Thermodynamics - A-level Physics

okay I have an admission I hate thermodynamics it was one of those things that always confused me in university and it might confuse you to select try and make sense of it all let's start off with the first law of thermodynamics and that is Q he calls you you might see a delta in front of that plus W now what are all these these are all energies this rule is true for gases so we need to be thinking about these energies in terms of gases so if you have a gas and you put heat into it you heat a gas that is what this Q is heat supply to gas that's fairly easy but you know that if you supply heat to a gas well the particles are going to move faster and if a gas is contained well great but if it's free to move and expand or whatever then it is going to exert a force and so that's what this is this a W indicate work done by gas now if work is done by gas then that means that it's losing some of that energy and so even though you're supplying maybe 10 joules of heat to a gas the gas might then be doing eight joules of work and so it's not keeping all 10 joules of that heat what happened to the extra 2 joules left over well that is turned into well kinetic energy of the particles or the molecules that make up the gas but we just say that this is gained or rather change if it's positive it's again in internal energy we know the that is proportional that's dependent on temperature isn't it so if you supply heat to a gas then it will do some work and so will not keep all of that heat but it will keep some of that heat some of that energy in the form of kinetic energy internal energy of the molecules now that might make sense but the difficulty is knowing how to apply that to different situations let's think about when a gas and the an isothermal change of process ISO means same thermal means wealth temperature basically and so in this case if the temperature is staying the same then that means that the internal energy of the gas is not changing at all so that means that the heat that you are supplying to the gas is all lost as it were by the gas again by the work that it does and knowing from thermal physics that PV is equal to NRT if you don't know that then have a look at my gas laws video then we can say that PV is proportional to T or in other words PV over T is a constant so you can compare before and after PV over T equals PV over T but we know that temperature is constant and so that means that we can take it out of the equation and so PV is equal to a constant as well so we can say that p1 v1 equals p2 v2 I'll write that down in a second let's go for another one adiabatic some people say adiabatic I like saying adiabatic those when no heat is lost or gained by the gas at all no heat is supplied to the gas and no heat is lost by the gas and so if that's the case we know that Q is equal to zero so if we take Q out of the equation put W over the other side or Delta U of the other side we know that the change in energy internal energy is going to be equal to minus W and that makes sense because if a gas does work then that means that it has to lose energy and it has to be losing the energy from the internal energy of its particles now for this one we can also say that p1 v1 is equal to p2 v2 however we have this little thing here that we raise the power of the volume by this is called the adiabatic constant and you'll always be told what that is that changes from gas to gas for a monatomic gas let's say argon that's equal to 5/3 I'm going to add one thing in here as well work done by a gas this is equal to 0 if V is constant so if a volume of a gas is constant then by definition it can't do any work you think about it work done is well force times distance and so if a gas isn't expanding or contracting then no work is being done on all by the gas and of course if work done is zero then that means that all of the heat supplied is being turned into the internal energy of the gas so Q equals Delta U there's one more thing that that is equal to P Delta V so pressure times the change in volume if constant pressure so those are the four rules that you have to remember for thermodynamics before you get started with anything else an isothermal change that means that the change in internal energy is zero so we can say P vehicle's PV adiabatic or adiabatic process no heat supply so that means internal energy is minus the work done if volume is constant work done is 0 if constant pressure work done is P Delta V what you can do is draw a graph of P against V and show what is happening to both pressure and volume for a gas during a change let's start off with our isothermal compression well we know that it's a constant temperature and so therefore P and V are well they're inversely proportional aren't they so if T constant is inversely proportional to V and so we get this shape graph now whichever way the pressure and volume are going we draw an arrow shown which way is going so here we go what's going on here well volume of the gas is decreasing the pressure is increasing so this is an isothermal because it's happening to constant temperature compression if the arrow is going the other way isothermal expansion and it would be just exactly the same line with the arrow going in the opposite direction pretty much anyway now what about if this happened at a different temperature though if this gas was at a colder temperature it's still a constant temperature but at a colder temperature then we know that the pressure and volume would be less and so basically the graph would look similar but it gets closer to the origin so this would colder temperature now we said that work done is equal to P Delta V and so times in pressure and volume together so what part of this graph gives us work done on or by the gas it's the area under the graph so the area under the PV graph is equal to work done however you have to think clearly and so I'm just doing it for the orange one here at the heart of temperature you just got to think is work being done or by the gas well it's compressing and so that means that work is being done on the gas that's what a PV graph looks like for an isothermal change what about an adiabatic change what is very similar it's just the characteristic really of an adiabatic change is that it gets closer to zero pressure okay so what if I had a PV graph and all I had was a straight line going up so we can see that the volume is staying the same or what do we say we know is the case if volume stays the same no work is being done at all so W equals zero so we can say that Q is equal to tell to you what about if I had a horizontal line well that's a constant pressure but changing volume and just like we said before work done is equal to P Delta V so again it is the area under the graph now you can see what I've done here is I've actually got two lines joined together kind of like vectors in a way and that's what happens usually when we have maybe an engine we don't just have one process happening we have lots of processes happening and they're all linked together the gas undergoes work and then it undergoes no work that the internal energy is changing etc etc so let's have a look at a PV loop so let's say that we have a gas here and what we do we do work on the gas like so let's say that it undergoes compression could be isothermal could be adiabatic so work is done by the gas and then it undergoes expansion and then work is done on the gas keep drawing arrows at the end you've got a drawing in the middle and then finally it undergoes maybe isothermal compression could be adiabatic let's extend this so let's just have a look at the area under the graph for when the volume it's increasing so that's from here to here so this area under the graph gives you work done by the gas okay we can say it's system but I like to think about it work done by the gas don't forget if the volume is increasing work is done by the gas but then on the way back when the volume is decreasing we have this area under the graph this is the work done on the gas or on the system and so if we have this much work done by the gas and then this work done on the gas then that means that we have a certain amount of Network resultant work done by the gas and this is kind of what happens with an engine we're gonna look at specifically a four-stroke engine and that's usually what you have in your car so what happens first of all is that well you probably know what happens you have a piston inside a cylinder and that goes up and down now what you can do is that you can suck in air and then you can exhaust the fumes out as well and we have an explosion as it were happening inside of the cylinder so what happens first is that the cylinder comes down and it comes down because it wants to suck in air and fuel into the cylinder ready to be combusted but the pressure isn't changing it's just sucking the air in so we represent that as just a straight line going across like that again we can say that the work is the area under this line we're not too concerned with out of the minute then what happens is that the piston is then pushed back upwards to compress the air and the fuel and then halfway up we have a spark that's why we need spark plugs and it causes combustion to happen and usually the spark happens about far something like here I thought I could draw sparks turns out I can't let me see that sort of dogleg going on there because the pressure is increasing very rapidly of course it can't stay like that can it because if the pressure is too great and the piston is allowed to move the piston is going to come back down again and that's what we see but it doesn't come down exactly the same way because of course we know that happens only if it's the same temperature but it's going to be at a hotter temperature so we're going to get further away from the origin like that so this is compression this is combustion and then we have our expansion as it explodes and that pushes the piston down that's what drives your car and then last but not least pistons down here we have all the exhaust fumes still in the cylinder we need to get rid of those that's what happens the piston goes up one more time and pushes just all of the air out again that's just at a constant pressure and so we have a straight line going across there that's supposed to be a straight line horizontally as we push the gas out is that a slightly higher pressure than what we suck the air & fuel mixture in with so there you go that's an introduction to thermodynamics and PV diagrams I hope that in my endeavor to unconfuse myself I've hopefully unconfused you a little bit as well if I have and you found this helpful please leave a like and if you think I've missed anything you've got any questions to put it in a comment down below don't forget to check out the rest of my videos on my channel for more help and I'll see you next time

Channel: Science Shorts

Views: 124,094

Keywords: school, answers, placement, tutorial, questions, college, practical, high school, lesson, edexcel, equations, ap, revision, explained, past, university, specimen, pv, sample, aqa, exam, technique, science, entrance, worked, paper, spoken, lecture, advanced, solutions, example, 11th, sat, shorts, experiment, work done, engineering, compression, ocr, adiabatic, practice, wjec, british, mark scheme, homeschooling, english, 12th, cie, first law, grade, expansion, summary

Id: F07SJRTzb0Q

Length: 12min 32sec (752 seconds)

Published: Sat May 11 2019