this video is a final run-through of the topics that are assessed in physics paper 1 of AQA GCSE combined science there obviously isn't a huge amount of detail because I'm trying to keep the video to a manageable length but all of the topics are covered before we get into the physics content a few last minute reminders for your exams firstly it's really important that you write in Black pen and that you keep your answers Inside the Box on the exam paper because your exam paper is going to be scanned so if you're writing in a different color or if you're going outside that box then there's a chance that your work won't be seen and the examiner won't be able to give you credit for it for all of your calculations it's really important that you're using the SI units watch out for sneaky questions where the exam board may have made it slightly trickier by say giving you a mass in grams instead of kilograms and then you needing to convert it for all of the extended response questions it's a good idea to be answering in the form of bullet points because there aren't any marks at all for answering in full sentences in GCC science and by giving your answer in bullet points it makes it clearer to The Examiner where you've put the marks and also it's just going to save you a little bit of time anywhere that you see the command word evaluate you need to give both sides of the question so offer some comparison and try to add value if you've been given a table with some data don't just quote values also it's really important that you write a conclusion all of the evaluate questions in AQA GCSE Science have one common Mark scheme and without giving a strongly Justified conclusion you can't get into that level three where you score five or six marks if you have to write a method say for one of the required practicals then make sure there's a logical flow to it a really good way of doing this is to give your answer in the form of a numbered list of instructions and then if you realize you've written things in slightly the wrong order you can just change the numbers that makes it really clear what you're doing also have a reader at the end and say will this actually make sense if you gave this method to a little year 7 who'd never heard of this practical before would they be able to follow it and collect the data that would allow them to answer the question there are a lot of working scientifically skills assessed in all of your science papers so these are things like identifying the three different kinds of variable knowing the difference between something being repeatable which is where you do it again using the exact same method and you still see the same pattern or reproducible where another group of scientists does the experiment or possibly uses a different method and they still see the same pattern know that when you're looking at your equipment the resolution is the smallest difference in results that you can identify you should know that computerized methods like digital thermometers are more accurate rapid and sensitive and you should also be aware that if you're asked to draw a graph then your line of best fit does not need to be a straight line I know that the maths Department keep telling you that it does but for GCSE maths that's true and for GCC science and the rest of the real world if your data curves your line of best fit should curve too all measurements contain error which basically means that not every single measurement that you make is identical there are two main kinds of error there are systematic errors which are caused by poorly calibrated equipment this could be things like having a balance where the mass is always reading half a kilo lighter than it should be so if you know that then you can just solve this by always adding half a kilo onto your calculations if this affects the balance when it has nothing on it and it's still saying minus 0.5 kilograms then this is called a zero error the other type of error is a random error which could be caused because your equipment isn't sufficiently precise or just because of natural fluctuation in the true value of that quantity so say if you're measuring the temperature of a room that temperature might change over time the way that we take account of this is that we make multiple readings and then we use those readings to calculate a mean every year after the science exams social media is full of people who are outraged that they've been assessed on math skills within the science papers so just to be clear over the course of all of the GCC science papers math skills make up 20 of the marks and in the physics papers 30 of the marks are supposed to be for mathematical skills at an appropriate level so that means if you're taking the higher tier then the math skills need to be at least GCSE Foundation tier and if you're taking Foundation tier for GCC science then the math skills have to be at least key stage three level to count as math skills so things like adding up numbers just don't count if you're watching this early enough in advance and you have the specification to hand then appendix 9 of the specification gives you a full list of everything that can be assessed for any of the base units so things like grams or liters or Newtons or volts there's a version of that unit that's a thousand times smaller called a millivolt or milliamp or whatever and then there's a version that's a thousand times smaller again which is the micro whatever and a thousand times smaller again which is the Nano and of course it goes the other way as well so a thousand times bigger would be kilo like kilogram or kilometer and a thousand times bigger again would be Mega and then Giga and all of these conversions are going to be by factors of a thousand The Only Exception is when you're thinking about centimeters because centimeters aren't a proper scientific unit but you should still know that there are 100 centimeters in a meter and that is that only one of these conversions where you're not multiplying or dividing by a thousand in 2023 like in 2022 you are going to be given the equation sheet in your exams but it's important to remember that the equation sheet doesn't tell you what the units are for any of these quantities and it also only gives you one version of the equation if you're not very confident with rearranging then it is worth memorizing some of the rearranged versions as well one of the first things I would always suggest you do in the first few minutes of the physics exam when you're just getting your head straight is to scribble down on the equation sheet what the units are for the different quantities now the reasons best known to AQA the first few equations on the equation sheet are actually ones that you're not going to need until paper 2 because they're forces equations but if we look at the ones that you are going to need to know the first thing that we should identify is that any type of energy will always be measured in joules with a capital J and that does include work done which is also just another name for energy speed is in meters per second and mass which features in these three equations is in kilograms that's a really common one where they're going to give you a mass in grams in the question and expect you to divide by a thousand to convert it height is going to be in meters and again you need to watch out for that because quite often they'll give you a height in centimeters and you need to convert that power is always in Watts time is always in seconds so remember that there are 60 seconds in a minute and 60 minutes in an hour because you might need to convert those as well charge is in coulombs potential difference is in volts current is in amps resistance is in ohms and then density is usually going to be in kilograms per meter cubed now this is one of the only ones where you might be asked to give your answer in a different set of units so occasionally there have been questions where instead of giving you a mass in kilograms and a volume in meters cubed they've given you a mass in grams and a volume in centimeters cubed and then asked for density in grams per centimeter cubed basically just look at the units they've given you and the thing to bear in mind is that the units in the equation marry up really nicely so if you've got mass in kilograms and volume in meters cubed the density will be in kilograms per meter cubed because that per that slash sign is just a divide sign the first Topic in physics paper one of AQA GCC combined science is the energy topic you should know that a system is an object or a group of objects and a system can store energy and there are a number of different ways in which it can be stored it can be stored as a magnetic store a kinetic store which means it's a moving object a thermal store so a hot object a gravitational potential store that's when you've taken an object and you've raised it against the action of a gravitational field a chemical storm where the energy is stored in the bonds so that could be a food or a fuel elastic potential stores are where an object has been stretched so say an elastic band or a hair tie or a spring electrostatic that's where we've got the attraction between positive and negative objects and finally nuclear stores it's possible for energy to be transferred between those stores in one of four ways energy can be transferred by heating because there's a temperature difference mechanically by a force electrically and finally by radiation remember you can't store energy as light or a sound but you may have energy being transferred through radiation and light and sound are types of radiation now conservation of energy is not actually part of this first little topic of energy but I find it challenging to explain the next bit without mentioning it first so we're going to do things slightly out of order we've mentioned that a system is an object or a group of objects and a closed system is where we have a situation where no matter so no stuff and no energy can get in or out within a closed system you can't create energy you can't destroy energy you can only transfer it so if we know how much energy is stored at the start of a scenario then the amount of energy that's stored at the end must be the same it might just be stored in different ways so for instance you might have started out with a chemical store and you finish with a thermal store but the total amount of energy will always be the same energy is always measured in joules and you always want to be writing joules with a capital J and I'd even suggest putting a hat on top of your J even if you don't normally just to make that super clear to your Examiner so we now have a bunch of different stores and calculations to go with them in 2023 these are going to be given on the equation sheet but only in one form and with none of the units listed a typical question near the start of the higher paper or the end of the foundation paper might ask students to calculate how much gravitational potential energy is transferred to an object when it's lifted up the first thing you're going to do is to write down the equation then underneath that you're going to write the numbers that correspond to each one of those letters the mass here is five kilograms the gravitational field strength is 9.8 and the height is 1.5 meters so if I multiply all those together then I'm going to get an answer of 73.5 and I'm going to write the units afterwards it doesn't matter whether you've written your final answer on the answer line that they've given you or if you've written it earlier in the answer space provided you haven't contradicted yourself if you have copied it out on the answer line make sure you don't make any transcription errors while you're copying and also with a question like this we want to watch out for those sneaky units again be careful that they have given you the height in meters it's possible that in some questions I'll give it to you in centimeters and you will need to convert there our next energy store is a kinetic energy store this is where we have a moving object things to watch out for here are that the squared sign in this equation only refers to the V so it's only the speed or the velocity that's being squared it's not the whole lot being put together again a typical question might ask you how much energy has been stored so you would write out the equation and then write it out with the numbers there in place of the letters if you don't have the ability to do fractions on your calculator just use 0.5 it means exactly the same thing as a half so I write in these numbers and then I do the calculation I multiply them together and I get an answer of 250 joules again with a capital letter then we're on to elastic potential energy and again here we've got a squared sign and again it only refers to the extension and again watch out for the fact that extension needs to be in meters not in centimeters or anything else it's worth pointing out here that for some questions you may need to calculate one of the other terms in this equation say for instance here you might need to work out the spring constant hopefully you're really confident rearranging equations but actually even if you're not you can still answer these questions because for reasons best known to themselves when AQA write the mark schemes they actually prefer you to substitute first and rearrange afterwards so if I had a question that asked me about the spring constant which is that k then what I could do is I could put in all the numbers I know and then solve for k so say I know that it's storing 4 500 joules of energy that's going to be equal to half times K the spring constant that I want to work out times 3 squared so if I simplify this through then I get 4 500 is equal to 4.5 K well I don't want 4.5 K I want 1K so I need to divide by four and a half and whatever I do to the left side I do to the right side and I get an answer of a thousand K solving the question like this will still work even without rearranging and it's still credit worthy it's actually what the mark scheme does first rather than rearranging next is specific heat capacity and this comes up in two parts of the specification so we will come back to it later capacity is a concept that you were first introduced to in Primary School looking at how much liquid you could fit in different containers it just means the maximum amount of something that an object can contain so then heat capacity is about the maximum amount of energy that a substance will absorb to make it change temperature and then the word specific tells us that we're referring to one kilogram of the substance and we're raising the temperature by 1 degree C before we get to the required practical we have this calculation and this one can throw people a little bit because it's got some extra symbols in it the triangle sign is called a Delta and it means change so this isn't two separate quantities on the left here this triangle e means change in energy and then likewise on the right we have change in temperature like density and speed this calculation is quite a nice one in that the units for the tricky bit I.E specific heat capacity actually tell you what to do with calculation if you wanted to calculate specific heat capacity as the subject you can see that the units for specific heat capacity which they will probably give you in the question if they were asking you to work it out are joules divided by kilograms degrees C so if you have to work out specific heat capacity you take the joules the amount of energy and you divide it by the mass and by the temperature watch out for the fact that you do need to have the change in temperature so you could be given a starting temperature and an ending temperature and need to work out what the difference between those two values are also again the mass has to be in kilograms so if it's in grams divided by a thousand the idea of this required practical is that you're given either a metal block or a container of liquid and you're trying to find out what the specific heat capacity is in other words how much energy does it take to heat up one kilogram by one degree C firstly you're going to have probably a metal block with two holes in it or you might be given a beaker of liquid you've got a balance because we want to know how much energy it takes to heat up one kilogram and your block might not have a mass of one kilogram so we might need to adjust our calculations to take account of that you've got some insulation because the experiment only generates accurate data if all of the energy that is given out or transferred by the heater is going into the block or into the liquid now obviously that's not realistic and some of it is going to dissipate and go into the surroundings but we want to minimize those energy losses as far as possible so you need to insulate the thing you're heating up we've got a thermometer so we can identify how much it is heated up by and realistically we're not going to heat it up by just one degree C because making small measurements reduces the Precision of the final value so to increase the accuracy of our results we're going to heat the block up by maybe 10 degrees or 20 degrees and then divide the amount of energy that has been transferred by 10 or by 20 to take account of that I've included up a pet here because often that thermometer is not going to make good contact with a metal block so in order to actually get a proper temperature reading you add a tiny bit of water in and that's going to come up to temperature with the block and then what you're actually doing is taking the temperature of the water we need some kind of electrical heater in order to heat up the block and then these last three are for us to calculate the amount of energy that's been transferred my preferred way of doing this is that you measure the potential difference using the voltmeter and you measure the current using the ammeter and you can use those two readings together to calculate the power then once you have the power you can use the stop clock to work out the amount of energy transferred alternatively you could also use a piece of equipment called a dual meter and that will tell you directly how much energy has been transferred but dual meters are not mentioned anywhere in the GCC specification and so I'm always a little bit nervous about whether the exam board will give you credit for it so if you can remember how to do the whole thing with voltmeter and ammeter that's the route I would go down the final Topic in this 6.1.1 energy section is power power is the rate at which energy is transferred or the rate at which work is done in other words how quickly is energy moved between stores the more powerful an appliance is the faster it can do work or transfer energy a typical early in the higher paper question might just ask you to calculate the power if an appliance like a kettle or a toaster or a washing machine transfers a certain amount of energy in a certain amount of time again here they might be sneaky and give the time in units other than seconds so just watch out for that so here in my question in the pink bubble I would say that the value 2000 must be the energy and I can tell that because it's got a j after it and obviously four is a time because it's in seconds so to calculate power I would do 2000 divided by four giving myself an answer of 500 watts now you'll notice that to get my answer here what I've done is a number in joules divided by a number in seconds so we can also say that a what is exactly the same thing as a joule per second and to be honest our lives would probably be easier if we called it a joule per second but physicists love to rename units after dead white guys so that's what we've done here then we move on to section 6.1.2 and the first thing we need to know about is efficiency this is the proportion of energy that has been usefully transferred and it can be expressed either as a decimal or as a percentage we've already said that conservation of energy describes how we must have the same amount of energy at the end as at the start however some of that energy will be transferred usefully in the way I want and some of it will be transferred wastefully so for instance here my light bulb might transfer a thousand joules of energy but a hundred joules of that energy is transferred as light radiation and the other 900 joules is transferred by a heating pathway and that's not what I want it to do so my light bulb would only be 10 efficient or we could also report that as it having an efficiency of 0.1 there are two different equations for efficiency but they're basically exactly the same equation just one of them is expressed in terms of energy and one of them is expressed in terms of power you might get a question that asks you about where the energy ends up when it's been wasted there will almost always be a transfer to a thermal store in the surroundings and when the energy has spread out so far that it's not useful anymore we just can't use it then we describe that as dissipating for the higher tier you should be able to discuss a number of ways in which we can reduce the wasted energy we can reduce heat loss by insulating something which is particularly relevant if you're thinking about houses and how they might have lost insulation or double glazing to help them be insulated we can talk here about the thermal conductivity of a material so for instance if you're looking at different kinds of brick just regular brick versus Breeze block then the higher the thermal conductivity is the higher the rate of energy transfer by conduction so in other words the better conductor something is the faster it conducts reduce wasted energy by reducing drag by streamlining that's particularly relevant if you're thinking about boats or cars or any other kind of vehicle we can also reduce wasted energy by reducing friction that could involve using wheels or lubricating moving parts if those frictional forces aren't reduced then they can cause energy to be transferred by Heating the final part of the energy topic is about National and Global energy resources you should be able to distinguish between renewable energy resources which are being generated faster than they're being used and non-renewable energy resources or you might be used to calling them finite resources which are being used faster than they're being generated all of these energy resources can be used in a variety of ways including transport and heating as well as electricity generation for each energy resource you should be able to discuss the advantages and disadvantages you want to think about things like are they renewable in which case they're not going to run out are they expensive do they have carbon dioxide emissions which you can then link to global warming are they reliable you can't use wind turbines if it's not windy and you can't use solar panels in the middle of the night also are there Geographic restrictions on where you can use them then also for nuclear power you want to think about how you would dispose of the radioactive waste and the risks associated with nuclear accidents so now we come to the second Topic in GCC physics which is electricity the first thing to be aware of is that you need to know all of the different circuit symbols and be able to use these to draw circuits and of course it's really important when you draw a circuit that you're using a ruler for your wires and that you're not leaving any gaps in those wires otherwise you wouldn't have a complete circuit one symbol to watch out for is when you get your resistor and your fuse it's really common to see people drawing the lines for the wires and then drawing the resistor on top of it so there's a line going through the middle but because that would actually then be the symbol for a fuse it's not correct and you would forfeit the mark so just really watch out if you need to draw that one there are a number of different equations in the electricity topic and bear in mind that even if you're taking your exams in a year when you are given an equation sheet you might not be told what quantities you're using in the question so it's really important that you know what all of the units are so that you can identify that if the question says 200 volts and 0.4 amps the 200 is the potential difference and the 0.4 is the current the other thing to be aware of is that for the higher tier in particular you may get questions where you need to stack more than one equation together to get to a final answer if you do have a question that asks you to work something out and you can't figure how to work it out using the information you've got then see what you can make and try to calculate something because the chances are it's one of these multi-step calculations and what you've done is the intermediate first step and that will still get you some credit you should be familiar with the idea that current is the speed of the flow of charge around a circuit and potential difference is about how much energy is being transferred by a component now the thing to remember with current is that even though we know now that current is the flow of delocalized electrons when the idea of electricity was first developed scientists didn't know about electrons and so they thought that it was positive particles that were moving when they started describing electrical circuits they described current going the wrong way so even though we now know that it's electrons we still continue to talk about this and it's what we call conventional current required practical 15 is about measuring the IV characteristics in other words the current and the potential difference of both a wire of different lengths and also resistors that have been placed in series and in parallel so in each one of these circuits we're using an ammeter connected in series otherwise it won't work to measure the current and a voltmeter connected in parallel otherwise it won't work to measure the potential difference and then using the calculation R equals V over I to calculate the resistance for each one of these so essentially what we find is that with a wire as it gets longer its resistance gets higher and then with the resistors in the series and parallel circuits if you're putting the resistors together in series then the overall resistance for the circuit can be found by adding up the values of the individual resistances of those resistors whereas if we're in parallel then it's a lot lower and we're going to look at that in just a second then required practical 16 is very similar so here you're asked to investigate the current which is I and the potential difference which is V for a variety of circuit components you might be asked to draw a circuit that would allow you to do this and as we said it's really important that your ammeter is in series and your voltmeter is in parallel just remember that your voltmeter is measuring potential difference and it can't measure the difference between the two sides of the component if it hasn't been attached on both sides of the component so here I've got a resistor in the bottom left which is what I would be currently measuring the current in the potential difference of so that's what I would swap out for a different kind of component having completed that practical I'd then be able to draw some different IV graphs having completed that required practical you'd be able to draw these three graphs so these are IV graphs you've got the current on the y-axis and you've got potential difference on the x-axis in other words we're changing potential difference and seeing what happens to the current so for an ohmic conductor like a wire at constant temperature or the resistor that I had on the previous slide as we increase the potential difference we increase the current and this happens in a linear fashion in fact this graph is directly proportional which means it's a straight line graph and it goes through the origin zero zero in other words if I double the potential difference I double the current a filament lamp looks a little bit different it's this slightly s-shaped curve and the reason for that is that it's not at a constant temperature as the potential difference increases the wire is getting hot and therefore its resistance is increasing even though as you increase potential difference current is still increasing it's no longer increasing by the same rate because the resistance is going up then finally we have the diode and there are two important things to note about the graph that you're drawing for this one the first one is that the diode will only allow current to flow in One Direction so you don't have anything on the graph at all below the x-axis if you unattach your power source and you reattached it with wires flipped over no current will flow then the second thing to note is that you if you only have a very small potential difference then you still don't get the current flowing the graph is going to track the x-axis for a little bit before it goes diagonally upwards next you should be familiar with light dependent resistors and thermistors now watch out because these graphs look similar to the previous ones but they're not we no longer have current on the y-axis instead we have resistance and what these graphs tell us is that a light dependent resistor will resist less as the light gets brighter so that means you could use it to turn on a radio in the morning as the sun comes up the resistance goes down the current starts to flow and your radio switches on and then for a thermistor as the temperature increases the resistance gets less so that's opposite to a wire next is series and parallel circuits in a series circuit there's only one path that the charges can follow whereas in a parallel circuit there's a junction where the current splits so some charges are going to go past one light bulb and some charges are going to go past the other light bulb for series and parallel circuits you need to be able to discuss the current so how quickly the charges are moving and also the potential difference which is the amount of energy that's being transferred to these different components if we think about the series circuit first and current we've said that current is about how many charges pass a particular Point per second and the image you want to have in your head is that this is like a traffic jam there are no gaps between charges and if one charge moves forward then the ones directly behind it will also move forward and everything is just going to carry on moving at exactly the same speed so because there's only one Loop and all of the charges are going the same way they're all going to go at the same speed so the current anywhere in that series circuit will be exactly the same regardless of where I put my ammeter now the parallel circuit is a little bit different because the charges are still going nose to tail like they're in a traffic jam but when they get to point a they're going to split so if these two branches are identical if my two light bulbs have just come out of the same packet then half of the charges will go one way and half of them will go the other way now the branches might not be identical so these could be two different light bulbs with different resistances um or I could have a situation where I've put two light bulbs in one Loop of the circuit but whatever the different currents are the current for the first branch and the current for the second Branch will always add up to be the same as that total current was before the charges split if we now think about potential difference that's about how much energy is being transferred imagine as these electrons go past the cell they're picking up some energy in the series circuit that I've drawn here they're going to have to split that energy between the two light bulbs because they're going to go past both of them so each bulb is only going to get half of the energy that was picked up from the cell but in the parallel circuit each charge or each electron is only going to go past one of the light bulbs so it doesn't need to save half of its energy to give to the other bulb it's all going to be given up in one go so whereas for current it was the same everywhere in the series circuit and it's split in the parallel circuit for potential difference it's going to be the other way around in the series circuit the potential difference is going to be split between the two components whereas in the parallel circuit each component is going to get the full potential difference and that's why the light bulbs in a parallel circuit are much brighter than those in a series circuit finally if we think about resistance you should be able to describe that in a series circuit to work out the total resistance we're just going to add up the resistance of each individual resistor so say if I had three resistors and they each had a resistance of two ohms then the total resistance would just be six ohms you'd literally just add them together whereas in a parallel circuit the total resistance is less than the resistance of the smallest individual resistor to actually work out what it would be we say that 1 divided by the total resistance is going to be 1 divided by each of the individual resistances added together but basically the most important thing is when you add up all of those together you're going to find out that the overall resistance is less than that of any individual resistor the British main supply of electricity is AC which stands for alternating current alternating current is current that repeatedly changes Direction in the case of the main Supply 50 times per second this helps to maintain power and it's also important because the Transformers that are used in the National Grid will only work with an AC Supply in contrast direct current which you get in a simple circuit with a cell or a battery is current that only Flows In One Direction as we've said the frequency of the main Supply in the UK is 50 hertz you should also know that the potential difference is about 230 volts be careful here because historically I think it used to be 240 volts and so that's a number that a lot of people have in their head or that they've heard their parents say and unfortunately 240 won't get you the marks in the GCC physics exams most appliances are connected to the main Supply using a three core cable which attaches to the supply using a three pin plug the outside of the plug is made from rigid insulating plastic to prevent you from getting an electric shock if you're unplugging something that's already switched on and the pins of the plug are made from an alloy of copper called brass as you know from chemistry Alloys are harder than pure Metals so this stops the pins from bending when you're plugging something into the wall you need to know the names locations colors and functions of the three wires which can be identified because of the colored plastic that's put around them the Live Wire is the brown wire and you can remember that it goes on the bottom right because bottom right is BR and so is brown the Live Wire carries the alternating potential difference from the main Supply the neutral wire is covered in blue plastic and it sits on the bottom left of the plug BL for bottom left BL for blue and it completes the circuit it has a potential close to zero Earth wire has green and yellow stripes and it's a safety wire to stop the appliance from becoming live if there's an electrical fault where the Live Wire touches the metal casing of an appliance the plug has one more safety feature which is the fuse so the fuse is just a thin metal wire and if too much current flows say if there's an electrical surge then that wire will melt and therefore that will break the circuit and just keep everyone safe be careful with your language here it's really common to hear people in everyday life talk about a fuse blowing and therefore we see students writing that the fuse has blown up or the fuse has exploded and it's nothing as dramatic as that and also you won't get a mark for saying that it is so you want to talk about the fact that the fuse is breaking or the fuse is melting most fuses that we see in plugs are either three amp fuses or 13 amp fuses you could be asked what's an appropriate fuse to use and you'd be given a multiple choice list so the idea is something that you want to have a fuse that will allow a slightly higher current to flow than what's normal before it melts so say if the normal current was 11 amps you would use a 13 amp fuse because that way it's not going to melt unless there's too much current the National Grid is a system of cables and Transformers which links power stations where electricity is generated to our domestic customers things like houses the Transformers are important because they allow this to be a really efficient process in those Central cables that run between pylons the electricity is traveling with a really really high potential difference and the reason for that is that there are two ways that we can get the amount of power that we need to our domestic customers firstly we could have a really high current that means the electrons are moving really fast and if that's the case because they're moving so fast the wires end up heating up and resisting and you end up losing a lot of energy to the surroundings it just dissipates into the atmosphere so high current doesn't really work for us because it wastes a huge amount of energy and therefore costs a lot of money the alternative is to have this really high potential difference but you don't want that high potential difference in the power station and the domestic Supply in the houses and the factories because high potential difference is really dangerous so what you have is firstly between the power station and the cables A Step up Transformer which increases the potential difference and then you have this really high potential difference going through the cables and at the other end you have a Step Down Transformer which is going to reduce the potential difference so that it is safe for consumers again the third Topic in AQA GCSE physics paper 1 is the particle model of matter this starts off with our three different states of matter and you should be able to recognize or Draw these simple diagrams to model the differences later I've drawn these in a more traditional way with the boxes the same size but I don't want to give you the impression that the number of particles changes during a state change because that's not accurate as we go from solid to liquid to gas as we add more energy the number of particles stays the same and so conservation of mass is still a thing they're just changing how they're interacting with each other and how spread out or close together they are in the solid the particles are packed in this regular structure in the liquid the particles are still close together and touching there aren't any gaps that you could push a particle into and that's why liquids are virtually incompressible but the particles are no longer in that regular structure in the gas the particles are separated by large distances you should be able to talk about the forces between the particles in a solid there are really strong forces holding the particles in place the forces in a liquid are a bit weaker the particles maintain contact so you don't have individual molecules or atoms flying off but those forces are weaker than they are in a solid and in the gas we've got really really weak forces you should also be able to talk about the particle movement as well so in the solid the particles are still moving but they're vibrating in fixed positions so the particles are staying on the spot and they're just vibrating back and forth in a liquid the particles are able to pass each other and in a gas the particles are constantly randomly moving in terms of how all of this affects the properties of solids liquids and gases it's difficult to change the shape of a solid because you've got these really strong forces and because all the particles are regularly packed together for a liquid it can change shape quite easily if you just pour it into a different container but liquids are virtually incompressible so in other words if you put some water in a syringe and you try to squash it as long as you've blocked the end so it can't come out you can't squash it because there aren't any gaps of the particles to move into and then a gas can easily expand to fill a space and therefore it's easy to compress because it's got these big gaps between particles you should be able to name all of your state changes so melting and freezing and boiling and evaporating which are different to each other so boiling is where all of the liquid is turning into a gas in one go and it happens at the boiling point and you're going to see bubbles so that's like when a kettle boils whereas evaporating is where just the particles of the surface are breaking away and becoming Vapor or a gas and so that can happen at a much lower temperature and you're not going to see bubbles so that's like when a puddle is gradually drying up as it evaporates in the sunshine and then condensing which is the opposite of evaporating and of course sublimating which is the one that people tend to forget that's where something turns straight from a solid into a gas it's very rare but carbon dioxide does it and so does camp for wax you should be aware that all of those State changes are physical changes in other words we're not making any new chemicals that weren't there before and if we reverse the heating or the cooling then we could go back to the original substance with the original properties if I took some ice and I melted it and then I boiled it but then I cooled it back down so it turned back into ice I'd still have the same ice with the same properties that it had at the start and in those physical changes those changes of state mass is conserved so if I start out with a kilogram of ice I'm going to finish with a kilogram of steam and then from there go back to my kilogram of ice thinking about how particles are arranged in solids liquids and gases leads us nicely onto density density is a measure of how much mass is contained within a unit volume of a substance so if I had a meter cubed of iron and water and chlorine then because the particles in the chlorine are so much more spread out than in the water or in the iron within that same volume I'm going to have far fewer particles and therefore the overall mass of that meter cubed is going to be much lower this is what we mean when we say that something has low density the calculation to work out density is mass divided by volume and this is one of these nice equations where the units give you the equation and vice versa so the units for density tend to be kilograms per meter cubed so in order to work out density we take the mass in kilograms and we divide it by the volume in meters cubed in the required practical we need to determine the densities of regular solid objects irregular solid objects and liquids so for each one of those we need to be able to calculate the mass and also the volume if we start off with a liquid you measure the volume of a liquid using a measuring cylinder so the way to do this is put your empty measuring cylinder on the balance and then tear the balance so that it acts as if the measuring cylinder is not really there and gives you a mass of zero kilograms and then you fill up the measuring cylinder with a certain volume of liquid and the bigger the volume is the more precise your data will end up being and then you see what the mass of that measuring cylinder and the liquid are and then you just read the volume of the measuring cylinder for a regular object like my cuboid here you're going to need to measure the length width and the depth using a ruler or to be even more precise you could use the calipers as I've shown here then you can work out what the volume is by multiplying the length by the width by the depth if they give you a cube then of course all of those are the same as each other then to calculate density we take the mass that we've measured using a balance and divide it by the volume that we've just calculated finally for an irregular object so something like my key ring here we're going to need to use this displacement can or you might also have called it a Eureka can so you're going to fill this can up with water until water is coming out of the spout and when water has stopped coming out of the spout you're ready to measure the volume of your irregular object you're going to fully submerge that irregular object in the displacement can and you're going to catch all the water that comes out of the spout in a measuring cylinder now the reason that this is going to work is that the volume of water that is displaced will be equal to the volume of the object so you now have the volume of that object and you can put that together with a mass that you've measured using a balance to calculate the density of the irregular object then we move on to internal energy the energy that's stored in a system by the particles is called internal energy and everything has internal energy even if it's not moving right now the internal energy is made up of two things the total kinetic energy of the system in other words the particles that are moving around and the total potential energy of the system as you're heating a substance the energy stored within the system will increase but that isn't always reflected by an increase in temperature if we take some ice and we heat it up we actually see a graph that looks a bit like this which might come as a bit of a surprise because you might expect the temperature to just go up in one nice straight line so what's actually going on here well we've got this flat line here where the ice is melting and another flat line here where the water is boiling whenever the substance is going through a state change the temperature doesn't change you can almost think of it like the substance can't multitask either it can get hotter or it can change state it can't do both at the same time assuming it's a pure substance so the red bits of the graph here represent places where the energy is going into the substance and it's making the particles move faster so if it's a solid like ice then the particles are vibrating faster on the spot and then if it's a liquid like water the particles are moving faster past each other and so the temperature is increasing if I want to know exactly how much energy it's going to take to do this then we need to know the specific heat capacity which we met earlier in the energy topic and that tells us how much energy is needed to raise the temperature of a substance by one degree C then the green bits of My Graph are where the state changes are happening here instead of that energy increasing the kinetic energy of the particles it's increasing the potential energy of the particles and so that's where my state change happens and if I want to know how much energy that's going to take then I need to know about specific latent heat which tells me how much energy is needed to change the state of one kilogram of a substance now that's not going to be the same amount of energy for melting as it is for boiling so we have slightly different names we have the specific latent heat of fusion when a substance is melting and a specific latent heat of vaporization when it's turning into a gas again this is an equation with quite sensible units in that energy is measured in joules and Mass is measured in kilograms so therefore specific latent heat is in joules per kilogram which is nice and easy to remember so if I was going to rearrange this equation to make L the subject it would be the energy divided by the mass so in other words joules divided by kilograms which is just joules per kilogram you should know that the molecules of a gas are in constant random motion and the temperature is related to the average kinetic energy of the molecules in other words if the temperature is hotter then the particles are moving faster provided we're working at constant temperature then pressure and volume of gas are inversely proportional this means if you've got a balloon and you keep it at constant temperature and you squash it so the volume is half of what it was before then the pressure inside will double however as volume decreases then actually in real life not just hypothetical physics world the temperature will increase the final Topic in AQA GCSE physics paper 1 is all about atomic structure and radiation the first part is common content with chemistry paper one so you've revised all of this quite recently anyway you need to be able to describe this nuclear model of the atom so there's a small dense nucleus containing protons and neutrons surrounded by electrons which orbit at fixed distances which we usually call shells you should be able to describe the relative masses and relative charges of each of those subatomic particles it's important wherever you see the word relative that you answer in the form of a number not just saying positive and negative but saying plus one for protons and -1 for electrons and zero for neutrons you are allowed to say that the mass of an electron is very small because that's what the specification says although if you do know a numerical value for it and you want to use that then that is also fine in terms of the numbers of these particles the bottom number for an atom is the atomic number and that tells you the number of protons which will be the same as the number of electrons in an atom and really in GCC physics we're only expecting to meet atoms we're not expecting them to ask you about ions that would be more appropriate to come up in your Chemistry exams then the mass number tells you how many particles there are in the nucleus so therefore you can work out the number of neutrons by subtracting the atomic number from the mass number so for instance with my lithium atom here to work out how many neutrons there are I would do seven take away three which is four and you can see in the diagram I've got my four green neutrons you should know that the radius of an atom is about 0.1 nanometers which can also be expressed as 1 times 10 to the minus 10 meters and you should also know that only about one ten thousandth of the radius of that atom is the nucleus we just draw it a lot bigger in diagrams so that we can then pick out the individual protons and neutrons now the reason that all of this atomic structure is relevant is that when we talk about radioactivity it tends to be different isotopes of elements that we're interested in Isotopes are atoms of the same element that have the same number of protons because the number of protons is what defines what element something is and they have different numbers of neutrons now if you've been revising from something other than an AQA approved textbook you might also have seen Isotopes defined in terms of their atomic number and mass number but your exam board want you to Define it in terms of subatomic particles and they'll often say in the question give your answer in terms of particles in the atom so you need to make sure you're talking about protons and neutrons in your answer not atomic numbers and mass numbers looking at how the model of the atom has changed over time is basically getting across this idea that scientists do experiments we collect data and then we update the models in light of that data so we start out with a British scientist John Dalton who described atoms as being solid indivisible in other words you can't break them up spheres he thought they were like snooker balls just solid you couldn't cut them in half because they were just a ball next is JJ Thompson who discovered the electron and developed the plum pudding model and this had the negative electrons set in a ball of positive charge to maintain that overall neutrality of the atom next comes Ernest Rutherford who does the alpha scattering experiment and here's the one scientist where you actually need to know the details of the experiment he did he took a very thin layer of gold foil and he fired alpha particles at it so alpha particles are two protons and two neutrons so basically the same thing as a helium nucleus so he fired these alph particles at the gold foil and he found that the vast majority of the alpha particles went straight through and this was evidence that actually the vast majority of the atom was empty space he also found that a very small proportion of the alpha particles were deflected back so they bounced back towards him and this was evidence for two things firstly that at the center of the atom was a small dense nucleus and secondly that that nucleus had a positive charge because the alpha particles they already knew had a positive charge and the alpha particles were repelled and we know that in terms of charges things with opposite charges attract and things with like charges repel so since they already knew that the alpha particle was positive the nucleus must be positive too then Niels Bohr did some calculations and some mathematics and he said that electrons orbited the nucleus at fixed distances which we now call shells and then various people did experiments which corroborated that then later there were experiments that led to the idea that the positive charge of the nucleus could be split up into smaller particles which all had the same amount of positive charge and we call those protons and then finally James Chadwick provided the evidence to show that there were also neutrons in the nucleus you need to know that some Atomic nuclei are unstable and the nucleus then gives out radiation as it's changing to become more stable and this is a random process called radioactive decay when we say that it's random what we mean is that we can't predict exactly which nucleus is going to Decay at any one time you just have to wait and see now as that decay happens and as the alpha particles or beta particles or gamma rays are given off we describe the rate at which that decay happens as activity which is measured in beckerels with a piece of equipment called a geiger mulletube all of these different kinds of radiation ionize and what that means is that they turn other atoms into ions by removing their electrons so you'll know from chemistry that there are ions made when particles lose electrons and when they gain electrons but in terms of radioactivity and physics we're always thinking about electrons being lost the first type of radioactive decay you need to know about is alpha decay an alpha particle is made of two protons and two neutrons so it's just the same thing as a helium nucleus I start off with my parent nucleus of my unstable isotope and then as it becomes more stable an alpha particle breaks off and is released and What's Left Behind is called the daughter nucleus now this daughter nucleus is going to be a completely different element because it's lost two protons and it's also going to be four Atomic units lighter because it's lost two protons and two neutrons alpha particles are very ionizing in other words they're very good at turning other atoms into ions they're not very penetrating because they're so big that they're really easily stopped they have a short range in air for the same reason they're quite big so they collide with molecules in the air and this stops them traveling further they can be deflected or pushed to the side by an electromagnetic field and they can also be stopped by a thin layer of paper beta Decay is also a particle this time it's a fast-moving electron and this is often confusing for people because we're talking about the nucleus so why would there be any electrons in there the best way to think of this is to imagine that a neutron is made out of a proton with an electron stuck to it so one of these neutrons is going to turn into a proton and in the same process it's going to release that fast moving electron and that is the beta particle again this element is going to turn into a different element because it's now got an additional proton and so that makes it a different element beta particles are less ionizing both because they're smaller and also because they've got a slightly lower charge it's just one electron as opposed to two protons however they're more penetrating because they're smaller so they can get further before they're stopped they have a medium range in air and they're still deflected by an electromagnetic field but they'll be pushed in the opposite direction to an alpha particle and they can be stopped by Thin aluminum foil finally we have our gamma radiation now this is not a particle it's an electromagnetic wave and we see it being released at the same time as an alpha particle or a beta particle but because it's not a particle when it comes to writing equations there isn't any difference to the parent nuclei and the daughter nuclei the gamma ray being released doesn't change what element something is and it doesn't change the mass of the nucleus either gamma rays are even less ionizing than beta particles and they're even more penetrating they have a huge range in air they can move basically infinite distances in a vacuum and they're not deflected by an em field and they can only be stopped by using thick lead so to summarize what we've said so far alpha particles can be stopped by paper the beta particle can go through the paper but will be stopped by the aluminum foil and then the gamma keeps on going until it meets thick lead now for these nuclear equations for the GCSE AQA exams you don't actually need to be able to predict what the daughter elements are but you do need to be able to make the atomic numbers and the mass numbers balance so for an alpha particle being released this is relatively straightforward radium-219 has a mass of 219 and the alpha particle has a mass of four so therefore the polonium daughter nucleus must have a mass of 2 1 5 because the two sides of the equation have to add up to be the same as each other and then likewise radium has an atomic number of 86 so if the alpha particle has an atomic number of two then the polonium must have an atomic number of 84. the beta particles tend to be where people fall down the mass is quite straightforward because the mass of an electron at least as far as this equation is concerned is zero so the mass remains 14. but the really common mistake we see is that people will do six minus one and end up with five instead for the atomic number of nitrogen and even if you don't have a periodic table in front of you you're probably aware that that's not right now if you think about this logically we're trying to make the two sides of the equation balance so what we're looking for is something take one is six so the something is going to be seven if I have a graph the y-axis could represent activity or count rate or the number of nuclei and I would handle all of these in exactly the same way let's say that my initial activity is 20 000 becquerels to work out what the half-life is I'm going to see how long does it take for the activity to fall to ten thousand becquarels in other words half of what it was to start with then I can read off my x-axis that this was 5 seconds or 12 years or whatever it is here I've just marked it with t now the interesting thing is that you don't actually have to start where X is zero you'd have to start at the start of the graph because the rate of change will remain constant so I could look at how long it takes to get from 10 000 to 5000 and that would still be the same amount of time and it would also be the same amount of time to go from 5000 to 2500. so if you're given a graph it's pretty straightforward you just pick any value from the y-axis and see how long does it take for that number to drop to half of that value the only advantage of starting where tonym is zero is that you then don't need to figure out what the difference is between where you start and where you finish because where you started is zero now the alternative is that they give it to you in terms of numbers so here I've got a sample where the activity drops from 64 000 beckerels to four thousand beckerels what you want to work out is how many times is that number halved and I would really strongly recommend that you write this out with arrows we start off with 64 000 which halves to thirty two thousand which halves to sixteen thousand which has to eight thousand which halves to four thousand now the problem is that people will then look at this and say this is five half-lives and it's not think about how many times has it halved so you need to count the arrows not the numbers here I can see that this is going to be four half-lives so the four half-lives are the 24 days from the question so four half-lives is 24 what's one half-life we divide by four and it's six days finally we need to talk about hazards so contamination and irradiation radioactive contamination is where you've got unwanted radioactive atoms that have stuck onto something else so they've made that other object radioactive and the risk there is because at some point those contaminating atoms are going to undergo radioactive decay and therefore you'd be exposed to that radiation now the exact level of Hazard is going to depend both on the half-life and the type of radiation that's being given out for instance if you've got some radioactive atoms on you but they're not going to Decay for the next 10 000 years it doesn't really matter whereas if they have a half-life of a few seconds then you're in big trouble now irradiation is different because it's not the radioactive parent nuclei that are on you or on the object that's been irradiated what's happened is that you've been exposed to the alpha radiation or beta radiation or gamma radiation that's been given off this could be hazardous to your health in that it might ionize your DNA and lead to mutation and lead to cancer but you are not becoming radioactive and if you move away from that radioactive Source there's no further risk of injury and we use irradiation really commonly when we're trying to sterilize things so say irradiating medical equipment to make sure there are no living bacteria on it so that's it for physics paper one of AQA combined science I really hope that that summary has helped you to feel a bit more confident and I'm wishing you so much luck for the exams if 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