Transcript for:
Physics Key Topics Overview

hi guys welcome to my all-in-one edexcel GCSE 91 physics video hope you find it super helpful I'm not sure how long it's going to be but I have gone through and tried to make sure I've hit every single specification point all my path for answers flash up and don't forget if you want to buy my revision guide which I have written containing my path answers you can get that on the website which will flash up now I hope you find this video super super helpful I hope your studies are going well don't forget to come follow me on insta to it on Facebook because I add lots of extra exam tips extra explanations and just general cool science start particularly on insta so I really would recommend giving that a go and I do go through pass papers there too so yeah I hope you find this video helpful let's get started so I've opened up my perfe answer revision guide for physics this is actually applicable for chemistry biology but we're going to scroll straight down to this page of the revision guide which is converting between multiples and submultiples of units and so this is a list you're gonna have to learn in the way that you've learnt what the role of the nucleus in the animal cell is for example you've got to learn this and don't forget because these questions come up all the time so as you can see we've got some perfect says down the left hand side so Giga mega kilo centi milli micro and nano meter do notice that these conversions are to convert from the standard SI unit into the new unit you're being asked for so you need to know that the standard SI unit for time for example is seconds distance is meters weights and forces newton's mass is grams frequency is hertz speed equals meters per second so you do need to have a working knowledge of these in order to use this table appropriately but let's actually go in now and look at a few examples I'm going to go through some proper exam questions too I'm also going to zoom in on the length part of this because actually that's one of the inter conversions you'll need the most so what is 0.2 centimeters in micrometers so in order to work out what we need to do we're going from centimeters here to micrometers here have a look at these numbers and work out the difference between the two and you can see that that is times 10 to the 4 now it is important that you understand that there are lots more micrometers compared with centimeters so let's take a different example one that's probably easier for you to understand look at 1 meter length you have those 1 meter rulers in your lab you should know that that is a hundred centimeters so you're going to have far more centimeters than meters and if we actually look at centimeters compared with micrometers hopefully you can see the same thing that you're going to have far more micrometers than centimeters which is why we do 0.2 times 10 and you can see the difference here is 10 to the 4 so we're going to do times 10 to the 4 to get an answer which is 2,000 micrometers looking now at B we want 12 megahertz in Hertz so we're going from megahertz to Hertz which is here again there were lots more Hertz than megahertz because megahertz is just a way of writing really big numbers in a nice easy way so in order to do that look at the difference between between the orders and you can see that it's times 10 to the 6 so actually 12 megahertz in Hertz has 12 million Hertz in C we want nine milliseconds in seconds so we're going from milli to seconds there were lots more milliseconds compared with seconds so you're going to do times 10 to the minus 3 here and we're using our table to tell us what to do so nine milliseconds is the same as zero point zero zero nine seconds now we want 47 kilograms in grams so we're going from kilograms to grams 1 kilo contains 1,000 grams just to give you context so we're numb our number is going to increase in size so we're going to do 47 times 10 to the 3 according to the table to get 47,000 grams so in order to be able to answer these questions you need both the knowledge of this table and to make sure you know what the SI unit is so these values all along here and you need to understand if your numbers going to get bigger or smaller and that's why this is quite a difficult task and it is important that you spend time learning this now we're going to take a look at the speed distance time topic so there are some very key equations that you need to be aware of so you know me I love formula triangles so I'm going to have my first one here which is D at the top then ask then T now you cover whatever it is that you're after so I'm after distance so if I cover that over with my thumb I can see that it's there for speed times time if I'm after speed then we're going to do distance divided by time and then lastly time is distance divided by speed so if they ask you for the equations sure use your formula triangles to actually work out what the equation is but you must write out the full equation to get the mark formula triangles do not count so as you can see on the left hand side I've drawn very poorly a distance time graph and I'm after the speed at point a so to work out the speed I know that I need to do distance divided by time I can read that straight off the graph so I can see that the distance traveled is 8 the times was 4 seconds so simple calculations the speed is therefore 2 meters per second meters per second due to the units we can see on the y axis which was that distance was given in meters and time was given in seconds it's important to understand these grasp fully because they could ask you for example how long was the car stationary for so in terms of stationary you're looking at the flat parts of the graph so the car is stationary here why because it remains that 8 meters throughout that time so how long was the car stationary for have a look along here and the answer here is 4 seconds this is a different type of graph so the velocity time graph or the speed time graph not to be confused with the distance time graph so we are being asked to find out the acceleration at Point a on the graph so acceleration is given by the equation acceleration equals final speed minus initial speed over time and I need to wait like that is a equals b minus u over t so looking at portion a of the graph what is the final speed well it's given here so read across and you'll see that it is five the initial speed is given here and we know that's zero so five minus zero the time taken was four seconds so the answer here is one point two five now be careful your units you're looking for meters per second squared for acceleration another thing they like to ask you is to calculate the distance traveled now this is a longer task and remember distance traveled is given by the area under the graph so let's calculate the entire distance traveled by this particular vehicle so the way I like to do this is by splitting it up you can work it out as a trapezium or you can work it out as two triangles and a rectangle which I personally find easier so I'm going to label them a B and C so area a is given by this formula well it's a triangle so I need to put in a half immediately times it by the height of the triangle which is five times it by its width do the calculation and that will therefore be ten meters area B is more straightforward it's just a rectangle so just make sure you're reading the correct lengths so it's five again this time times it by three you get a value which is 15 meters finally area C we're back to our triangle so we need 1/2 times the height of the triangle which is 5 times the width of the triangle which is 2 so we get 5 meters so if you add that up you get a total area of 30 meters and that therefore is the distance traveled you I've picked up some questions so question one a toy car falls down a ramp and hits a cushion the graph shows how its velocity changes with time so this is effectively a speed time graph constant velocity on the graph is shown by what does the word constant mean well it means same so where on this graph is it the same velocity now you can kind of imagine a y-axis and the numbers going up the y-axis so it could be something like 0 10 20 30 on the velocity front so where does that not change but it's obviously going to be the horizontal portion of the line because at that point the speed or the velocity is going to be the same everywhere so the answer here is be the horizontal part of the line and with these sorts of questions rather than reading the options through and getting confused I would look at the question first of all work out what you think is the answer and then see if that is an option below the distance traveled is shown by well again I told you to work our distance that's the area under the graph line let's look at the options and that's a the average velocity of the toy car is given by so remember your equation velocity or speed equals distance divided by time therefore that is be a bus travels along a straight road the graph shows how the velocity of the bus changes during a short journey okay so we've got a velocity time graph so again distance will be given by area under the curve the acceleration will be given by the gradient anyway state the velocity of the bus after 25 seconds so you're looking up here the answer here is 6 meters per second how long is the bus stationary during its journey now I've seen loads of people get this wrong because they think that it's stationary here and here well no that's absolutely because look at the velocity it's 12 so stationary means that it was standing still so standing still means that the velocity must have been zero which is just this chunk here and therefore your answer here is 10 seconds state the equation linking acceleration changing velocity and time taken they're giving you the exact wording so you want to write it out as they've written it so you're going to write acceleration equals change in velocity over time taken or you could have written a equals v minus u over T calculate the acceleration of the bus during the first tensor give the unit that's worth three marks so we'll have a look back up here and we're looking at this portion of the graph the first ten seconds were looking at the gradient so that equation was acceleration equals V minus u over T so that is final speed which is 12 minus initial speed which was zero over time taken which was ten so twelve taken zero over ten is one point two the units of acceleration are meters per second squared state the equation linking average speed distance meat and time taken average speed equals distance move divided by time taken the bus moves the total distance of 390 meters during the journey calculate the average speed of the bus so speed equals distance divided by time distance is 390 divided by look on the graph for whatever the time was and it was 60 seconds pop that into your calculator and your answer will be 6.5 meters per second the bus travels further in the first 30 seconds of its journey than it does during the last 30 seconds of this journey explain how the graph shows this well remember that I told you distance is given area by area under the graph line and you can see that the trapezium for the first 30 seconds is way bigger than the one for the second 30 seconds so therefore clearly the bus traveled further so for the first mark state the fact that distance is given by area under the graph and then for the second might compare the two areas under the line and you're done question for the diagram shows some people waiting in a cute supermarket the queue moves forward each time a person leaves the checkout person expands seven minutes in the queue before reaching the checkout and the graph shows how distance changes with time for person X so it's the last person in the queue and notice that it is a distance time graph so what is the initial length of the queue and we're looking for that in meters so all you have to do is read off here on the y axis so read along and you'll see if my pen wasn't as fat that it is six point one meters they expend how you could use the graph to work out the number of times person X so stationary so explain is going to be a very crucial word here and we also need to work out the number of times the person is stationary so station mu means that they don't move so when aren't they moving well it's all the flat portions of the graph so that's one two three four five six seven so you're going to say seven times that they were stationary and we know this because the flat part of the graph indicates zero speed state the equation linking average speed distance move done time taken so out of the way I'm going to write my formula triangle DST so therefore average speed equals cover the ass distance moved over time taken part to calculate the average speed of person X in the queue give the unit don't forget to give the unit so the distance moved we know is six point one meters the time is seven minutes don't get caught out here I know the graph looks confusing but it tells us up here that the person spent seven minutes in the queue I'm going to multiply it by sixty because the SI unit for time is seconds so obviously there are 60 seconds in a minute hence why I'm timesing it by sixty pop that into your calculator and you'll get a value which rounds to 0.01 five to three significant figures and we know that the distance was in meters I've already told you that I've converted to seconds which is why the unit here is meters per second 9.6 by measuring the length of the skid marks in an accident investigator determines at the distance a car traveled between the brakes being applied in stopping was 22 meters the investigator used a sled to determine the friction the investigator then calculated that the car decelerated at 7.2 meters per second squared calculate the speed of a car just before the brakes were applied give your answer to two sig fig okay so this is sue bat so we're using this equation which is V squared equals u squared plus 2ei s and now it's just a matter of identifying what everything is so the distance here is as the acceleration is well as minus 7.2 but that's the acceleration so that's a looking at the speed of the car just before the brakes were applied so we're looking for the initial speed which is U so that's the question mark and we know that it stopped so that means the final speed was zero so just substitute in all these values so it's literally zero squared which is zero equals u squared plus two times minus seven point two times the distance which was 22 so zero equals u squared minus three hundred and sixteen point eight how do we get u square bites off well add it to both sides so three hundred and sixteen point eight equals u squared and then square root that answer in order to calculate your u so u will equal 18 to two significant figures I've just done what you heard the physics equation sheet because I actually knew this equation off the top of my head so I didn't use it but they've given it in a slightly different form and I just want you to see that it does work it doesn't matter which way around you do it it does work so V squared squared again is 0 minus u squared is what we're looking for equals two times the acceleration which was minus seven point two times the distance which was 22 and then if you pop that into your calculator you get a value which is well there will be the same went it will be three hundred and sixteen point eight minus and then because they're both minor so you can get rid of the minus and then square root it and you'll get 80 against it does work either way we now need to discuss the difference between weight and mass so people constantly get this confused they talk about how much they weigh when actually they're talking about their mass and I'm going to explain what I mean by that so weight first of all is given by the unit and which stands for Newtons mass is given by the unit kilograms and so shows away we can see that they have different units now it's worth noticing that your mass is unchanged which doesn't matter where you stand with which planet you're standing on where you are in space your mass will remain the same so if your mass is 50 kilograms on earth it will be 50 kilograms on Mars and that's because it's really a description of what you're made up of however with weight weight has to take into account gravity and therefore when gravity changes your weight will change if we look at the equation linking the three you find that weight equals mass times gravitational field strength so if you have a mass of 50 kilograms you're standing on the earth in which has an approximate gravitational field strength of 10 your weight will therefore be 500 Newtons so this would be the case on earth however on the moon if we do the same calculation we find that although your mass is still 50 your gravitational field strength is hugely reduced on the moon too in fact only 1.6 meaning that your weight on the moon is only 80 Newton's so this is a huge change your weight has drastically decreased on the moon whereas your mass has stayed the same so there in everyday life we talk about how much we weigh we say 50 kilograms remember that's inaccurate and they were actually discussing our mass and as long as you can use this equation weight equals mass times gravitational field strength we'll have no problems we now need to look at the topic of force so just remember first of all the effect that our force has on an object and that can be that the force changes the object speed it can change the object's direction or it can indeed change the object shape you need to be able to list the different types of force so I've written them out here and now we just need to have quick chat about what all of those things mean so first of all notice that some of these forces are contact forces and these are forces which act between two objects that are actually physically touching each other non-contact forces is obviously as the name suggests when they don't touch so let's actually have a look at this normal reaction first of all because that's quite a strange one this normal reaction force or reaction force or normal contact force it's all the same thing that's when an object is on the ground and it experiences a force which is perpendicular to the surface and what that means that's where the word normal comes from because remember in the light topic a normal line is the lines for example drawn like this so here's your glass block and here are some normal lines because they're 90 degrees to the surface of the glass block so in a similar way the reaction force is felt perpendicular to the surface and I'll show you an example that later with a car so don't worry too much now friction is a nice straightforward one that's when two objects slide past each other they experience a friction force so for example a toy car sliding down a slope will experience friction where its wheels touch the slope air resistance or drag that's to do with objects moving through the ass it could be a car being driven along the road and our particles collide with the car creating a small force which acts to slow it down the faster the cars traveling the more particles that will hit per second hence air resistance increases and that's why things like cars have to be streamlined to reduce air resistance looking at non-contact forces so let's start with the magnetic force and that is experienced by any magnetic material inside a magnetic field do you remember that opposite magnetic poles attract so North and South Poles will attract like poles such as to north poles or to south poles repel an electrostatic forces all to do with static charge so we're talking about the buildup of charge and it's experienced by any charged particle that's held within an electric field so a gravitational force as the name suggests is to do with gravity and it's experienced by any mass which is found within a gravitational field and remember these masses may be attracted towards each other due to this gravitational force the easiest example here is the Sun and the earth they're both masses they're both very large objects and they're held in position to to the gravitational force between them weights I've already touched upon remember that is a force acting downwards from any object and it incorporates both the mass and gravitational field strength up thrust this is when we're talking about water so for example a boat sitting on water although its weight is acting downwards there's the force of the water pushing back up so it's the equivalent of the normal reaction force so we call this up thrust and if weighted and up to us to equal then we know the boat will float the nuclear force is as Anna suggested with the nucleus of atoms or the nuclei of atoms and it's the strong attractive force between the protons and the neutrons within the nucleus that helps hold the nucleus together remember that force is measured in Newtons and that you often see a force diagram and that's basically an object which is moving and it will have arrows and the size of the arrow represents how large the forces so obviously the larger the arrow the larger the force remember in your textbook I often talk about balanced forces that is when two forces acting on an object they'll be acting in opposite directions to each other and remember that they have to be the same size for them to be balanced forces and that basically means that an object which is standing still I stationary I'm going to speed up it will stay that as it is and it also means an object which is traveling at a certain speed when it has balanced forces acting on it will continue to travel at that speed it won't speed up it won't slow down so their fault would move them out unbalanced forces it makes sense so one of the forces opposing the other force is larger than the other so if the object is stationary it will start moving and if the object is traveling at some speed but only the speed up or slow down so one of these forces that we're talking about I'm going to use the example of a car driving along the street to help illustrate this so you have a car that starts on the stomach and it's driving so the forward force is going to be the driving force from the engine and that will be causing the car to either accelerate or just carry on traveling at the same speed opposing that driving force will be several other forces first of all air resistance otherwise known as drag now what is that basically if you've got a car moving you've got air whistling past it and these air particles they'll collide with the car and they've generate a tiny tiny force which acts in the opposite direction to the direction the cars traveling and by doing that they oppose the motion of the cast they act to slow it down and we have lots of other forces opposing motion so there's friction remember friction is the force that occurs between surfaces so friction of this example will be between the car wheels and the road so remember it's that friction which is useful because Allah allows the car to grip onto the roads and in certain conditions like icy conditions wet conditions or if the ties have been too worn you'll decrease the friction between the tires in the road and that can lead to dangerous occurrences like the car skidding so a bit of friction is important you have other forces acting on the car you have weight remember that's the downward force due to gravity and don't forget this force it's called the normal reaction and that occurs between the tires and the road surface and it occurs upwards so perpendicular to the road and basically all the normal reaction is is it's the force which stops objects kind of being forced into the earth so it acts against gravity that's quite a hard one to imagine but just remember that it occurs at 90 degrees to the surface another physics equation you need to be aware of is farm so f stands for force a stands for acceleration and M stands for mass so let's work out the various variations so force equals we can see that it's acceleration times mass therefore after acceleration tougher that up you can see that it is force divided by mass and lastly mass equals force divided by acceleration touching quickly on the units mass should always be in kilograms forces Newtons so acceleration is meters per second squared so make sure you have that sorted and we'll look at a couple of examples now so in this question we're calculating the resultant force required to accelerate a 30 kilogram object at one point five meters per second squared say yeah we have mass we have acceleration we're looking for force so force equals acceleration times mass it's a simple experience of substituting in those values to 1.5 times 30 which gives you a value of 45 Newtons question two the force on a moving object is 1,000 Newtons calculate the mass of an object if it is accelerating at two point five meters per second squared so we've been given the force in the acceleration masses given by F over a our forces of thousand Newton's divided by the acceleration which is two point five giving us a value which is 400 don't forget your units it is in kilograms we wore to a battle of velocity we tend to use the example of a parachutist jumping out of an aeroplane and you'll often see lots of diagrams of areas of different sizes I'm going to try and talk to you about that now so let's start at the top was slightly not arrow pain and our parachutist is looking out he's about to jump the moment he jumped the only force acting on him is wet so that will cause him to accelerate towards the Earth's surface now the faster he accelerates the the bigger the air resistance because remember as he's dropping he's going to be hitting lots of air and that those air particles are going to be acting in the opposite direction to his motion trying to slow him down and the faster he travels the more particles will be hitting per second so the overall force of drag or air resistance will become very large so at a certain point you will find that the size of the air resistance or the drag will match the size of the weight force acting downwards and we call that terminal velocity because all that means is that the two forces are balanced and the parachutist will no longer accelerate he'll just continue falling at a constant speed and that is what time the velocity is basically mean accelerating or decelerating just traveling at a constant speed before too long the parachutist will choose to open his parachute because he won't obviously want to splatter on the ground when he opens his parachute you see a massive increase in the surface area of the parachutist and therefore when more air particles will be will be trapped inside the parachute and therefore air resistance won't increase hugely and he'll jerk upwards as a result of that and you'll see that he'll slow down however because his speed decreases that actually causes air resistance to decrease because if you think about it the slower he now travels the fewer air particles will be hitting him and opposing his motion and therefore he'll slow down and eventually his weight and the size of the drag air resistance force will become the same and we call that time we've lost again because he's now traveling at a constant speed now remember that tangled city will be a much lower speed than the initial one and that's due to the fact he's opened up his parachute a student investigates terminal-velocity she uses a tall glass tube filled with oil she drops the metal ball into the tube and the ball falls through the oil use ideas about forces to explain how a falling object can beat terminal velocity doesn't matter what this question is if it's someone jumping out of a plane or a ball being dropped into a tube of oil if it mentions terminal velocity this is your perfe answer so first of all you want to state that the ball has a weight and that will be its downward force acting upwards will be the force of drag as the ball accelerates the drag force will increase and eventually the weight will equal drag at this point the ball no longer accelerates it travels at a constant speed and there is no further acceleration and this means that terminal velocity has been reached so that's your purpose answer try and write it in sentences I've written it like that for ease of iPad use describe how this unit could find out if the ball reaches terminal velocity as it falls through the oil in your answer you should include the measuring instruments that the student will need the measurements she should take and how she could use her measurements to find out if the ball has reached terminal velocity you may include a label diagram in your answer so clearly we need to use a timer to time how long the ball falls for and we need to use a ruler to make sure that the ball is falling over the same distance every time in terms of the measurements taken you need to take a measurement of time for the amount of time that it took for the ball to pass between two points you obviously want to work out that distance using your ruler and then repeat and calculate average to increase the reliability to make it even more specific you could have used a light gate and then lastly once you've worked on the distance and the time you can obviously calculate speed using speed equals distance divided by time so we're going to start by looking at circular motion so any object that moves in a circle has circular motion whether that's a car driving around a racetrack whether it's a satellite orbiting the Earth these broke both have circular motion now in this topic we need to assume that they're traveling at a constant speed however it's now importantly you understand the difference between speed and velocity because people use them interchangeably but they're not quite the same thing speed is how fast you're traveling and you can attach a unit to it such as five miles an hour ten meters per second that sort of thing now a velocity is a vector quantity where speed is a scalar quantity and all that means is that velocity has both the size so yes you could be traveling at five meters per second ten miles now however because it's a vector quantity it also has a direction and that's what makes velocity different to speed they both have a size 10 meters per second but velocity has that added thing which is that it has a direction so now we talk about object moving in a circle and because although it's traveling at constant speed that object is constantly going to be changing direction and therefore has a changing velocity and the point is the object's velocity is directed at a tangent towards the circle that sounds horrible for maths hopefully you know that a tangent is just a line that touches the edge of a circle that just goes off an angle and therefore an object moving around a circle the fact that it's constantly changing direction the point is it will constantly change directions so that velocity is aimed at a tangent if that is horrible ignore what I'm saying I'm yet to see a question that mentions tangents just know that an object traveling at constant speed constantly changes its velocity now we need to touch on a separate point which is that the objects traveling at a constant speed is also accelerating and that's like what is she saying I was so stressful how can it be travel at a constant speed but also accelerating because acceleration to me means speeding up no we need to look at the technical piece of what the right acceleration actually means now acceleration is given by the equation which is change in velocity over time taken and like I've just said although you can be traveling at constant speed the velocity changes because you're constantly changing direction therefore in acceleration if you're constantly changing your velocity you're changing your direction therefore you must be accelerating so that is another point they may ask you an object traveling at a constant speed is accelerating let's move on now any object that's traveling in a circle must be acted upon by resultant force the reason why is because otherwise the object would just fly off in any direction but we know that that object's was gonna keep moving round and round and Master must be being acted upon by a force which is keeping it moving in a circle and we call that the resultant force and because it's moving in a circle we call it sensory Pizza force now this is a question that commonly comes up is gonna up the question may ask what three things affect the size of the resultant force or the centripetal force so obviously first of all the mass of the object is going to have a big role to play because the life of the object the greater the force needed to keep it moving second of all the speed or its velocity velocity of the object it's going to impact on the size of the force needed to keep it in that place because the faster its traveling the greater its velocity the larger the force needed to keep it moving in a circle and lastly the radius so remember that is the distance from the center of the circle to the edge the size of that radius is going to impact than the size of that force because obviously if the circles bigger it's going to need a smaller force to keep it moving if that's something complicated don't worry too much about whether it's bigger or smaller just learn the three things which affect the resultant force and that is the velocity of the object the mass of the object and the radius of the circle and doesn't matter if it's a car driving on a track or if it's a satellite orbiting a planet these things all have a role to play now sometimes they ask you about the force which is acting on the object causing it to move in a circular motion so if it was a car traveling on a track then that's going to be friction between the tires and the road surface if you're talking about a satellite orbiting a planet than the force keeping it in that circular motion is going to be gravity what is the momentum of a bullet of mass 50 grams traveling at 300 meters per second so momentum is given by this equation which is momentum equals mass times velocity this is a nice straightforward example the only thing to notice that you need to convert your grams into kilograms we do that by dividing by a thousand times pi I've lost you which has been given as 300 and our final answer is 15 notice the unit which is kilograms meters per second and we can see that if we have a look at the equation while mass is kilograms velocity is meters per second so I'm just shoving them together to get my unit getting more difficult now this is a recoil velocity style question but don't worry I'll show you the easiest way of doing this so in this example amber is standing still and fires a rifle the bullet has a mass of 0.04 five kilograms and is traveling at 350 meters per second if Emma has a mass of 60 kilograms with what velocity does she move backwards so make this comment first of all momentum is conserved which means it doesn't change and because of that you can then write this comment momentum before equals the momentum after and we're going to use this to lay out our answer so what is Emma's momentum before well we've been told she's standing still which means that her velocity is zero times it by her mass 60 we also know that the rifle has intense pink yet it's going to be fired in that case the velocity of the bullet is zero and its mass is 0.04 five kilograms looking at their momentum afterwards so her momentum will be her mass which is 60 times her velocity which is X that we don't know the answer to yet Plus that of the bullets that's going to be 0.04 5 multiplied 5 350 and that's a matter of sorting out that equation so those cancel out / 0 + 0 zero we have 60 X plus 0.05 times three hundred and fifty to get 15.75 how do we solve that equation well we take away 15.75 from both sides to get the 60 X by itself how do you make X by itself divided by 60 on both sides so her velocity is minus zero point two six five meters per second in this question a boy of mass fifty kilograms and ice skates throws a ball of one kilograms in front of him at a velocity of two point five meters per second what is the boy's velocity so stating the same as usual momentum is conserved it's worth writing that because usually forget the calculation wrong that has to mark available for writing it Munson's before equals momentum after the momentum before is zero because neither the ball all the boy has moved its the momentum after will be the mass of the boy times his velocity plus the mass of the snow balls times its velocity they're canceling dance 50 X plus 2.5 take 2.5 away from both sides and then to get X by itself you want to divide by 15 again on both sides so the boys velocity is minus 0.05 meters per second which means he's chomping in the opposite direction here's another type of momentum calculation these will always bounce in the same way if so an object colliding with an object another object and they stick together then this is the method you need to use so in this situation of carven mass 1,000 kilograms traveling at velocity of 20 meters per second collided with stationary motorbike of mass 500 kilograms the vehicles moved together after impact that is crucial they were effectively stuck together calculate their velocity so as always momentum is conserved momentum before equals momentum after so let's have a look at momentum before so what is the momentum of the car before well it's it's mass which is a thousand times its velocity which we've told is 20 meters per second the motorbike has a mass of 500 kilograms but it's velocity is zero because it is stationary the momentum afterwards will be their combined masses which is 1,000 plus 500 that's both the mass of the car and the motorbike times their velocity which is what we're after so let's start sorting this well out so that would become 20 thousand five hundred times zero is zero and that becomes 1500 fat's to get the X by itself we divide both sides by 1500 so X is thirteen point three meters per second starts the velocity with which they move off at a car traveling at 20 meters per second collides with the stationary lorry and is brought to rest injured 0.02 seconds a woman in the car has a mass of 50 kilograms what force does she experience and we need a different equation this time which is that force equals in momentum over time so forces what we're after so that's X and the change in momentum is the initial momentum minus the final momentum so the initial momentum of the woman will be the velocity of the car times a mass and that makes sense because after all she's in the car and then we take away the final momentum while it's coming to a rest which means although her masses staying the same the velocity obviously reduces to zero because the car stops and time we've told is 0.02 seconds use your calculator so 20 times 50 is 1,000 minus zero over zero point zero two and therefore the force she experienced is 50,000 and the unit's a force units of that solutions nine assume is pain again with some empty tins he throws a wet cloth of massive from one five kilograms at the tins the wet cloth moves at a velocity of six meters per second state the equation linking momentum mass and velocity so that is mentum equals mass times velocity calculate the mentum of the wet cloth and give the unit don't forget to do the other half of the question so momentum equals mass which you know 0.15 times the velocity which is 6 when you put that into a calculator you'll get 0.9 and we can see that mass is kilograms velocity is meters per second which is why this is simply the unit if you can't remember it just work it out from the equation the wet cloth sticks to 10 1 the mass of the 10 1 is 0.05 kilograms the cloth in 10 will move away together calculate their velocity so I always do this in the same way which is momentum before it cause momentum after now we've just worked out the momentum of the cloth which is 0.9 because the tin wasn't moving it must mean Altima see that its momentum is zero and then the momentum after well they've stuck together so we need to add up their masses so that's 0.05 plus 0.15 times by their common velocity so let's just do the maths and sort this equation out so now we simply do X equal 0.9 divided by 0.2 giving you a value which is 4.5 meters per second the student Oh is a bigger weight loss at the remaining teens the wet cloth sticks to tens two and three and they move away together the student concludes I see the cloth the same way so the velocity of tens two and three is the same as the velocity of ten one do you agree with this conclusion and it explained why so clearly the student is wrong and that's because the variables haven't been controlled so the mass of the cloth could be different the mass of the tins could be different and the cloth velocity hasn't been measured so try and use your common sense with this one 9 the stopping distance of a cars - some of the thinking distance in the braking distance table 4 shows how the thinking distance and braking distance vary with speed what is meant by the braking distance of vehicles finally a definition that you just have to learn off by heart so that's the distance traveled under the braking force or you could say it's the distance traveled from the moment the brake is applied till when the car comes to a stop the data in table 4 refers to a car in good mechanical condition driven by an alert driver explain why the stopping distance of the car increases if the driver is very tired right if the driver is very tired clearly they're not going to be able to respond as quickly so their reaction time will increase and this will therefore increase the thinking distance and therefore because they've told us how stopping thinking and braking distance or length therefore the stopping distance will increase - the student looks at the data in table form writes the following thinking distance is proportional to speed braking distance is proportional to speed explain whether the student is correct no they're not correct because although when the speed increases the thinking distance increases by the same factor the braking distance does not and then you want to show some numbers here so you could say for example the increasing from 10 meters just per second to 20 meters per second increases the thinking distance from 6 meters to 12 meters but the braking distance increases from 6 meters to 24 meters okay new for this specification is the types of energy they've kind of changed the naming system so some have stayed the same and some are new so we're going to go through each of them in turn talking about what they mean and some examples so starting with chemical energy this is to do with energy stores and its associated with chemical bonds and really your examples here are things like food remember food is a storage of energy the same is true for batteries kinetic energy this is to do with moving an object so anything that has movement has kinetic energy so a man running down the road a bus being driven along the street they both have kinetic energy gravitational energy as the name suggests is energy associated with an object gaining height so anything that has been lifted will gain gravitational energy for example a chairlift a ski resort an aeroplane these will have gravitational energy and Apple being picked up for example next up elastic energy this is the energy stored when an object is stretched squashed or twisted rubber bands of the obvious place here catapults balloons that have been inflated anything that you distort and then pings back to its original shape will contain elastic energy nuclear energy will become very important when we look at nuclear fission because this is the energy associated with those reactions so remember the fuel uranium-235 which you do need to know that is a huge store of nuclear fuel and therefore nuclear energy thermal energy as the name suggests is to do with heat anything that has gotten hotter has gained thermal energy so a hot cup of tea for example magnetic energy is new for you guys and this is the energy stored when light poles are pushed closer together or when unlike poles are pulled further apart so light poles remember is two South Poles being pushed together unlike poles would be the North and South Pole so just name any magnets here so just a simple bar magnet a fridge magnet that he puts on the fridge door these would all count as magnetic energy stores and lastly electrostatic that's the energy stored when light charges are move closer together or when unlike charges are pulled further apart so very similar to the magnetic energy and your example here is it's hard but it's things like clouds remember when there were lightning storms there's a huge buildup of static charge Van de Graaff generators the same place wherever you build up static charge you will have electrostatic energy the diagram shows some electrical appliances which appliances designed to transfer electrical energy to thermal energy so we're putting an electricity we're guessing he's out of it it is clearly the kettle B which appliance is designed to transfer electrical energy to kinetic energy so again we're putting in electricity we're getting movement energy out which would be the food mix that a in all appliances energy is conserved what is meant by the phrase energy is conserved and that is the energy cannot be created or destroyed it is simply converted from one form into another the lamp has an efficiency of 20% explain what this means and that means that 20% of the energy input has been transferred usefully so in the case of the food mixer 20% of the electrical energy in will have been converted to kinetic energy or you could have argued that 80% of the energy input has been wasted draw a label Sankey diagram before the lamp okay so some key diagram to always look like this now you do find that a lot of it is wasted so that's why the waste energy arrow down here will be much broader than the useful energy and now let's label all of them so the lamp coming in to that will be electrical energy the useful energy out will obviously be light energy and then they'll be wasted energy in the form of heat because do you notice that lumps get pretty hot because we haven't been given any numbers you can't add any numbers so don't worry about that we now need to look at efficiency so using this example a filament bulb was supplied with 80 joules of energy 10 joules of energy was out that is light the rest was wasted calculate the efficiency of the light bulb you need this equation so efficiency equals useful energy out over divided by total energy in and I like to make it a percentage so times by hundred but that's not altogether necessary so useful energy out we know was the light so that's 10 total energy input was 80 times it by a hundred so you can pop that into a calculator or just cancel it down my car's just done so the efficiency is 12.5% right I'm going to go through some miscellaneous calculations to show how you should be approaching questions if you're not quite sure which equation to use so my first piece of advice is the moment they tell you to start during your exam turn over to a blank page and start scribbling out all your formula triangles so you're pretty much making yourself your own personal formula sheet even though that will probably take you about a minute it will be totally worthwhile because it means when you get to a particular calculation of which there are many in the physics exam you don't need to be scrambling around in your brain for the correct one you just refer to your formula sheet and it should let you just be sitting there waiting for you so let's have a look at what we have here though so student pushes a trolley of weight 150 Newton's up a slope of length 20 meters the slope is 1.2 meters high calculate the GPS that's the gravitational potential energy of the trolley and calculate the work done by the student so what do we have here let's identify look at the unit it's a Newton which means it's both weight and force we know the slope length is 20 meters and we know that it is 1.2 meters high so gravitational potential energy is given by mass times gravitational field strength times height we see an immediate issue which is that we don't have the mass of the object we only have its weight so we need to do a preliminary calculation using this formula triangle which is weight equals mass times gravity we're looking for mass so that's going to be weight divided by gravity now the weight we've been told is 150 Newton's gravity on earth is approximately tan you could put 9.8 but I'm going to write 10 and therefore you're going to write the mass as being 15 kilograms now we're ready to use the original equation so mass is 15 gravity I just said is 10 the height it says that the slope is 1.2 meters high hence 1.2 is substituted in here so once I've substituted that and I get a value which is 180 it's an energy value so it needs to be in joules so I've rubbed the best out so we've got more space so and B we're calculating the work done by the student if she pushed the trolley with a force of 11 Newton's so we have work done we have force so the formulas on Google so we need this time is work done equals Force Times distance so we're looking for work done let's write out the equation equals Force Times distance the force we know is 11 Newtons the distance look higher up in the question she pushed it up a slope of lunch 20 meters that's why we substituted in 20 here pop it into the calculator and you know that it is 220 work done remember has the same units as energy which is why I'm putting 220 joules next equation I'm interested in is kinetic energy which is half of mass times velocity squared I prefer not to put this into a triangle I think it's easier if you keep it exactly as it's written and I'll show you how to calculate the different parts so you can actually see that it can be rearranged easily in this first question we're being asked to calculate the kinetic energy of a tennis ball traveling at 46 meters per second with a mass of 500 grams so as always write out our equation which is caiis half MV squared be careful with your units because 500 grams isn't in kilograms so I'm going to put that into kilograms so remember 500 grams is is 0.5 kilograms so start substituting in your values so it's 1/2 times 0.5 times 46 squared so start to simplify so half of 0.5 is obviously 0.25 46 times 46 or 46 squared is 2 1 1/6 times that by 0.25 and that gives us a value which is 529 it's an energy value so it's in joules calculate the velocity of a bus traveling through town with a mass of 5040 kilograms and kinetic energy of I'm not going to read this number of five hundred and twenty seven thousand six hundred and forty three jaws so writing out the equation ke equals 1/2 MV squared we know the ke this time so I'm just gonna pop that in here I find it easier to answer these questions if I rearrange later so 1/2 of mass so that's 1/2 times 504 Oates and we're looking for velocity so we keep that as B squared the easiest thing now is to work out half of the mass which is two thousand five hundred and twenty times that by V squared now we need to get V squared by itself so the way in which we do that is divide both sides by 2 5 200 so I'm going to do five hundred and twenty seven thousand six hundred and forty three divided by two thousand five hundred and twenty which gives me a value of 209 point three eight two dot equals b squared and then we just need to square root to make sure we just get a value for V so B equals fourteen point four six nine notice I haven't rounded too early I'm now going to round to three sig fig so that becomes fourteen point five and it's a velocity which means the units of meters per second so the meaty topic of conduction convection radiation this is all to do with thermal energy transfer so things heating out things cooling down starting with conduction now I'm sure you're aware that metals are good conductors the reason being is due to their delocalized electrons or you can say they're free electrons but delocalized sounds a bit fancier remember the structure of a metal you'll need to know this for both chemistry and the electricity topic of physics so to be honest it's worth learning so remember that a metal is made up of positive ions surrounded by a sea of delocalized electrons so what happens in the process of conduction is that when heat is applied the positive metal ions start by racing and they gain kinetic energy what you find then is that the kinetic energy is transferred from the hot parts of the metal to the cooler parts by the presence of these delocalized electrons so as the electrons move through the matter they carry the thermal energy with them vibrating the positive ions more and before long you can see how an entire metal object becomes hot now the opposite of a good conductor is an insulator and these are clearly going to be poor conductors so to point out things which are poor conductors first of all plastic and wood why because they have no delocalized electrons and that is the key point here air is also a poor conductor it is a good insulator and not just because it doesn't have any delocalized electrons but cleaning its ass so the particle is going to be extremely far apart so the chance of them vibrating and colliding with other particles to pass on the kinetic energy is going to be very slim hence why air is a good insulator so touching on some exams style questions so polystyrene why is it a poor conductor and that's because it contains it all air spaces and air is a good insulator because it has no delocalized electrons and the particles are very far apart so you want to link every single object you're given whether it's a poor conductor insulator next up for example a duvet how does a duvet keep you warm it's full of feathers or synthetic material that looks a bit like cotton wool and the point is again that it traps air air is a good insulator it's a poor conductor so less heat transfer will take place meaning that you stay warmer so what we learn we have learnt that metal is a good conductor wood and plastic are poor conductors and that notice that solids will be better conductors than gases now convection is a very different type of heat energy transfer it only occurs in liquids and gases you cannot get convection in a solid it literally makes no sense whatsoever and the key example we always use is a radiator in a house now why is wonderful drawing comes out so here's our rubbish radiator and it's nice and hot because it's got hot water flowing through it and what happens is that heat causes particles of air above the radiator here to gain kinetic energy now because they're gained kinetic energy the air expands which means that that the particles occupying more space now because they're occupying more space it means that they become less dense so they rise up to here as they rise they become cooler by definition they become denser and therefore they sink and before you know a convection current has been set up whereby the process keeps repeating and that's how a radiator in the corner of a room can actually heat up the entire room so we've got coffee in here and we don't want it to be getting cold too quickly so what can we do well I've already said you can put it in a polystyrene cup wipe to prevent conduction losses however because the coffee's hot can you imagine that bizarrely it will heat the air above the coffee and it will literally set up a mini convection current above the coffee that keeps repeating itself speeding up the process of cooling down that coffee what you want to do here is add a lid and that's to prevent convection currents being set up the other great thing about adding the lid is you add a layer of air in here and remember air is a good insulator so you're preventing heat losses by both convection and conduction by adding a lid last up radiation infrared radiation which we will have met in the electromagnetic spectrum it is a wave which means it is not dictated to by the state of matter what I'm trying to say here is that it can travel in solids and liquids and gases and that kind of automatically makes it the most difficult to try and prevent heat losses via however there are certain things you can do and that's all to do with the color of objects so do try to remember that white shiny surfaces reflect infrared radiation which is why in very hot countries like certain parts of Spain you might see the houses a white same in same was true for Greece and that's because the white colour reflects infrared radiation anyone with dark hair black hair you'll notice that your head gets really hot in the Sun why because black matte objects are very good absorbers of infrared radiation and notice that the hot of the object or the grace of the difference between the object and the surrounding temperature the greater the rate at which heat is lost putting this more into contact so looking at a thermos flask am I even gonna try and draw this Oh God okay so that's appalling you John but I can still point out a few things so first of all the most obvious thing here to point out is that it has a vacuum now remember that a vacuum is a space with no particles and we know that both conduction and convection require particles in order to occur so we say that there is no conduction or convection that can occur within this vacuum and as always unlinking the property with which type of heat transfer it prevents there are silvered surfaces which line the bottle or line the flask and the point of that is that it reflects infrared radiation back at the contents of the fast keeping it hot or keeping it cold the lid prevents convection currents which I've already touched upon and you can say that the whole thing is made out of plastic because plastic is a good insulator and that's the way you get these answers nice and scientific is by pointing on the various parts of it and what it stops so looking at heat transfers within a home how can we stop all our heat escaping so first of all in the loft you can have loft insulation which contains lots of air spaces air is a good insulator preventing conduction heat losses you could talk about the cavity walls so cavity meaning a hole like a cavity in your teeth so there's holes within the walls which contain air again because air is a good insulator do you notice though that convection currents can still be set up in air which is why you often find there's also foam because that solid foam stops the convection currents being set up you could talk about the radiators now having a silvered surface and that's to reflect infrared radiation back at the room and lastly double glazing remember that's when you have two window panes which trap air between them helping to prevent conduction question 9 the diagram shows how an weaves near the coast on a warm day explain why our me man leaves explain why our moves are shown on the diagram hopefully the fact that there are errors and we're talking about a gas which is air and heat hopefully that's screaming convection at you so again this is a puffer aren't so you need to devote learn your convection answer so first of all states that energy is transferred from the Sun which heats the land this means that above the warm land becomes warmer so the particles gain kinetic energy and effectively the air expands and becomes less dense this means that that heated air rises and therefore there's a space so cooler denser air falls in order to replace that warm air and you've therefore created a convection current which repeats so this is a convection current answer which you need to have just learn off by heart although they have given you a different scenario to the usual radiator in a hot room so try and make it applicable to the questions so let's start by looking at the warm land initially the warm land heats the air and that means that the air has gained kinetic energy so the air expands becomes less dense and Rises so as that warm air moves over the sea it becomes cooler and therefore denser so that cooler denser air falls and replaces that air that was above the warm land and then by doing so you've created a convection current which keeps repeating itself explain how brainy emotion provides evidence that air is made of small particles so remember brownie emission our example here our pollen grains in water or smoke particles and remember that these large particles will move with random motion and they collide with smaller invisible particles and that's the particles which we find in a there's always something about renewable non-renewable energy so non renewable energy is things like fossil fuels so coal natural gas oil these are things which are running out they're very good at providing us with energy but if we carry on using them at the rate we're using them now we're going to run out and because they come from fossils they are not going to be replaceable so we're turning to renewable energy things like solar wind hydroelectric geothermal all those sorts of things are newer energy and you need to be able to state the advantages and disadvantages of both they all tend to have the same kind of disadvantages which is that they're unreliable so for example if there's no wind that we know wind power no Sun no solar no waves no wave power the only type of reliable energies out there are things like hydroelectric because that's remember of when water falls down a dam that's tend to be quite reliable unless has been no vane geothermal tends to be very reliable but remember there's only very certain places on earth where you can harness geothermal energy places like Iceland where there's a gap between the two tectonic plates so places like in England you won't be able to get geothermal energy they could ask you how energy transfers occur for example inside a hydroelectric power station they do like to ask you that's because remember water gains gravitational potential energy because it's at the top of the dam as it falls over the dam it will gain kinetic energy transferred into electrical energy so therefore power houses this might seem quite busy but I'm just literally racking my brain to think of the questions I'll ask you they could ask you how a fossil fuel power station works and actually any power station kind of works in the same way which is that either if they're burning biomass or the burning fossil fuels what you're doing is you're heating water that water turns into steam the steam turns a turbine a turbine turns a generator and generator generates electricity so those are all the steps involved in generating electricity nuclear powers quite an interesting one because it's not technically renewable because you can't make more radioactive material however such tiny amounts are needed it means that it's very unlikely to run out so there is a huge potential for nuclear power however disadvantages include the fact that there's radioactive waste which needs to be stored very carefully because if that escapes into water supplies that could cause cancer there are high decommissioning costs and that means very expensive taking the whole Power Station apart once it's been used for about 20 years or so high startup costs obviously follow so it's going to be expensive to make those batteries in the first place you so the wave topic let's try and cover as much of this as possible there are two types of wave you needs to know about transverse and longitudinal waves learn the perfect definition for both transverse waves of vibrations are cut perpendicular to the direction in which the wave was traveling key examples here are water waves light waves and any member of the e/m spectrum infrared radiation ultraviolet x-rays etc longitudinal waves vibrations this time occur parallel to the direction which the wave is traveling your key example here is a sound wave being prepared to draw a longitudinal wave and notice that there are periods of refraction and compressions it's like a slinky but it comes close together that's compression where it moves further apart that's a ref action now the wave equation states that wave speed equals wave frequency times wavelength waves to be measured in meters per second frequency measured in hertz and wavelength measured in meters what is the frequency of the wave well it's the number of waves per second if you're labeling a wave be prepared to draw the amplitude which is from either the middle lines for the top of the wave or to the bottom of the wave it's not from top to bottom that's two times the amplitude the wave length is the distance between two peaks or two troughs the time period of a wave is the time taken to produce one wave and it's given by the equation of the frequency equals one over time period so let's talk about reflection that's all to do with waves bouncing so remember the key thing here is to no difference between the angle of incidence and the angle of reflection so the wave coming in and hitting the substance is the incident ray it bounces off and what you find is the angle at which it bounces off so the angle of reflection will equal the incident angle don't forget to lower your LUMO line which is a line it's like an imaginary line drawn at 90 degrees to the boundary surface the refraction is all about a way of changing direction when it enters a new medium and that's due to either slowing down speeding up so it's due to a change in wave speed what you find here is if you have a light wave entering a glass block it will slow down and it will bang towards the normal on exiting the block it will speed up again and Bend away from the normal so be prepared to draw that and also be aware of what happens when water waves move from a different depth so when they go from deep water you find that they're traveling really quickly and that the waves are very far apart from each other as they enter shallower water they slow down and the waves come closer together and you'll actually see the wave front getting closer together if you're struggling to remember that just remember cars on a road they'll be further apart when they're traveling nice and quickly and then when you're stuck in a traffic jam and they're traveling slowly they're basically on top of each other what is an echo well it's a reflected sound wave remember ladies echo sounding for working out to the depths of the ocean so there's some dance some sound waves they work all the time it takes for the sound wave to be transmitted and then be received again by the transmitter you use the equation speed equals distance over time so you know the speed of the sound wave you've recorded the time just make sure you have the time because obviously if you use the full time then that's the time it took tip for the wave to hit the bottom and come back up whereas the distance is only from the transmitter to the bottom which is why you have to have the time what is ultrasound well ultrasound is a longitudinal sound wave which is above the range of human hearing so it's above 20,000 Hertz try and be really detailed with your answers their infrasound is sound which is too low for the human ear to hear so it's below 20 Hertz the last thing to do with sound waves is remember that pitch relates to the frequency of the sound wave so the number of waves per second the higher that number the higher the pitch so the squeeky of the sound like whereas amplitude so that's the height of the wave is all to do with the loudness of the sound so the higher the amplitude the louder the sound so if we take my brother's finger here I do that can you see it has a very low amplitude cuz it's very short that means it's a quiet sound but because the frequency is very high it has a high pitch so it's a very sky whereas if I were to do this we've got a much large amplitude so a loud sound but because the waves are so infrequent it's a very low sound you - a microphone is connected to an oscilloscope to display a sound wave the diagram shows the trace on the oscilloscope screen the oscilloscope settings are in the y-direction 1 square is 1 volts and in the x-direction 1 square 0.001 seconds so let's make a few annotations so that must mean that up to there is 2 volts and this is 0.01 seconds which means the entire wave cycle 0.200 2 seconds so how many periods are shown on the trace i remember the period is the time for one wave so that's a complete wave so there's the first wave then you can see that repeats three times which is why the answer here is three and what is the frequency of the sound wave so frequency is given by F which is 1 over the time period so we simply do 1 divided by zero point zero zero two which gives us a value of 500 and they've already given us the units which was in Hertz on the grid below sketched the trace of a sunway with a smaller amplitude and a higher frequency than the wave shown by the dotted line so a small amp amplitude means that it will be quieter so our wave needs to be less high so it needs to be much lower high frequency means that the waves need to get closer together so I'm going to draw it like this and try and keep it nice and even so I'm only going up horn waving down one way but because they're closer together I've definitely drawn a high frequency the fact that they're lower means that it's a smaller amplitude let's now think about images formed in a plane mirror so here's our plane mirror a straight line with cross hatchings mine shows that it's a plane mirror remember we have a normal line which is drawn at 90 degrees so let's take an object such as a flower and think about the image that will be formed by that mirror so here is our object so it's the actual flower that we're looking at so how does that object actually become an image in our eye well light from an object strikes the plane mirror and is reflected from the mirror surface and then after that reflection the light strikes the eye of the observer but what can we actually say about that image well first of all the image is obviously the same size as the object and it's worth making a note of everything on my team and these are quite common sense points so don't be surprised by them secondly obviously the image will be the same color as the object the image will be the same distance behind the mirror as the object is in front but crucially the image will be laterally inverted which means the left will appear on the right and the right will appear on the left lastly this image will be virtual which means that it can't appear on a screen now one hard question that people do strugglers is working out how to draw the image when you've been given the object such as in this case so here's our plane mirror now the easiest thing to do here is use your ruler to draw a straight line from the object which hits the plane mirror then draw a second line just beneath that and then you want to reflect that back off the mirror so you could do it really neatly by drawing normal lines here measuring the angle of incidence but to be honest you can probably get away from drawing it by eye like I'm doing now and then at the end of these lines you need to draw the eye which is actually viewing the object so something like this is perfectly adequate well then to finish I'm just going to change color you want to take an approximation probably use your ruler to measure this distance here so that you can draw the image behind from the same distance to make it make sense and then just use your ruler to draw a dotted line here and really this distance from the object to the mirror should be the same as from the image to the mirror and if I rub out those kind of explanation lines really this is what your final answer should look like while the convex lenses otherwise known as a converging lens and if you know what the word converging means it is actually a just a normal English word it means coming together so if you've got a couple of rays and they pass through a convex or converging lens then they'll be brought together and they'll meet behind the lens an example of use of this is the magnifying glass now the focal length is the distance between the center of the lens and its point to focus the principal focus is the point where the parallel rays meet after they pass through the lens and then lastly the difference between a real image and a virtual image is that a real image can be formed on a screen and a virtual image cannot now if I move and I show you this hopefully this will make it a bit clearer so with the convex lens we can see that when the parallel rays pass through it they then come together ie they converge and the point where they converge behind the lens is called the focal point and that's what's going on here they're meeting and we look at someone that's long sighted so this is what happens to older people they start needing glasses to read print like reading books whatever so this time you've got the parallel rays coming in they're hitting the lens but the lens is weakened over time so rather than meeting on the retina here they're meeting behind the eyeball behind the retina so you're gonna get a very blurred image here so instead you need a convex or converging lens which is actually going to cause those rays to come in so that when they're brought on to the human lens they meet nicely on the back of the eyeball here on the retina so let's have a go at constructing various very diagrams this arrow here is supposed to be the object this is the lens and these are the focal points so based on my instructions first of all you want to draw a horizontal line from the top of the object use a ruler and a pencil for this and then you want to take it to be the focal point like that oh gosh I really struggle underneath I pad next time you want to draw a diagonal line which runs from the top of the object re through the middle of the lens and we'll see where it crosses supposed to cross through the middle of the lens and then where the two lines that is your image and then you need to compare it to the original object now because it's underneath the line we know that the image is inverted if I join it accurately the actual heights of the arrow so from here to here compared with that believe it or not would have been shorter so therefore we know that the image formed is smaller or we could write diminished if you're feeling fancy because that means smaller and then because it's formed after the lens then we know our image is real if they ask you for an example of a lens which works like this then you can talk about the eye or camera or something like that this one's slightly different we can see that the object arrows now move between the focal point and the lens so we're going to see a slightly different thing happening but it doesn't matter you can still use the same process horizontal line to the lens then we're going to take the line down through that focal point and then from here on this is why it's a bit confusing because if I then draw the diagonal line here you can see those rays are never ever going to meet on the right hand side ever they're just going to keep going and going going so what you want to do is take a your ruler and carry on the lines and make them dotted make it nice and straight unlike what I'm doing do the same here and then eventually they'll cross and that's actually where your image will be formed up here so I'm going to draw the arrow so now let's discuss what we can see well because it's the same not the same height but it's upright it's the same direction as the object arrow was facing we know that it's upright because that R is much larger we know that it's enlarged but because it's formed before the lens we know it must be a virtual image and an example of this would be the magnifying glass and your arrows will go this way dispersions to do with light splitting up and spreading out so if you get white light remember it's made up of lots of different colors if you shine it into a prism it will split into those colors remember the order of the rainbow and I'll help you with knowing what colors the light splits into so Richard of York gave battle in vain red orange yellow green blue indigo violet now red is refracted the least because it's so down the least violet is refracted the most because it was travelling the fastest and therefore slows down the most five eschew uses of ik tango glass block to determine effective index of glass the diagram shows array of red light in there as enters the glass block at P the normal at P is shown as a dotted line which as we would expect as perpendicular complete the diagram by drawing the way that continues inside the block labeling the angle of incidence and the angle of refraction and drawing the Ray that leaves the block so use your ruler here and a pencil so remember as it enters the block it enters the more dense medium so it bends towards the normal which is why I'm doing it here we need to draw a second normal line down here try and draw that more straight than I have and then remember as the Ray leaves it is again parallel to that Ray that came in which is why I'm doing it at this angle now we must label the angle of incidence and the angle of refraction so the angle of incidence is between the normal and the incident ray the angle of refraction it's going to be hard to show that that's in there and that's between the normal and the way that has been refracted the student measures values for the angle of incidence and the angle of reflection R so I is given as 60 degrees I was given as 34 degrees so we need to complete the table wine setting values for sine I and sine R so that's quite straightforward just put into your calculator sine 60 and when you've done that that is zero point eight seven to two decimal places put into your calculator sine 34 and that is zero point five six to two decimal places state the equation linking refractive index angle of incidence and angle of refraction remember refractive index is given by n so it's any course sine I over sine R so in terms of calculation effective index sine I is 0.87 sine R is zero point five six and when you put that into your calculator you get a value which is one point six two two significant figures how should the student continue the investigation to obtain a more accurate value for the refractive index of gloss so that's worth three marks to make three separate points so the most obvious point to make is to increase the reliability you want to repeat and calculate an average you want to vary the angle of incidence so you could do 50 degrees 70 degrees 80 degrees and then that's you could draw a graph plotting sinai against sine and remember that the gradient will give you the refractive index the electromagnetic spectrum now which is a little sub topic of the waves topic I do like this topic remember that it is a family of waves and they vary just in terms of their frequency and their wavelength so let's go through those ways first of all so starting with the longest wavelength wave so waves which look more like this so they have long wavelengths because you remember the wavelength is the distance between two peaks or two troughs as opposed to having short wavelengths whereby the wavelength is this big as opposed to this big so we're starting from the longest wavelength that is radio waves then we have microwaves infrared radiation visible light ultraviolet x-rays and gamma rays and I'll provide a link below this video which I love one of my two T's told me about it a couple years ago and it's I think it's sold by some South Korean guides if you start playing it as 90 seconds listen to the chorus through three times and you'll have to order nailed in your head otherwise I'm sure there were lots of mnemonics that people can remember and comment below if you know of a good one tell us a frequency clearly gamma rays will have the highest frequency because there's more waves per second with gamma rays because they're much closer together where is microwaves and radio waves will have a much lower frequency because their waves are much further apart the key point to notice with this topic is the various uses of all of these so we'll start with radio waves remember these are used in communication microwaves are also used in communication so satellite communication they're also obviously used in cooking food most of us have microwaves in our kitchen then we have infrared radiation and that is also used in communication and in this case we're talking about remote control so communicating between the remote control that you're pointing at the TV for example so infrared radiation is being given up there remember we met infrared radiation when we're talking about the heat topic so conduction action radiation because infrared radiation is simply heat being given off from objects and therefore everywhere variation is the type of radiation used in ovens because obviously they get hot to cook our food and they are giving off infrared radiation visible light as well as the usual use of so that we can see stuff with them it's also used in optical fibers and photography now we're getting down to the other end of the spectrum so when the wave start getting much closer together the next way we're going to be looking at is ultraviolet so if UV remember these arrays that come from the Sun they have various uses such as tanning beds so some beds that will have UV rays being emitted but also you can use it to check where the banknotes have been forged or not to hold up to the UV light and you can tell if they're authentic then we're looking at x-rays obviously they're used in medicine to create images of the human body and lastly gamma rays gamma rays are used to sterilize surgical equipment because there's such high energy they kill things effectively so that could be killing bacteria on surgical equipment it could be killing cancer cells so used appropriately they're very useful if they're used inappropriately they can actually cause cancer so that's worth noticing the way they'll ask the questions is it will say stuff like microwaves are used in communication named two other ways also used in communication you could have said infrared radiation you can set radio waves all visible so be aware here what dangers associated with infrared radiation well because it's effectively heat if they gets too hot it can cause skin burns if you touch it dangers associated with ultraviolet that will be skin cancer because again if used inappropriately back and cause cancer as well as the rays from the Sun if we're exposed to them for too long it can cause cancer how can we reduce danger from exposure to ultraviolet well wearing sun cream wearing sunglasses to protect our eyes limiting our exposure time coming up wearing clothes protect ourselves from x-rays we need to limit our exposure time hide behind LED screens wear protective and the same is true of gamma-rays you really don't have to be exposed to these race for too long they could ask you what all these Rays have in common and that is that they are all transverse waves so in all cases vibrations are cut perpendicular to the direction which the waves traveling remember that for proper definition of a transverse wave they clearly all transfer energy they may all be reflected refracted and diffracted so they can all bounce of objects they can all change directions they go through a different medium and lastly they travel at the same speed which is 300 million meters per second we start by looking back in history and look at the original structure of the atoms so we can actually understand all our findings that we know to be true today so originally there was the Thomson plum pudding model now you don't need to know too much about this but just know that a plum pudding this was in the 1800 by the way so you can imagine a Christmas pudding if you don't know what a plum pudding is I don't know one what is so big sphere of sponge embedded with different types of fruit and in the case of the plum pudding those were plums and Thompson stated that the sponge was made out of positive charge and that those plums embedded within that sphere of sponge were the electrons now we know that this is false because we now know the modern-day structure of the atom with its nucleus and its shells with the electrons circling and we're going to talk about the gold foil experiment to help us understand why this new item became the accepted model so Rutherford fired alpha particles and we'll talk about alpha particles soon at gold foil now he found that most of them passed straight through and this was strange because really if the atom was structured like a plum pudding there's no way these alpha particles should have passed straight through but because they pass straight through it told him that the atom is largely empty space which we know to be true some alpha particles were deflected and because an alpha particle is positive we had told him that they had to hit something also positive and that they had been repelled and that made him understand that the nucleus was positively charged which we know to be true because that's where the protons are found lastly very few of the alpha particles were deflected in this way and this told him that the nucleus was very small so do link together what he did with what findings he found out and the conclusions he drew from those findings and that'll help you score really highly so we now have our structure of the atom we know that it has a nucleus containing protons and neutrons remember this is also known as the nucleon number and that's just the name given to all the particles found within the nucleus and surrounding the nucleus are the shells of electrons where the electrons orbit so just to remind ourselves that the mass of a neutron and a proton is one a mass of an election it's very small so 1/2 thousand or 1,800 depends where your teachers taught you and that the neutrons because they're neutral have no charge protons have a positive one charge and electrons have a negative 1 charge now looking at the periodic table just remember that the atomic number is the number of protons and the neutron and the mass number is the number of protons plus the number of neutrons this will become really important when we now come to look at isotopes you've probably met isotopes in chemistry so things like carbon-12 carbon-14 remember that these are atoms of the same element with the same number of protons the different number of neutrons if we look at carbon-12 and carbon-14 in the periodic table they both have an atomic number of six which makes sense because they're the same element which means that they must have the same atomic number their mass numbers are different now carbon-14 has two extra neutrons when compared with carbon-12 so when we look at radioactive isotopes we're just talking about isotopes which are unstable and tend to give off radiation so ionizing radiation may be given off and at them and this is a random process and it may give off alpha beats or gamma radiation and that's what we need to talk about now so I'm going to start by chatting you see what whopped alpha beta and gamma radiation is and then I'll show you some decay equations so alpha radiation first of all so what happens when alpha radiation happens is that two protons and two neutrons are lost from a particular atom and clearly therefore you will have a new element whose atomic number remember that's the proton number is to fewer than it was before and the mass number because it's lost two protons and two neutrons will have gone down by four with beta radiation what's happened this time is that a neutron has turned - a proton and stayed within the nucleus of that particular atom so because a neutral and a proton has the same charge clearly the mass number will be unchanged for the proton number will have gone up by one C will have a new element now gamma radiation is very different because it's an electromagnetic wave so you see no change in mass or atomic number due notice that alpha radiation is the largest it has the largest mass because it's made up of two protons and two neutrons and it is the most ionizing whereas gamma is the least ionizing an ionizing this is the ability to cause something else to become an ion which is a charged particle so if we compare the properties of alpha beta and gamma notice that alpha as I've already said is the most ionizing b2 is in the middle gamma is the least ionizing in terms of their penetrating powers that's how easily they passed through substances you notice that alpha is stopped by a was stopped by aluminium foil and gamma is stopped by the several centimeters of lead or several meters of concrete in terms of their range and an alpha has a range of only about 5 to 10 centimeters beta has a few meters range whereas gamma has an indefinite infinite range in a the table shows the nature of alpha and beta particles so alpha we know is a helium nucleus made up of two protons and two neutrons and B 2 is a fast-moving electron explain why alpha particles and beta particles have different penetrating powers so basically let's start by looking at the difference so state first of all that alphas are much larger because they're made up of two protons and two neutrons so they have a heavier mass they have a higher charge this means that they cause more ionization which means they can't penetrate as far and that's due to all energy which has been lost as they've caused lots of ionization and also point out that the alpha particles are more likely to collide with the atoms because they are bigger or you could give the converse argument for beta so you could say they're smaller they have less charge they cause less ionization they have a lower mass so in this first alpha decay equation let's have a look we need to work out the new mass number and the new atomic number of thorium so we're starting with uranium-238 that is its mass number well I've just told you that you lose two protons and two neutrons in alpha decay which means the GNAT mass number must have therefore decreased by four which is why it's new mass number is two three four because you've lost two protons it means that the atomic number decreases by two to ninety and that is your answer in the second example I'm after the new mass number of lead so we've lost two protons two neutrons again which is why the mass number will have decreased by four to become to one two and this time we're looking for the original atomic number or proton number of polonium so we know that we've lost two protons to get to eighty two which means the original atomic number must have been to more than eighty two so the answer here is 84 in this beecher equation we're looking at what happens to sodium so I've already told you that in beta decay a neutron turns into a proton and stays within the nucleus of an atom because of that the mass number is unchanged so we're going to stay having a mass number of 22 the neutron turns into a proton and stays within the nucleus and because of that we have an increase of 1 the atomic number so that turns into 12 and because we've increased our Tomic number we clearly have a new element and if you look in the periodic table you'll see that the element with the atomic number of 12 is neon so in this question we're looking for the new mass number and a new proton number again so we have a neutron turning into proton meaning that the mass number is unchanged which is why I'm going to write a 14 here because we've gained an extra proton it means the atomic number will have gone up by one forming seven and because you have a new atomic number we have a new element and that element of according to the periodic table is nitrogen now we're going to be moving on to looking at half-life and background radiation so let's first of all state what is background radiation and it is radiation which is always present in our surroundings and so you may need to list several sources so that could be cosmic rays from space it could be radioactive rocks like granite it Vanacore more could be food and drink and medical sources such as x-rays now what is the unit for measuring radiation it is the Becquerel capital B little Q and what instruments to be used to measure the level of radiation whether that's background or not we use the Geiger Miller detector or counter do you remember if you're given a radioactive source and you need to determine its radioactivity you must work out the level of background radiation and then - and then subtract that from the value you get from your guy Camela counter to make sure that you remove it from your calculations so half-life now let's define it first of all remember this is the time taken for half the radioactive nuclei to decay and you might have used coin tossing at school to help you model this the reason why that works really well is because radioactive decay is a random process and as is tossing the coin and with tossing coins you've no idea you can't predict whether it will land on heads or tails and it's the same with radioactive decay you have no idea which of the radioactive atoms will decay fast in terms of using tossing a coin as a model does have several limitations and this is really the number of times you can do it I mean realistically probably at most about thousand times whereas with radioactive decay you're looking at millions of atoms which need to decay and now I'm going to show you a question involving half-life and I'll talk you through the half-life calculations because that probably the most difficult part of this topic the sample of sodium 24 has an activity or fourteen hundred becquerels on the access sketch a graph to show how the activity of this sample changes over the next 40 hours the half-life of sodium 24 is 15 hours so at time equals zero we're going to have an activity which is 1,400 we know that the half-life is 15 hours so look along 15 hours half for 1,400 is 700 so we need a cross bar when another 50 ours has taken place so at 30 hours that's 700 will have hard again for 350 so we need a mark here and now you can attempt to draw a curved line which I'm appalling up oh it's not too bad granite is a rock it contains a radioactive isotope of uranium that decays very slowly explain how scientists can use this radioactivity to find the edge of a piece of granite so first of all make a note that there is a known proportion of activity when the rocks are formed this means that you can now measure the proportion of uranium now and then you compare the activity of the original uranium value with the activity now and this can help you determine the number of half-lives which have elapsed and then you calculate the age at the Forum reference to half-life suggests why the age of a piece of granite could not be fine using uranium isotope with a half-life for 15 hours and that's obvious because the half-life is too short 15 hours as meant that the decay has occurred far too quickly and so the activity now would be far too small to measure you know 25 has a half-life of one minute what fraction of it remains after three minutes so what you have to do here is work out how many laughs half-life's have occurred and because it's three minutes and one goes into three minutes three times three half-lives have occurred and then all you have to do because you're finding out a fraction is do 1/2 times 1/2 times 1/2 to find that 1/8 remains after three minutes question 3 then on 1 3 3 is the radioactive gas used for diagnosing lung problems in 15 days it's activity falls to 1/8 of its original value what is its half-life so we need to work out how many half-lives occurred to get to 1/8 so what you need to do is you might have to do trial and error but you need to just do as many half-lives as you need to in order to get to 1/8 and the answers actually going to be three of them so three half-lives occurred in 15 days so how long did it take for one half-life to occur where you just need to do 15 divided by 3 and the answer is 5 to 5 days is its half-life question for the half-life of radioactive isotopes alien 24s 15 hours the sample has the camera to the - 240 counts per minute this can't mate 60 hours later we'll be okay so this time again we need to find out how many half-lives have occurred so I'm going to do 60 hours divided by 15 and I see that four half-lives have occurred so therefore after one half-life 120 counts would remain but there were four of them so I'm going to times it by a half four times and that is the same as you hundred forty times answer and that is 15 as my answer so it's count me after four half-life's have occurred so I've divided 240 by 2 or times them by half four times and I have got 15 as my answer so that's 15 counts per minute question five a radioactive isotope of silver has a half-life of twenty minutes a sample gives a rate of six thousand four hundred counts per second at nine o'clock what time will decant maybe about two hundred counts per second okay this sounds hard but again just use trial and error to work out how many half-lives have occurred so I'm just gonna G six thousand four hundred times a half and I've got 3,200 so I'm going to times it by half again and I'm gonna keep going until I get to two hundred and actually what I found here is that it has taken five half-lives in order to get to 200 counts so all I did was times 6,400 by 1/2 five times and I got to 200 so like I said that means five half-lives have occurred and each half-life took 20 minutes so therefore five half-lives takes a hundred minutes so all you need to do now is work out what 100 minutes past 9 o'clock is and remember there 60 minutes an hour so that brings us up to 10:40 that one was quite hard but you can work it out like I said just use trial and error to work out how many half-lives have occurred practice doing as many questions that these as possible in order to get good at answering the different types a question remember when you're reading off graphs to look at you can beat off the half-life by looking at how long it took for the counts to reduce by half and then you need to read a cross to see how long that took take into account background radiation because you'll see that the graph line will never actually touch the x-axis and that's because background radiation exists and remember but that comes from things like cosmic rays x-rays rocks just general stuff really and remember half-life is actually the amount of time taken for half the radioactive nuclei to decay so after 42 days the activity of a sample of phosphorous 30g has decreased from four hundred becquerels to fifty becquerels what is the half-life of phosphorous 32 so we've gone from having an activity of four hundred becquerels to fifty so we need to work out how many half-lives took place so there's the first one which would have taken us down to two hundred becquerels the second one would have taken us down to one hundred becquerels and then the third one would have taken us 250 back around so we can see here that three half-lives have taken place and what was our time frame well three half-life's must have occurred in 42 days so then it's a simple expedient of doing 42 divided by 3 to work at how long each half-life took and that answer is there for 14 days looking at uses of radiation alpha radiation is used in smoke alarms I'm going to chat see how this works so you have an alpha source in a smoke alarm which is giving off alpha particles and what happens is those upper particles collide with particles in the air and they ionize them creating a small electric current which is picked up by the detector now I've already told you that alphas range in air is very limited so the moment there's a fire there's now smoke particles in there and they obstruct the Alpha and they stop it reaching the detector so they're detected get zero reading and therefore the alarm goes off Beauty radiation is used in aluminium foil thickness to kitchen for the eruption sandwiches in so there were two rollers and there's a detector either side so you've got a source of beta detector on the other side and there's a certain amount that should be picked up and if that detector reading is too low it tells you that the aluminium foil is too thick so the rollers pressed together to make the foil thinner and so the detector can pick up the correct amount in terms of in use in medicine do you remember that we use radioactive sources in medicine such as radio active iodine and we call this a medical tracer and that's because it's used to diagnose things like kidney problems so the patient takes a sample of radioactive iodine it flows through their body and as a detector which picks up how much radiation there is and you'll see a characteristic graph meaning where the meaning stays high and that tells you that there's a blockage within the kidney so they do make great diagnostic tools do notice some crucial properties there and that is that you've got to have a an isotope which decays into a stable product which makes sense because you don't want it giving off radiation it has a medium-sized half-life you don't want it to have such a short half-life that you can't actually pick up how much there is because it's already decayed away but you also don't want it to be so long but you're staying radioactive for many years to come because that is dangerous and this is because of all the dangers relating to radiation to remember ionizing radiation causes mutation within ourselves a mutation is the first step leading towards cancer so it's pretty nasty stuff we can reduce our risk and limit our exposure to radiation by wearing protective clothing by standing behind LED shields by using tongs to handle the radioactive material and for using things like photographic film you see these internal medical badges the radiographers wear and that will show up if they've been exposed to too much radiation looking at use of radioactive carbon - carbon 14 in carbon dating so that's when you determine how old plant material is for example so carbon 14 is radioactive and during a plant's lifetime remember when it photosynthesizes it takes in carbon dioxide so a certain proportion of the plant will contain carbon 14 now once it dies clearly it won't be photosynthesizing anymore and over time that radioactivity decreases and by comparing the radioactivity of a sample with the radioactivity of a living version of that particular plant you can work out how odd it is that's a very clever dating technique you so nuclear fishin let's be very specific with our definition remember that it is the splitting of an atomic nuclei do say atomic nuclear if you don't say that you don't get the mark so where is this carried out its carryout artificially within a nuclear reactor which which means we need to add a fuel and we tend to use uranium-235 so we have our nuclear reactor which contains uranium-235 to get the nuclear fission process underway we have to fire a single Neutron at that uranium nuclei the uranium nuclei splits into two daughter nuclei and I'm being very specific with my wording and you should be using this exact wording in your exams releasing three two to three neutrons these go off and collide with other uranium nuclei which splits forming two more daughter nuclei and three more neutrons are released and before you know it's a chain reaction has been set up and this can obviously become extremely dangerous if it isn't controlled hence the Chernobyl disaster back in the 80s so how do we try and improve the safety of the nuclear reactor well we have several mechanisms such as control rods these dip into the nuclear reactor and what they do is they absorb neutrons so clearly if you take away neutrons they can't collide with any uranium so the whole reaction slows down you also have a moderator and what a moderator does is that it slows the neutrons notice the difference control rods from remove neutrons moderators slow the neutrons so they lower the kinetic energy meaning that they collide with less energy and therefore subsequently of the reaction is made safer you might find there's a coolant and that's just called water which should remove some heat from the reaction and you'll also see that it is contained within a big lead box or concrete and that's to improve their safety to make sure that people that work in the nuclear reactor don't get exposed to radiation now the type of energy released is going to be kinetic energy and the kinetic energy is converted to thermal energy which is used to heat water that water turns into steam and steam turns the turbines and the turbines the generator and that's how the electricity is actually produced from a nuclear reactor there are disadvantages although this is a great way in which we can produce electricity only a very small amount of uranium in order to generate huge amounts of electricity it is extremely dangerous the radioactive waste remains radioactivity for many years it needs to be stored safely underground and make sure that it can't contaminate water sources which we drink it has high decommissioning costs and that's to do with how you actually take apart the nuclear reactor once you are done with it and that's obviously due to the safety aspects and it also has high startup costs we're building it in the first place will be extremely expensive you 11 in a nuclear reactor uranium-235 nucleus absorbs a neutron and fish and occurs complete the equation below that shows a typical fission reaction please don't find this totally overwhelming we can work it out so we've got uranium-235 going in and a neutron is fired at it leading to I think that's barium and Krypton being produced as a smaller nuclei and three neutrons given off so basically it's like just like maths or algebra or something you just need to make sure that whatever you started out with which was uranium-235 everything else produced after that has to add up to 235 you can't just randomly lose matter so effectively what you need to do is add 142 to one and also to one because that's the neutrons accounted for and then if you add those all up and take them away from 2 3 5 you'll get an answer which is 91 and that needs to go here and then the same below you've now used got a proton number an atomic number of 92 and we can see zero against the neutrons so we'll need to worry about is the Krypton 36 so just do 92 take 36 and you'll get an answer which is 56 be explained how nuclear fish and can lead to chain reaction so for the first mark you want to say that the neutrons are released that these are then absorbed by other uranium nuclei and this causes further fish and reactions for another much he could have said that neutrons are slowed by the moderator but I don't really know why you think to write that particularly see the diagram shows a nuclear reactor on the diagram label the control rods and the shielding so the black things hanging down here are the control rods and the shielding is the Box on the outside which prevents excess radiation getting out explain while shielding is needed you need to talk about the fact here that the reactor material or the waste is radioactive and then a second mark that can lead to cancer which is very dangerous so that's why you obviously need to shield it and keep it away from people and the reason why you do need shielding is because remember that radiation is very penetrating it will pass through a huge amount of air so you do need that concrete box around it to prevent it getting out 12 the diagram shows the main parts of a nuclear reactor so we've got our control rods moderator fuel rod shielding reactor vessel and coolant in do you remember the control rods absorb excess neutrons and the moderator removes the kinetic energy and the moderator reduces the kinetic energy of the neutrons the fuel rod will obviously provide the source of uranium anyway draw a straight line linking each part of the reactor to its correct purpose so I've already accidentally said what the control rod does which is that it absorbs neutrons I'm going to go to a fuel rod because that's the next easiest one to answer remember that contains uranium the moderator I said decreases kinetic energy which is the same as slowing them the reactor vessel keeps radioactive material inside the reactor that makes sense so the coolant transfers thermal energy that also makes sense which of these is a nuclear fission product should remember that's what is made after nuclear fission takes place and remember nuclear fission is the splitting of the atomic nuclei which releases neutrons which is why see as the answer here describe the process of nuclear fission so neutral a neutron is fired at the uranium-235 nucleus and the nucleus splits producing two daughter nuclei and up to three extra neutrons and I've actually already said three or four months worth of things and you can say that kinetic energy is released state three ways in which nuclear fission differs from radioactive decay she remember nuclear fission is a human artificial process whereas radioactive decay occurs naturally so start by saying that decay is a random process which occurs naturally whereas fission is not random it's due to humans firing neutrons uranium-235 you want to talk about the fact that fish and produces two daughter nuclei whereas decay only produces one and remember that alpha and beta or gamma are produced by decay whereas a nuclear fission either alpha or beta or gamma are released instead it simply daughter nuclei and neutrons so let's start by looking at the solar system so the solar system contains all the planets that you guys have heard of and in order starting closest to the Sun you have Mercury Venus Mars Jupiter Saturn Uranus Neptune and sometimes people can't please her but I don't think we're supposed to cancer anymore because it is too small to be counted as a planet you saw me do my thing of me remembering this the way I remember the order of the planets is my very easy method just speeds up naming and that is a great mnemonic for helping you remember the order of the planets now more terminology we need to look at things like the galaxies galaxies may have called billions of stars so it's a collection of billions of stars what is the universe where the universe is made up of billions of galaxies so we on earth are living on a teeny tiny planet which exists in a galaxy the galaxy you do need to know the name of it's the Milky Way and the Milky Way is just one of billions of galaxies which make up our universe so yeah it's kind of mind-boggling I try not to think about it too much because it freaks me out so we know that the various objects in space are held in position due to the gravitational force however what does the size of that gravitational force depend upon firstly the mass of an object and this is why the gravitational force on Jupiter is much greater than that on earth because Jupiter is so much larger has a way bigger mass compared with earth and secondly it's the distance between the objects so the closer the objects are the greater the gravitational force so we've already touched on it but what is that gravitational force responsible for so it causes moves to orbit planets planets to orbit the Sun artificial satellites to orbit the Earth and lastly comets to orbit the Sun and try and remember that comets have very elliptical orbits so if this is the Sun the comet's orbit is incredibly oval-shaped it's not really circular at all we call this shape elliptical and the comet travels fastest when it is closest to the Sun and that's all to do with that gravitational force being stronger which we mentioned above when the Comets further away such as this point over here it will be traveling fast slower just to touch on one more thing so looking at moons and artificial satellites a satellite is any object which orbits our planet or the Sun so let's make a note here and there were two categories artificial so ones that we put into space and natural and that's why I've highlighted moons because these count adds the natural satellites the next thing we need to look at is quite a complicated part of the specification which is why does a satellites velocity change even though it travels at a constant speed and that kind of sounds like you're arguing with yourself but you do need to know the difference between speed and velocity here even though they sound like the same thing so the real difference between speed and velocity is whether they're scalar or vector quantities so speed is a scalar quantity velocity is a vector quantity and a scalar quantity just has a size or a magnitude whereas a vector quantity has both size so the magnitude and a direction and that's key so although you could have a car traveling along the road we know that drawing is not my strong suit we could say that it speed is 10 miles per hour so you could also say because velocity also has a size you could say that it's velocity is 10 miles an hour but to make it into a prop back to quantity we need to say it is traveling at 10 miles per hour in a North Lee say a word direction or you could say to the left so notice in order for it to be a vector quantities such as in the case of velocity it needs both the size and a direction so going back to our initial question which I'll write out again so why does the satellites velocity change even if it is traveling at a constant speed this is because the satellite constantly changes direction because it's moving in a circle despite the fact that it is traveling at a constant speed hence its velocity is changing you next up let's consider our historical models of the solar system so in today's modern-day world we understand that we have a Sun that forms the middle of our solar system and then orbiting the Sun is a collection of planets I'm not really gonna be able to draw but we know their planet closest to the Sun is mercury followed by Venus Mars so my very easy method just speeds up naming planets and that these are all orbiting the Sun so they go round and round in circles with their slightly elliptical orbits notice that they're nowhere near as elliptical as our comet so if we were to look up you can see how incredibly squashed that circle is for the comment planets have far more circular orbits so this is the modern-day model of the solar system we call it the heliocentric model it puts the Sun in the center of our solar system and the man who was responsible for this was called Copernicus great name this is why I think I'm not going to achieve anything in particular because I don't have a great name think about Einstein Hawking Newton maybe I'm imagining that they have great names but they sound great Lyndsay less great okay so Copernicus the reason why he was able to count this model is because he actually could see what was going on so he used telescopes to suggest the new model and today I'll telescopes are even stronger and so that she managed to prove that his model is correct so historically speaking what was the other model of the solar system well it was the geocentric model and as you might imagine because we had no evidence to the contrary we put ourselves as humans on Earth in the center of the solar system so this puts the earth in the center of our solar system and the reason for this really is because we were basing all our judgments on what we could see with our naked eye and it's because our naked eye really can't see very far we can actually see other features of our solar system so we weren't able to observe asteroids moons of other planets very distant planets so ones like Neptune and to name someone who actually thought that the geocentric model was correct Fattah Mele what and Watson tells me that Tom Lee I can't say is the guy responsible is that a silent P right now let's now look at the origin of the universe what the statement origin of the universe and there are two theories you need to be aware of these are the Big Bang Theory and the steady state theory and we're going to summarize these now so the Big Bang Theory's it's probably the one you're most familiar with and that states that the universe is expanding after exploding suddenly in a huge explosion which we call the Big Bang and it means that the universe started from a very small point and that space-time and matter were created in the Big Bang I'm going to write that down now whereas the steady state theory suggests that the universe has always existed that it continues to expand and that matter is continuously created throughout time notice that there is a similarity in these in that in both theories we state that the universe is expanding it's just the beginning of the universe is different so let's highlight the things which are the same for both while they're both state that the universe is expanding the difference is to do with how matter was created so what evidence do we have for these while it's split broadly into two categories so evidence number one is redshift number two is cmbr which is cosmic microwave background radiation so let's take each of these in tan so the redshift is quite a hard concept to understand but basically because we think that the universe is expanding it means that we think that distant galaxies are moving farther away from us so if we're stood on earth over here I'm gonna pick one star in one galaxy over here and remember the Stars are luminous they emit light which can be picked up by telescopes on earth now according to the fact that the universe is expanding and that the galaxies are moving away from us hopefully you can see that this star would end up in a new position further away now if the light being given off that original star has a wavelength of something like this are you happy that when it moves further away that that wavelength gets stretched so it has a longer wavelength and if we were to look at the spectrum of colors when you have a longer wavelength it means that the light emitted is going to be red up hence why we describe this as redshift so the light has been red shifted so really redshift provides evidence that the universe is expanding so redshift can be used to support both the Big Bang Theory and the steady state theory and this is really important that you realize this so both of these theories can be supported by redshift looking now at CMB our so we've already said that this is cosmic microwave background radiation it's detected in all directions in space it has a constant temperature of approximately minus 270 degrees Celsius and it's thought to be the remains of thermal energy from the Big Bang spread thinly across the whole universe now as you might imagine because we've mentioned Big Bang in cmbr hopefully you can see that the Big Bang Theory is supported by both redshift and cmbr because they do like to ask you about this however cmbr does not provide evidence for the steady state theory you we now need to look at the life cycle of stars both very large stars and stars which are smaller like our Sun first of all we need to understand the term nebulae now in every there is just a big cloud of dust and gas and that's beginning other stars like we're going to take a small star to begin with such as our Sun and we're going to look at its life cycle so to begin with that nebula that big old ball of gas and dust needs to be brought together how to through gravitational pull so it pulls together all that dust and gas and then at that point it can start burning fuel and the fuel in question is hydrogen because the hydrogen nuclei come together we call that a nuclear fusion because they're fusing and at least a huge amount of both light and thermal energy at this point the stars in its main sequence it's like the adult part of its life it will eventually run out of hydrogen fuel and it will start burning helium and then it will swell up to form a red giant then at this point it turns into a white dwarf and and when it totally runs out of fuel it will turn into a black dwarf large stars so very very large stars have a slightly different lifecycle they start in the same place which is that they have a nebula so the huge dust cloud gas cloud will be pulled together through gravity then it will actually its main sequence and will start burning through its hydrogen fuel again nuclear fusion is occurring however this time when it runs out fuel it will talons for huge events supergiant's and then at this point a huge explosion occurs called a supernova I love that word and then depending on the size of the star it will leave the form a neutron star and it's a star very very massive it will form a black hole which you probably heard of from films so how do scientists observe the universe before that's to use of telescopes we've already pointed out all the issues with using our naked eye honestly really with our eyes we can't really see anything so what are the three types of telescope you need to know about while firstly it's optical telescopes the second is the radio telescope and the third one is the space telescope so let's have a look at each of these so the optical telescope to do with optics so we're looking at light so it's used to observe visible light from space they tend to be quite small which enables everyday people like ourselves to observe the night sky relatively easily and for people who aren't particularly skilled in this field we call ourselves amateurs so it allows amateurs to view the night sky relatively easily and crucially they can only be used at night because they're observing light being emitted from stars in our galaxy and their use is affected by poor weather conditions so if it's foggy you won't be able to see anything or if there's a lot of light pollution radio telescopes as the name suggests are used to detect radio waves coming from space they can be used in all weather conditions they're very large and expensive as you might imagine things which tend to be more technologically advanced do gain a lot in terms of how much they cost lastly then Space Telescope's hopefully you can see we're getting more complicated now what you find is that objects in the universe emit electromagnetic radiation such as infrared rays x-rays gamma rays and although these are often blocked by the Earth's atmosphere they can be detected by telescopes which have been placed in space so in orbit around the Earth I said quite a lot there so I'm gonna write it all so you can actually see it written down running out of spaces I'm just gonna carry on chatting about Space Telescope's down here so they can be used to observe the whole sky they can be operated both during the day and night clearly they're gonna be very expensive and they're actually quite difficult to set up and because they're in orbit in space we can't go and fix them if they break only astronauts can fix them so properly trained space people so we've compared the three microscopes the optical telescopes the one that we'd use to observe the night sky radio telescopes and all the way up to space telescopes which are the most technologically advanced we've seen there were lots of disadvantages with the optical telescope the fact that they can only be used in the night time the fact that they're affected by poor weather conditions but how could we improve the picture produced by an optical telescope well we could use a wider aperture that's all to do with the width of the lens you could use a better quality lens and clearly pick the best conditions to actually use it in not that you do this but if you actually went to a desert you'd be more likely to see the sky more clearly because there'd be less light pollution gonna write that here just for completeness sake and now we're going to look at some past paper questions because this topic is quite tricky and I want you to see the sort of questions they're going to ask to a scientists no longer accept the geocentric model of the universe that it was the accepted theory for hundreds of years explain why the evidence available at the time supported the geocentric model so what is the geocentric model well that's that historical but incorrect model of the solar system which is that earth is at the center and as we know to be the case today the Sun is at the center of our solar system and all the planets and moons orbit the Sun so explain why the average events available at the time supported the geocentric model so why would people think that earth is at the center well firstly because they have no scientific evidence the only evidence they had is what they could see with their eyes and what was this evidence they could see with the eyes well it showed to them that the Sun and the moons appeared to move across the sky in the same direction and due to that people thought that them sitting on the earth meant that they were in the middle of the solar system and that the Sun and the moons were just zipping past be the Big Bang Theory and the steady state theory are two theories about the origin of the universe the discovery of CMB led scientists to accept only one of these theories explain why redshift supports both of these theories but at CMB supports only one of them so we need to make some very basic comments first all about what these two theories state and what they have in common so you want to start by saying that both theories state that the universe is expanding but they're only the big bang theory states that there wasn't really a beginning where there was a huge explosion which sent all the matter whizzing out into space and so if we now look at our pieces of evidence let's first of all take redshift now light from distant galaxies is redshifted which means it has a longer wavelength therefore indicating that these galaxies are moving further away from us so actually the redshift Theory does indicate that the universe is expanding and therefore supports both the Big Bang Theory and the steady state theory however this CMB which is present has to provide evidence for there being a beginning and that's why it only supports the Big Bang Theory and it is unable to support the steady state theory Oh No haven't been concise enough falling off the edge of the page here see part I a star with a mass very much larger than the Sun okay so what do we know about our various stars and depending on their size about their life cycles so let's take a large star to begin with so it goes through its main sequence which is actually longer than that of a small star and then it swells up to form a red supergiant followed by a supernova which is a huge explosion and then either a black hole or a neutron star depending on its size however a smaller star such as our Sun enters a shorter main sequence swells up to form a red giant but not a supergiant and then a white dwarf followed by a black dwarf I do love these names so let's have a look in these options so a star with the mass was really much larger than the Sun has a longer main sequence than the Sun and ends as a white dwarf no there's no mention of a white dwarf in my larger star notes B has a longer main sequence in the Sun and ends up as a black hole yes that's definitely a potential C has a shorter main sequence Nora already know that's wrong D has a shorter main sequence no that's wrong again which is why the answer here has to be B which row has two correct statements about black holes so a black hole is something which a kind of freaks me out because basically nothing can escape it nothing at all no light nothing nothing nothing nothing nothing nothing and that's why we can say that no electromagnetic radiation can escape and if you have a look at what I have written in terms of forming a black hole you can see that it comes from very large star going through this red supergiant phase followed by a huge explosion so let's have a look and see if any of these options match up with that one thing I do want to point out is a nebula is completely wrong because that's the very beginning of the star's life so it wouldn't be a nebula collapsing so that's why a is 1 B allows nothing to escape yes that's true a very large star collapses yes that's true C allows nothing to escape but yes that's true I've already said why it's not our nebula collapses so C is incorrect and then D only allows electromagnetic radiation to escape no I've said that even light can't escape so that's why B is the only right answer here we do need to know a lot of that stars they three figure to show some lines in the absorption spectra from four different galaxies a B C and D and formula poetry source all the spectra are aligned unto the same scale so for a b c and d explain using figure to which galaxy is furthest away from us do notice that we've got this wavelength here identified and that's because it's the only thick line and really that's our reference point we're going to be comparing the thick line on a b c and d and the other thing to notice is which galaxy is farthest away from us and that means is the most redshifted because remember when galaxies move away from us their wavelength effectively gets stretched like this the wavelength is longer and then if you look at these wavelengths along the bottom of the scale you're looking for the longest wavelength so which one is the most redshifted and the longest wavelength you'll find towards the right-hand side the furthest right of these lines which is why I hope you can see this which is why this is the answer see because it has been the most red shifted and we're going to write about that now dalek CC is furthest away because it has been red shifted the most and is therefore traveling at the fastest speed cool and that is done part 2 in Figure 2 the reference wave length is shown at 390 nanometers estimate the change in reference wavelength for the light from galaxies D so going back up right we have our reference wavelength which is here this is gonna be hard for you to see I'm just going to change color and it's moved to here so you just want to use this scale to help you identify it and if you have a look you'll see that that is approximately 20 nanometers but they would have accepted any answer between 19 and 25 calculate the speed V of galaxy D used this equation we've been given the speed of light and just be really careful with your units because we've got nanometers up here meters per second here so actually although they've given us the equation we do need to just be a little bit careful so I'm gonna write the equation out V or C Delta Lambda over Delta 0c we've been told is 3 times 10 to the H that's the speed of light and according to this equation we need to multiply it by Delta Lambda which we've just worked out in the question above is 20 nanometers and then be careful you need to convert nanometers to meters and the way in which you do that is by doing times 10 to the minus 9 so we're going to pop that conversion in here and then divide the whole lot by Delta zero which we know is 390 nanometers so 390 and then we multiply that by 10 to the minus 9 again to avoid any issues with units and we get a final answer which is huge it's 15 million four hundred thousand meters per second the figure 3 shows a photograph of galaxy D the photograph was taken by student at his home state two ways that list can improve the observation techniques so that the quality of the image is improved some of this will be common sense so for me that would be move your telescope to a better viewing conditions and that's actually why a lot of these satellites are found in deserts this tends to be very little light pollution very clear skies so I'm just gonna write because I like this answer move the telescope to better viewing conditions eg in a dry desert something else you could say is use a better quality objective lens can you hear the ice-cream man it's not even one and if anyone's listened to my first chemistry video you'll know that was a dog banging and a builder doing something else London is so noisy something else you could have said is used a wider aperture camera or use longer exposure time while the telescope is locked on to a star so we're done well done for staying all this way I'm really impressed if you manage to watch the video all in one don't forget about my revision guides my perfect answer vision guides are available on my website right now at WWII so don't you care you can click on this card to buy yourself a copy of my revision guide which makes the perfect a compliment to these videos [Music]