel to the metal to neutralize the positive charges remove the Earth wire first then remove the positive Rod the metal sphere is left with a negative charge electrostatic phenomena rub the plastic ruler on the cloth or sleeve of your jumper the ruler will become charged we have assumed that the ruler has become positively charged charged if it is held close to some small uncharged pieces of paper some electrons within the paper will be attracted to the edges closest to the ruler there will be an attraction between these negative parts of the paper and the positive ruler a com attracts your hair when you are brushing because the friction causes negative charges to transfer between the comb and your hair the comb and hair become oppositely charged which is why they are attracted together a negatively charged balloon attracts the positive charges in the stream of water the stream of water bends toward the balloon because the attractive force between unlike charges is greater than the repulsive force between like charges a negatively charged balloon strikes a wall because it repels the negative charges in the atoms on the wall this causes the negative charges on the wall to move away the balloon the attractive force between unlike charges is greater than the repulsive force between like charges which is why the balloon sticks to the wall lightning during the thunderstorm the attractive force between the bottom of the clouds and the positive charges at the ground causes negative charges to travel from the cloud to the ground this creates an electric current the electrical energy is transferred to light heat and sound Energies [Music] [Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure Electric current the electric current is related to the flow of charge so the definition of electric current is the amount of charges passing a point per second we can be wrote the equation as I equal Q / T where I is the current in ampere Q is the amount of charge in kulum T is the time in second so 1 ampere is 1 k/ per second the direction of current is same to the direction of moving of positive charges while the direction of current is opposite to the direction of moving of the negative charges conductors conductor is material that allows charge usually electrons to flow through it easily this is because it has free moving electrons electrons flow easily through all metals we therefore describe Metals as being good conductors of electricity in metals some electrons are free to move between the atoms under normal circumstances this movement is random that is the number of electrons flowing in any One Direction is roughly equal to the number flowing in the opposite direction there is therefore no overall flow of charge and no electric current if however a cell or battery is connected across the conductor more of the electrons now flow in the direction away from the negative terminal and towards the positive terminal we say there is now a net flow of charge this flow of charge is what we call an electric current that its direction from positive to negative terminal the greater the flow of charge the greater the electric current insulators insulators is a material that does not allow the flow of charge through them very easily this is because it has no free moving electrons electrons do not flow easily through Plastics rubbers glasses or woods so they are poor conductors of electricity in insulators all the electrons are held tightly in position nucleus and are unable to move from atom to atom charges are therefore unable to move through insulator measuring current a simple circuit is shown on the diagram this is a cell this is a light bulb this is an Amer it is used to measure the current in a circuit it must connect in series good amateur should has low resistance to allow more charges to flow through it this is the connecting wire which is made of copper because a copper is a good conductor we can draw the diagram of this circuit in the symbol of each component as shown this symbol is a cell long longest side is indicated a positive shortest side is indicated a negative the convectional free moving electrons in a copper flow from a negative terminal to a positive terminal of a cell while the convectional current flow from a positive terminal to a negative terminal of a cell this symbol is a light bulb this symbol is an Amer direct current or DC direct current is the current to flow in One Direction This current draws from a dry cell or battery cells and batteries provide currents and voltages that are always in the same direction and have the same value this is called direct current or direct voltage if we draw this as a graph it would be a straight horizontal line alternating current or AC alternating current is the current to flow forward and back backward this current draws from Main's electricity of house and Generator its value increases and then decreases and then does the same again but in the opposite direction if we could draw these changes as a graph they would look like a wave [Music] [Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure electromotive force or EMF electromotive force is the electrical work done by a source in moving a unit charge around a complete circuit we can wrote the equation as e equal W / Q where e is the electromotive force that is measured in volt W is the electrical work done in Jewel Q is the charge in kums potential difference or voltage potential difference is the work done per unit charge that passing charge through a component we can rote the equation as v = w / Q where V is the potential difference that is measured in volt W is the electrical work done in Jewel Q is the charge in kums so 1 volt equals 1 Jew per Kum we often use cells or batteries to move charges around circuits we can imagine them as being electron pumps they transfer energy to the charges the amount of energy given to the charges by a cell or battery is measured in volts if we connect a 9vt cell into a circuit and current flows 9 JW of energy is given to each Kum of charge that passes through the cell as the charges flow around a circuit The energy they carry is transferred by the components they pass through for example when current passes the wires energy is transferred to the surroundings as heat when current passes through a bulb energy is transferred to the surroundings as heat and light measuring electromotive force and potential difference this is the symbol of a voltmeter the voltmeters have two types as digital and analog voltmeter is used to measure voltage across battery cell and components the voltmeters are connected across or in the parallel with a cell and a light bulb good voltmeter should has very high resistance to block the the current to flow through it a simple circuit is shown on the diagram this is a cell this is a light bulb this is an Amer a voltmeter connected across a cell or battery will measure the energy given to each Kum of charge that passes through it a voltmeter connected across a component will measure the electrical energy transferred when each of charge passes through it connecting cells in series each cell has an EMF of 1.5 volts four cells connected in series so total EMF is 1.5 + 1.5 + 1.5 + 1.5 is equal to 6 Vols if one cell is reversed as shown so the EMF of two cells is cancelled then the total EMF is 3 volts connecting cells in parallel each cell has EMF of 1.5 volts four cells connected in parallel so a total EMF is 1.5 volts which is equal to the EMF of one cell [Music] [Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure resistance resistance is defined as the ratio of potential difference to current we can write the equations as r equal v/ I where R is the resistance in ohms V is the potential difference in volts I is the current in ampir so 1 ohm is the 1 VT per ampere the resistance is the value in the material that resists the current to flow through it so for a given potential difference the higher the resistance the lower the current resistor the resistor is the component in the circuit that is used to control the current two types of resistor as a fixed resistor and a variable resistor the fixed resistor and its symbol is shown on the diagram the variable resistor and its symbol is shown on the diagram the variable resistor is used to vary the current in the circuit resistance and resistor in the circuit a battery transfers the energy to the charges for driving the charges to move around the circuit all components in a circuit it offer some resistance to the flow of charge in the connecting wires allow charges to pass through very easily losing very little of their energy the flow of charge through some components is not so easy and a large amount of energy may be used to move the charges through them this energy is transferred usually as heat and other form of energy in a fixed resistor the electrical energy is transferred to heat when the charge passing through it a light bulb the electrical energy is transferred to heat and light when the charge passing through it if the circuit has no the resistor the current is too high and the bulb breaks the variable resistors as it is possible to alter their resistance in the circuit a variable resistor is being used to control the size of the current in a bulb if the resistance is increased the current will be smaller and the bulb will glow less brightly or not at all if the resistance is decreased there will be a larger current and the bulb shines more brightly the variable resistor is behaving in this circuit as a dimmer switch an experiment to determine the resistance of a resistor using a voltmeter and an Amer set up the circuit shown in figure turn the variable resistor to its maximum value close the switch and take the readings from the am and the voltmeter alter the value of the variable resistor again and take a new pair of readings from The Meters repeat the whole process at least six times place the results in a table we can find the resistance of a resistor by three methods first calculate the resistance of a resistor R by the equation are equals v/ I for each pair of the results and find the average of the resistance second plot the graph of voltage against current which shows the graph is a straight line graph passing through the origin the resistance of resistor is the gradient of the graph the steeper the slope the greater the resistance of the wire third plot the graph of the current against voltage the resistance is 1 over gradient the Steep of the slope the smaller the resistance of the wire factors that affect the resistance of metallic conductor temperature as temperature increases the resistance of a metallic conductor increases this is because the atoms in a conductor gain the kinetic energy and vibrate more which reduces the rate of flow of charges this causes the current to decrease and the resistance to increases types of materials different materials have different resistance for example silver a best conductor so it has least resistance the resistance of a material increases as follows as the diagram below length of a conductor wire the longer a wire conductor the greater its resistance this is because electrons have to collide with more ion atoms and increasing the rate of flow of charges and so there will be more resistance so the resistance of a wire is directly proportional to its length cross-section area of a wire wire the thicker a wire the smaller its resistance this is because there is more space for electrons and so more electrons can flow so the resistance of a wire is inversely proportional to cross-sectional area cross-section area is equal to Pi radius squar or Pi diameter squar over 4 so cross-section area is directly proportional to the square of radius and square of diameter so the resistance of a wire is inversely proportional to square of radius and square of diameter exam style first a pieces of wire has a resistance of 0.45 ohms calculate the resistance of another piece of wire of the same material with a third of the length and half the cross-section area resistance is directly proportional to the length a third of the L length means that the length decreases three times so the resistance also decreases three times therefore the resistance is 0.45 to divide by 3 is equal to 0.15 ohms resistance is inversely proportional to the cross-section area a half of the cross the cross-section area decreases two times so the resistance increases two times therefore the resistance is 0.15 * 2 is equal to 0.3 ohms second YP has a resistance of 330 Ohms a diameter d and a length l a second piece of y q is made of the same material as P the diameter of y q is 0.50 * D and its length is 5.0 * L calculate the resistance of y q res distance is inversely proportional to the diameter squared as diameter of Q is 0.5 * D meaning that diameter of Q is half of diameter of P so the resistance of Q is four times of the resistance of P therefore the resistance of Q is 4 * 330 is equal to 1,320 ohms resistance is directly proportional to the length as length of Q is 5 time of P so the resistance of Q is also five times of the resistance of P therefore the resistance of Q is 5 * 1,320 is equal to 6,600 ohms ohms law ohms law states that the potential difference or voltage across a metallic conductor is proportional to the current passing through it when temperature remains constant so we can write that V equals i r the metallic conductor obeys the ohms law it is called that ohic conductor an experiment to explain the current voltage graphs for a resistor of constant resistance set up the circuit shown in figure alter the value of the variable resistor and take the readings from The Meters repeat the whole process at least six times reverse a cell and repeat the all steps as the previous place the results in a table and plot a graph of current against voltage the graph is a straight line passing through the origin this shows that the current is directly proportional to the voltage its slope is constant so its resistance remains constant this side of the graph comes from the result that we reverse a cell an experiment to explain the current voltage graphs for a filament lamp set up up the circuit shown in figure alter the value of the variable resistor and take the readings from The Meters repeat the whole process at least six times reverse a cell and repeat the all steps as the previous place the results in a table and plot the graph of current against voltage the graph is not a straight line this shows that resistance of the bulb changes the slope of the graph decreases this shows that the rate of increasing voltage is greater than the rate of increasing current this means that the resistance of filament to increase this is because at higher current and voltage to create the more heating effect causing the resistance of filament to increase this side of the graph comes from the results that we reverse a cell an experiment to explain the current voltage graphs for a diode set up the circuit shown in figure alter the value of the variable resistor and take the readings from The Meters repeat the whole process at least six times reverse a cell and repeat the all steps as the previous place the results in a table and plot a graph of current against voltage this strangely shaped graph shows that diodes have a high resistance when the current is in One Direction and a low resistance when it is in the opposite direction this side of the graph shows that a diode has low resistance causing the current to flow this side of the graph comes from the results that we reverse a cell the resistance of a diode has very high resistance causing no current to flow this shows that the diode only allows the current to flow in one way [Music] [Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure circuit diagram and circuit components you will be expected to know what each component is and how it behaves in a circuit power supply cell and Battery they Supply electrical energy to the circuit and driving charges around the circet this causes the current this is the symbol of a dry cell which gives a directed current the longer side is positive the shorter side is negative this is the symbol of a battery which consists at two or more cells it gives a directed current this is the symbol of power supply this is the symbol of a general directed current power supply this is the symbol of a general alternating current power supply fixed resistor and variable resistor they are used to control current higher resistance means lower current this is the symbol of a fixed resistor this is the symbol of a variable resistor potential divider or potentiometer it is a kind of a variable resistor that consists of a coil of wire with a sliding contact Midway along it this is the symbol of a potential divider or potentiometer its resistance can be varied by moving the sliding contact which changes the length of the wires and the resistance of potential divider a switch is used to turn on and off the circuit the first symbol shown is for an open switch which means that the circuit is closed the second symbol shown is for a closed switch which means that the circuit is open lamps or light bulbs glow as the currents flow through this is the symbol of a light bulb an amiter is used to measure the current is a circuit it is connected in series with a circuit this is the symbol of an Amer a voltmeter is used to measure the potential difference or voltage an electromotive force or EMF F in a circuit it is connected in parallel with a circuit this is the symbol of a voltmeter fuse is used to prevent excessive current in the circuit and act as a safety against fire this is the symbol of a fuse a heater is a device that converts electrical energy into heat energy this is the symbol of a heater a thermister is a temperature sensor this symbol is a thermister thermister have a negative temperature coefficient this means their resistance decreases as the temperature increases the graph shows the M's decreasing resistance with increasing temperature in the circuit diagram when temperature increases the thermister resistance decreases causing the current from the battery to increase this causes a light bulb to shine brighter thermister are used in temperature sensitive circuits in devices such as fire alarms they are also used in devices where it is important to make sure there is no change in temperature for example in freezers and computers a light dependent resistor or ldr is a light intensity sensor this symbol is a light dependent resistor a light dependent resistor ldr has a resistance that changes when light is shown on it in the dark its resistance is high while when light is shown on it its resistance decreases the graph shows ldr's decreasing resistance with increasing light intensity in the circuit diagram when the light shine on the ldr the ldr's resistance decreases causing the current from the battery to increase this causes a light bulb to shine brighter light dependent resistors are often used in light sensitive circuits in devices such as photographic exposure equipment automatic lighting controls and burglar alarms diode is a component that allows current to flow in One Direction This is the symbol of a diode current is allowed to flow in this direction for the first circuit diagram the current flows from the positive to negative terminal so the diode allows the current to pass through causing a bulb turns on and shining bright for the second circuit diagram the cell is reversed this causes the current from the positive terminal to be blocked by the diode causing a bulb turns off diodes are used in rectifier circuits that convert alternating current into direct current a light emitting diode or LED glows when current flows through it in the correct direction this is the symbol of a light emitting d diode it glows when current flows in this direction for the first circuit diagram the current flows from the positive to negative terminal the light emitting diode allows current to pass through causing it to Glow brightly for the second circuit diagram the cell is reversed this causes the current from the positive terminal to be blocked by the light emitting diode causing it to not glow a motor is a device that converts electrical energy into kinetic energy this is the symbol of a motor generator is the device that converts kinetic energy into electrical energy this is the symbol of a generator a relay coil is used to control a large current using a small current this is the symbol of a relay coil for the circuit diagram a small current flows from the lower battery this causes is the relay coil to magnetize which switch close switch S1 this allows a large current to flow from the larger power supply causing the light bulb to turn on a Transformer is the device that converts the voltage from high voltage to low voltage or low to high this is the symbol of a transformer a magnetizing coil this is the symbol of a magnetizing coil a magnetizing ing coil is a coil of wire that creates a magnetic field around itself when a current passes through it an osilloscope this is the symbol of an oscilloscope an oscilloscope is a laboratory instrument commonly used to display and analyze the waveform of electronic signals [Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure series circuits a series circuit is a simple Loop circuit with no branches or Junctions there is only one path for the current to follow in a series circuit containing three identical bulbs as shown a single switch placed anywhere in the circuit can turn all the bulbs on and off if a switch S2 opens the other bulbs will stop working and if it closes the other bulbs start working if a switch S one opens the other bulbs will stop working and if it closes the other bulbs start working if any one of the bulbs breaks it creates a gap in the circuit and all of the other bulbs will stop working we can conclude that the current has the same value in all parts of the circuit current is not used up the size of the current in a series circuit depends on the voltage supplied to it and the number and type of the other components in the circuit if a second identical cell is added in series the voltage will double and so the current will also double causing bulbs are brighter the energy Supply by the cell is shared between all the bulbs this means that the voltage from cell is shared across each bulb in a series circuit sum of voltage across each bulb is equal to voltage across a cell so the more bulbs you add to a series circuit the less bright they all become decorative lights are often wired in series each bulb only needs a low voltage so even when the voltage from the main Supply is shared out between them each bulb still gets enough energy to produce light unfortunately if the filament in one of the bulbs breaks then all the other bulbs will go out series circuit summaries definition of series circuit the current at every point in the series circuit is the same and the sum of potential difference across each component is equal to the electromotive force the battery the current from the battery i t is equal to the current through each component so I T = i1 = I2 equals I3 the sum of the potential difference across each component is equal to EMF of the battery so EMF e equal V1 + V 2 + V3 the total resistance of series circuit is equal to the sum of the resistance of each component so r t r 1 + R2 + R3 this can be proven mathematically as follows from v = i r and e = v 1 + V 2 + V3 substitute e equal r t i t v 1al r 1 I 1 V 2 = R2 I 2 and V3 = R3 I 3 we can cancel out all of I because they are equal so r t equals R1 + R2 plus R3 the disadvantage of a series circuit if one component fails the all components stop to work a switch in a series circuit controls the entire circuit not individual components work example one calculate the total resistance resistance current from the battery and potential difference across the resistor X and Y calculate the total resistance of a circuit using the following equation RT = RX + r y substitute RX = 4 r y = 6 so r t = 10 ohms calculate the current from a battery using the following equation i t = EMF / r t substitute EMF = 12 and r t = 10 so i t equal 1.2 amp in the series circuit the current from battery is equal to current through each resistor so i t = iix = i y equal 1.2 amp calculate the potential difference across the resistance X using the following equation VX = IX * RX substitute iix = 1.2 RX = 4 so VX = 4.8 volts calculate the potential difference across the resistor y using the following equation v y = i y * r y substitute i y = 1.2 r y = 6 so v y = 7.2 volts you see that v x + v y = 12 V which is equal to EMF of the battery if we added a resistor Zed in a series circuit like has shown calculate the total resistance current from the battery and potential difference across the resistor x y and Zed calculate the total resistance of a circuit using the following equation RT = RX + r y + r z substitute RX = 4 r y = 6 and r z = 6 so r t equal 16 ohms you see that total resistance to increase calculate the current from a battery using the following equation i t = EMF over r t substitute EMF = 12 and r t = 16 so I equals 0.75 amp you see that the current from Battery to decrease in the series circuit the current from battery is equal to current through each resistor so i t = iix = i y = i z = 0.75 amp calculate the potential difference across the resistor X using the following equation VX = IX * RX substitute iix = 0.75 RX = 4 so VX = 3 Vols calculate the potential difference across the resistor y using the following equation V y = i y * r y substitute i y = 0.75 r y = 6 so v y = 4.5 Vols you see that VX and v y to decrease calculate the potential difference across the resistor Zed using the following equation VZ = i z * RZ sub substitute i z = 0.75 RZ = 6 so VZ = 4.5 Vol you see that v x + v y + v z = 12 volts which is equal to EMF of the battery we can conclude that when more resistor are added in series to a circuit total resistance increases this causes the current from Battery to decrease therefore potential difference across each resistor decreases parallel circuits a parallel circuit is a circuit with branches or Junctions providing more than one path for the current to follow in a parallel circuit containing three identical bulbs as shown switches can be placed in different parts of the circuit to switch each bulb on and off individually or all together if a switch S3 opens only the bulb on the same Branch to turn off if a switch S1 opens all bulbs to turn off if one bulb breaks only the bulbs on the same Branch are affected this means the current from the cell is shared in each the branch so the number of electrons that flow into a junction each second must be equal to the number that leave each second this means that the currents entering a junction must always be equal to those that leave at the junction p in the circuit the current that enters Junctions is 0.6 amp and the current that leaves is 0.4 amp + 0.2 amp equal 0.6 amp each branch of the circuit receives the same voltage so if more bulbs are added to a circuit in parallel they all keep the same brightness this means that the voltage Remains the Same across each branch and is equal to the voltage across the cell or battery the lights in your home are wired in parallel this is why you can switch lights on and off separately and the brightness remains unaffected when other lights are turned on or off also if a bulb breaks or is removed you can still use the other lights parallel circuit summaries definition of parallel circuit the sum of the current ENT a junction is equal to the sum of the currents that leave the junction the potential difference across each component is the same and equals to the EMF of the battery the current from the battery I T is equal to the sum of the currents through each component i t equal i1 + I 2 + I3 the potential difference across each component is the same and equal to the EMF of the battery EMF e = V1 = V 2 = V3 the total resistance of the parallel circuit is 1 / r t = 1 / r 1 + 1 / R 2 + 1 / R3 this can be proven mathematically as follows from i t = i 1 + I 2 + I3 and I equal V / R substitute I t = e over r t i 1 = V 1 / r 1 I 2 = V2 / R2 and I3 = V3 / R3 we can cancel out all of v and E because they are equal so 1 / r t = 1 / r 1 + 1 / R2 + 1 / R3 the advantages of a parallel circuit the PD across each component is the same and equal to the EMF of the battery if one component is failed the other components will still work each component can connect a switch to turn on and off independently work example two calculate the total resistance of the circuit current from Battery current into a resistor X and Y calculate total resistance using the equation as shown substitute RX = 6 and r y = 4 the result is 2.4 ohms in the parallel circuit the potential difference across each resistor is the same and equal to the EMF of the battery so EMF e = v x = v y = 12 VTS calculate i t using the following equation i t t = e over r t substitute EMF = 12 and r t = 2.4 so I T is 5 amp calculate iix using the following equation iix = VX / RX substitute VX = 12 and RX = 6 so iix = 2 Amp says calculate i y using the following equation i y = v y/ r y substitute v y = 12 and r y = 4 so i y = 3 amp you see that iix + i y = 5 = i t adding 4 Ohms resistor in the parallel circuit count calculate the total resistance of the circuit current from Battery current into a resistor x y and Zed calculate total resistance using the equation as shown substitute RX = 6 r y = 4 and r z = 4 the result is 1.5 ohms you see that the total resistance to decrease the potential difference across each resistor is the same and equal to the EMF of the battery so EMF eal v x = v yal v z = 12 V calculate i t using the following equation i t = e over r t substitute EMF = 12 and r t = 1.5 so I T is 8 Amp you see that the current from the battery to increase calculate iix using the following equation iix = VX / RX substitute VX = 12 and RX = 6 so iix equal 2 Amp you see that the iix to be the same calculate i y using the following equation i y equal v y / r y substitute v y = 12 and r y = 4 so i y equals 3 amp you see that the i y to be the same calculate i z using the following equation i z = VX / RX substitute v z = 12 and r z = 4 so i z equals 3 amp you see that the current from Battery to increases and sharing into new resistor Zed you see that iix + i y + i z = 8 = i t we can conclude that when a new resistor are added in parallel to a circuit total resistance decreases the current and PD across X and Y Remains the Same each resistor will have the same PD the current from the battery increases to share the new added resistor Zed work example three calculate the total resistance of the circuit current from Battery current into a resistor x y and Zed voltage across the resistor x y and Zed calculate the total resistance of the resistor Y and Z in series combination first by using the equation R in series = r y + r z substitute r y = 2 and r z = 5 so the result is 7 ohms calculate the total resistance of circuit by using the equation as shown substitute R in series equal 7 and RX = 6 so r t equal 3.23 ohms calculate i t using the equation i t = EMF / r t substitute EMF equal 10 and r t equal 3.23 so i t equal 310 amp VX = EMF = 10 Vols calculate iix using the equation iix = VX / RX substitute v x = 10 and RX = 6 so IX = 1.67 amp i y = i z = i t - IX = 1.43 amp calculate v y using the equation v y = i y * r y substitute i y = 10 / 7 and R Y = 2 so v y = 2.86 Vols calculate VZ using the equation VZ equal i z * RZ substitute i z = 10 / 7 and r z = 5 so v y = 7.14 Vols you see that v y + v z = 10 vol equals EMF of the battery short circuit when the switch S is opened three lamps x y and Zed light up when the switch is closed two lamps X and Y turn off this is because most of the current flows through the switch S and very small current flows through lamps X and Y causing they are not brighter which is called a short circuit [Music] [Music] [Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure potential dividers when two resistors are connected in series the potential difference across the power source is shared between them the current Remains the Same any points in series circuit so the current from the power supply is equal to the current through R1 and R2 so the potential difference across each resistor are proportional to its resistance the resistor with the largest resistance will have a greater potential difference than the other one if the resistance of R1 is increased it will get a a greater share of the potential difference whilst the R2 will get a smaller share of potential difference if the resistance of R1 is decreased it will get a smaller share of the potential difference whilst the R2 will get a greater share of potential difference this causes the ratio of R1 / R2 = V1 over V2 potentiometer or riat a potentiometer or riat is a single component that consists of a coil of wire with a sliding contact Midway along it as shown the sliding contact has the effect of separating the potentiometer into two parts as an upper part and a lower part both of which have different resistances their resistances are proportioned to its length of both parts their potential differences are proportional to its resistance of both parts so their potential differences are also propor to its length of both parts if the slide in contact is at point B the resistance of the lower part is zero and so the potential difference across it is also equal to zero while the resistance of the upper part is maximum and so the potential difference across it is equal to EMF if the sliding contact is moved upwards the resistance of the lower part will increase and so the potential difference across it will also increases while the resistance of the upper part will decrease and so the potential difference across it will also decrease if the slideing contact is at Point a the resistance of the lower part is maximum and so the potential difference across it is equal to the EMF of the power supply while the resistance of the upper part will be zero and so the potential difference across it will also zero potential divid ERS are used widely in volume controls and sensory circuits using ldr and therms potential dividers and thermister the thermister and resistor are connected in series as shown in a diagram the potential difference across the power source is shared between them the current from the power supply is equal to the current through the thermister and resistor so the potential difference across each component is proportional to its resistance when the temperature increases so the resistance of thermister decreases the thermister will get a smaller share of the potential difference while the resistor will get a greater share when the temperature decreases so the resistance of thermister increases the thermister will get a greater share of the potential difference while the other resistor will get a smaller share potential dividers and ldr the light depending resistor and resistor are connected in series as shown in a diagram the potential difference across the power source is shared between them the current from the power supply is equal to the current through the light depending resistor and resistor so the potential difference across each component is proportional to its resistance when the light intensity increases so the resistance of ldr decreases the ldr will get a smaller share of the potential difference whilst the other resistor will get a greater share when the light intensity decreases so the resistance of ldr increases the ldr will get a greater share of the potential difference whilst the other resistor will get a smaller share [Music] candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure heating effect of current heating effect of current in wires or appliances causes by their resistance and current passing through them the wiring in a house is designed to let current pass through easily as a result the wires do not become warm when appliances are being used we say that the wires have a low resistance and lower heating effect these wires are made of a copper with insulating a light bulb when current passes through the very thin wire filament of a traditional light bulb it becomes very hot and glows white the bulb is transferring electrical energy to heat and light energy other common appliances that make use of the heating effect of electricity include kettles electric ovens toasters electric fires and hair dryers they want wires more usually called heating elements to become warm the wires of a heating element are designed to have a high resistance so that as the current passes through them energy is transferred and the element heats up electrical power in mechanic power is defined as the work done or energy transfer per unit time we can write the equation as p = w / T we can recall the formula for the voltage as V equal W / q and W = V Q we can recall the formula for the current as i = q over T substitute w = V Q and Q over t = i so we can write the formula for the power as P equal VI I we can recall the formula for the voltage as V equals i r substitute V equals i r so we can write the formula for the power as p = i^ 2 R we can recall the formula for the current as I = V / R substitute i = v / R so we can write the formula for the power as p = v ^2 / R electrical energy rearranging the energy and power equation the energy can be written as E equals p t we substitute P equals VI I so we can be written the formula for the electrical energy as E equals V T everyday appliances transfer electrical energy from the mains to other forms of energy in the appliance for example in a washing machine this will transfer electrical energy into a kinetic energy the amount of energy an appliance transfers depends on how long the appliance is switched on the power of the appliance measuring energy usage energy usage in homes and businesses is calculated and compared using the kilowatt hour the kilowatt hour is defined as a unit of energy equivalent to 1 kilowatt of power expended for 1 hour calculating energy usage with kilowatt hour the kilowatt hour can also be defined using an equation e equal p t where e equal energy in kilowatt hour p equals power in kilowatt T equals time in hour therefore 1 kwatt = 1,000 wat * 3,600 seconds = 3.6 * 10 to the^ of 6 JW since 1 kilowatt equal 1,000 watts and 1 hour equal 3,600 seconds the kilowatt hour is a large unit of energy and used for calculating the cost of energy used in homes businesses and factories appliances are given power ratings as shown on the diagram this power tell consumers that the amount of energy between 2500 jewles to 3,000 jewles transferred by electrical work from the main power supply to the kettle every second this energy is commonly measured in kilowatt which is then used to calculate the cost of energy used for for example a kettle transfers 2,500 WT of electrical power to heat in 90 minutes how much will this cost if 1 kilowatt hour costs 14.2 pennies convert from 2500 watts to kilow the result is 2.5 kwatt convert from 90 minutes to hours the result is 1.5 hours calculate the energy used in kilowatt the result is 3.75 kilowatt then calculate the price the result is 53.2550 [Music] [Music] to have a thorough understanding of the syllabus details outlined in the accompanying figure electrical hazards from Main electricity Main's electricity is potentially lethal because it can carry high voltages and currents even voltages as low as 50 volts can pose a serious Hazard to individuals especially if the current is high enough common hazards include damaged insulation if someone touches an exposed piece of wire they could be subjected to a lethal shock overheating of cables passing too much current through too small a wire or leaving a long length of wire tightly coiled can lead to the wire overheating this could cause a fire or melt the insulations exposing live wires damp conditions if moisture comes into contact with live wires the moisture could conduct electricity either causing a short circuit within a device which could cause a fire or posing an electrocution risk excess current from overloading of plugs extension leads single and multiple sockets when using a main Supply if plugs or sockets become overloaded due to plugging in too many components the heat created can cause fires main electricity Main's electricity is the electricity generated by power stations and transported around the country through the National Grid main electricity is an alternating current Supply an alternating current is the current flow back and forth in the UK the domestic electricity supply has a frequency of 50 htz and a voltage of about 230 volts a frequency of 50 htz means the direction of the current changes back and forth 50 times every second this creates the waveform like as shown main circuits main circuits usually consist of three wires the Live Wire the neutral wire and the Earth wire the Live Wire provides the path along which the electrical energy from the Power Station travels this wire is brown insulated and carries the alternating voltage of negative 230 volts and POS positive 230 volts making the current flow backwards and forwards through the circuit the neutral wire completes the circuit and it is kept at 0 volt this wire is blue insulated the Earth wire usually has no current in it it is there to protect you if an appliance develops a fault it provides a path for current to escape without passing through the user this wire is yellow green insulated ring main circuits provide a way of allowing several appliances in different parts of the same room to be connected to the mains using the minimum amount of wiring in a figure is shown the ring main circuit in a room this is a socket set in wall these wires are connected to a consumer unit plug and sockets plugs and sockets in different countries look different but the principles rules of electrical wiring are similar in the UK three pin plugs are used to connect appliances to the main circuit the three pins are for the Live Wire the neutral wire and the Earth wire this pin is connected to the neutral wire the middle pin is connected to the Earth wire this pin is connected to the live wire and then connected to the fuse when wiring a plug check the following wires are connected to the correct terminals using the color code on the left the cable is held firmly by the grip the correct fuse is fitted the three pin plug is connected to the Appliance and then plug in the socket when it is used in a figure a kettle is plugged into a main socket it is connecting to a main circuit then the electrical energy comes from a generator in a power station to the kettle in a figure you see that the Earth wire to connect with the metal casing of the appliance a fuse is placed at the live wire a switch is placed at the Live Wire fuse is connected in the live wire for safety fuse is a thin piece of wire this is a symbol of fuse if too much current flows through the circuit the fuse will gets overheats and melts to blow and break the circuit this prevents the the user getting a shock the cable overheating and catching fire and saving devices in electrical appliances once the fault causing the increase in current has been corrected the blown fuse must be replaced with a new one of the same size before the appliance can be used again circuit breaker is used in your consumer unit are often in the form of trip switches if too large a current flows in a circuit a switch automatically opens making the circuit incomplete once the fault in the circuit has been corrected the switch is reset usually by pressing a reset button there is no need for the switch or circuit breaker to be replaced as there is when fuses are used switch is connected in the live wire for safety with the switch open and connected into the Live Wire the current cannot reach the appliance the user is safe from electric shock with the switch open and connected into the neutral wire the current can reach the faulty Appliance the user is not safe from electric shock Earth wire Earth wire is a safety wire that connects the metal body of an appliance to Earth this prevents the appliance from becoming live if the live wire comes loose and touches the metal body a current immediately flows to Earth and blows the fuse this means that the appliance is then then safe to touch if the appliance has no Earth wire and the live wire comes loose a current could flow through the user causing an electric shock double insulated appliances some appliances do not have an earth wire this is because their outer case is made of plastic rather than metal the plastic acts as an extra layer of insulation around the wires which prevents any current from flowing to the user even if the live wire comes loose inside the appliance fuse values the plug is usually fitted with either a 3 amp 5 amp or 13 amp fuse the value tells you the current needed to blow the fuse it must be greater than the normal current through the appliance but as close to it as possible so that the fuse will blow as soon as the current gets too high if you know the power of a Appliance you can use the equation of P equal v i to work out whether a 3 amp 5 amp or 13 amp fuse is needed for example calculate the correct fuse that should be used for 2,300 wat 230 volts Kettle current equals power over voltage equals 2,300 over 230 is equal to 10 amp so the correct fuse for this Kettle is 13 amp calculate the correct fuse that should be used for 550 W 230 v television current equals power over voltage equals 550 over 230 is equal to 2.4 amp so the correct fuse for this television is 3 amp the TV would still work with a 13 amp fuse but if a fault developed its circuits might overheat and catch fire without the fuse [Music] blowing candidates are expected to have a thorough understanding of the syllabus details outlined in the accompanying figure wave forms of an alternating current an alternating current of main electricity has a frequency approximately of 50 hertz then the current flows backwards and forwards 50 times per second causing 50 complete cycles of the waveform to be traced out on the screen of an osilloscope uses of an oscilloscope to display waveforms the timebase setting controls the horizontal movement of the waveform the number of Cycles displayed on the screen can be adjusted by changing the timebase setting decreasing the timebase setting will increase the number of Cycles visible on the screen and vice versa the Y gain controls the vertical movement of the cycle the amount of vertical movement can be Amplified by increasing the Y gain setting when the time base is off and a DC voltage is connected to the the Y input the trace appears as a single spot above the middle of the screen when the time base is off and an ac voltage is connected to the Y input the trace will appear as a vertical line when the time base is turned on with no DC voltage connected to the Y input the trace will be a horizontal line in the middle of the screen when the time base is turned on with a DC voltage connected to the Y input the the trace will be a horizontal line above the middle of the screen when the time base is turned on with an ac voltage connected to the Y input the trace will oscillate vertically while moving horizontally across the screen creating the characteristic waveform of an alternating current measuring PD and frequency with an oscilloscope the Y input of the oscilloscope in the diagram is connected to an AC Supply from the waveform on the screen and the settings on the oscilloscopes controls the voltage and frequency of the supply can be found as follows measuring Peak voltage the Y gain control is set at 5 volts per CM this is 1 cm in a vertically the peak voltage is represented by the distance marked amplitude on the waveform as the amplitude is 2 cm so Peak voltage equals 2 cm * 5 Vol per CM the result is 10 Vols measuring period and frequency the time base control is set 10 milliseconds per CM this is 1 cm in a horizontally as the horizontal peel to Peak distance is 4 cm Peak to peak time equals 4 cm * 10 milliseconds per CM = 40 MCS which is equal 0.04 seconds this is the time taken for the trace out one complete cycle of the waveform it is known as the period so the number traced out per second or frequency can be calculated as frequency equal 1 / period equal 1 / 0.04 which is 25 Hertz an electric circuit contains a 700 ohms resistor and a light dependent resistor the electromotive force of the battery is 12 volts an oscilloscope is connected across the fixed resistor figure 6.2 shows the oscilloscope including the settings of the time base and the Y gain controls line Q shows the position of the trace on the oscilloscope when the switch S is open the switch S is closed and the trace on the oscilloscope moves to the position shown by line p in Figure 6 6.2 determine the potential difference across the 700 ohms resistor from a diagram the Y gain control is set at 2 volts per division the line p is two divisions from the line Q so the PD across the 700 ohms resistor is two divisions times 2 volts per division which is equal to 4 volts you got one mark from this determine the resistance of the ldr the PD across the ldr is 12 - 4 which is equal to 8 Vols in the figure 6.1 it is the potential divider circuit so we can use the equation of R1 / R2 = V1 / V2 to substitute R1 equals ldr's resistance V1 equal ldr's PD = 8 Vols R2 = 700 ohms and V V2 = 4 Vol so ldr's resistance equals ldr's PD / V2 time R2 ldr's resistance equal 8 over4 * 700 is equal to 1,400 ohms you got three marks from this the intensity of the light incident on the ldr gradually increases State and explain how the trace on the oscilloscope screen moves as the intensity of the light increases causing the ldr's resistance to decrease this causes the PD across ldr to decrease and the PD across 700 ohms resistor increases so the trace on the osilloscope screen moves upward you got three marks from this I hope you found this video helpful if you did I would be grateful if you would subscribe share like and leave a positive comment your support will encourage me to create more content thank you