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Current time:0:00Total duration:18:29

Video transcript

I've done a bunch of videos where I use words like pressure and let me write these down pressure and temperature temperature and volume and I've done them in the chemistry in the physics playlist and then I especially in the physics playlist but even in the chemistry you play this I also use words like kinetic energy kinetic I'll just write e for energy or I use force force and velocity and you know a whole bunch of other other types of I guess properties of things for better or for worse and in this video what I want to do is I want to make a distinction because it becomes important when we start getting a little bit more precise especially when we get more precise in thermodynamics or I guess you know the study of how how heat moves around so these properties right here these are properties of a system or we could call them these these are macro states of a system macro States macro states and these these could be macro states so for example amede let me make it clear when I call a system if I have a balloon if I have some balloon like this and it has a little tie there and you know maybe has a string this has these macro States associated with it there is some pressure in that balloon remember that's force per area there is some temperature for that balloon temperature and there's some volume to the balloon obviously but all of these these help us relate what's going on on that inside that balloon or what what that balloon is doing and kind of an everyday reality before people even knew about what an atom was or maybe they thought that there might be such an atom but they never proved it they were dealing with these macro states they could measure pressure they could measure temperature they could measure volume now we know that that pressure is due to things like you have a bunch of atoms bumping around and let's this is a gas it moves to balloon it's going to be a gas and we know that the pressure is actually caused and I've done several I think I did the same video in both the chemistry and the physics playlist I did I'm a year apart so you can see if my my thinking has evolved at all but we know that the pressure is really due by you know just the the bumps of these particles as they bump into the walls of the side of the balloon and we have so many particles and at any given point of time some of them are bumping into the wall of the balloon and that's what's essentially keeping the balloon pushed outward giving it as pressure and it's volume we've talked about temperature as essentially the average kinetic energy of these of these balloons is a function of these particles which could be either the molecules of gas or if it's a if it's an ideal gas it could be just the atoms of the gas maybe it's you know atoms of helium or or or or neon or something like that and all of these things these describe the microstates so for example I could describe what's going on with the balloon I could say hey you know there and let me I could just make up some numbers I could you know there the pressure is pressure is right you know let's say it's five Newton's per five Newton's per per meter squared or some number of Pascal's the units aren't what important that the in this video I really just want to make the differentiation between these two ways of describing what's going on I could say the temperature I could say the temperature is 300 Kelvin I could say that the volume the volume is I don't know maybe it's one liter and I've described the system but I've described it on a macro level now I could get a lot more precise especially now that I we know that things like atoms and molecules exist what I could do is I could essentially label every one of these molecules or let's say atoms in in the gas or that's in contain in the balloon and actually at exactly this moment in time you know I can say at time equals zero molecule you know atom one has you know it's it's a momentum is equal to X and its position its position in three d dimensional coordinates are x y&z right and then I could say atom number 2 its momentum I'm just using row for momentum or P hey V its equal to Y in its position is ABC and I could go and give you this I could I could list every atom for in this molecule obviously we're dealing with a huge number of atoms on the order of 10 to the 20 something so it's a it's a massive list I would have to give you but I could literally give you the state of every atom in this balloon and then if I did that I would be giving you the microstates or I would give you a specific microstate of the balloon at this time now the if when a when a system and I'm going to introduce a word here because this word is important especially as we go is in thermodynamic equilibrium so let me write that down equilibrium equilibrium and we learned about equilibrium from the chemistry point of view and that tells you you know the amount of something going in the forward reaction is equivalent to the amount going in the reverse reaction and when we talk about macrostates thermodynamic equilibrium essentially says that the macro state is defined that they're not changing if this balloon is in equilibrium at time one its pressure will be its pressure temperature and volume will be these things and if we look at it a second later its pressure temperature and volume will also be these things it's an equilibrium none of the macro states have changed and actually I'll talk about a second in order for these macro states to even be defined to be well defined you have to be an equilibrium I'll talk about that in a second now at second number at time equals zero you might have this whole set of you know I went and I listed 10 to the 28 something microstates of all of the different atoms in this molecule but then if I look at this this this these gases a second later I'm going to have a completely different microstate right because all of these guys are going to have bumped into each other and and given each other different you know their momentum and all sorts of crazy things could have happened in a second here so I would have a completely different microstate so even though we're at the reminded thermodynamic equilibrium and our macro state stayed the same our microstates are changing every you know gazillion of a second they're constantly changing and that's why for the most part in thermodynamics we we tend to deal with these macro states and actually most of thermodynamics or at least most of what you'll learn in a first-year chemistry or physics course it was it was devised or it was thought about well before people even had a sense of what was going on at the macro level that's often a very important thing to think about sometimes when and we'll go into concepts like entropy and internal energy and things like that and the end you know you kind of wrack your brain how does it relate to atoms and sometimes and we will relate them to atoms and molecules but it's useful to think that the people who first came up with these concepts came up with them not really being sure what was going on at the micro level they were just measuring everything at the macro level now I want to I want to go back to this idea here of equilibrium of equilibrium because in order for these macro states to be find the system has to be in equilibrium and let me let me explain what that means if I were to take a let's say I were to take a cylinder and we will be using this cylinder a lot so it's good to get used to this cylinder good cylinder there and it's got a piston in it and that's just a it's kind of the roof of the cylinder can move up and down the roof of the sill this is the roof of the cylinder the cylinder is bigger but let's say this is a kind of a roof of the cylinder and we can move this up and down and essentially we'll just be changing the volume of the cylinder right I could have drawn it this way I could have drawn it like a cylinder no my drawing skills are I could have drawn it like this and then I could draw on the piston like this so there's some depth here that I'm not showing we're just looking at the cylinder front on right and so at any point in time let's say the gas is between the cylinder and the floor of our container you know we have a bunch of molecules of gas here a huge number of molecules and let's say the we have a rock on the cylinder that a cup essentially let's say we're doing this in space so everything above the piston is a is a vacuum actually let me just erase everything above so let me just erase this stuff just so you see it's where we're doing this in space and we're doing it in a vacuum this so let me write that down so all of this stuff up here is a vacuum which meant essentially says there's nothing there there's no pressure from here there's no particles here just empty space and in order to keep this we know already we've studied it multiple times that this gas is generating you know things are bumping into into this the wall of this the floor of this piston all the time they're bumping into everything right we know that's continuously happening so we need to apply some pressure to offset the pressure being generated the gas otherwise the piston would just expand it would just move up and the whole gas would expand so let's just say we stick a big rock we stick a big rock or a big weight on top let me do it in a different color we put a big weight on top of this piston where the width the force crimp is completely offsets the force being applied by the gas and obviously this is some force over some area right the area of the piston over some areas so we could figure out its pressure and that pressure will completely offset the pressure of the gas but the pressure of the gas just as a reminder is going in every direction the pressure on this plate is the same as the pressure on that side or on that side or on the bottom of of the of the container that we're dealing with now let's say that we were to just evaporate this well let's say so let's not say that we evaporate the rock let's say that we we just evaporate half of the rock immediately right so all of a sudden our our weight that's being pushed down or the force that's being pushed down just goes to half immediately let me draw that so I have maybe bed better off just cut and pasting this right here so copy and paste it so now I'm going to evaporate half of that rock magically let me take my eraser tool and I just evaporate half of it and now what's going to happen well this piston is now applying half the force it can't offset the pressure due to this gas so this whole thing is going to be pushed upwards but I did it so fast I did it so fast you could try it I mean it's you know this would be true of a lot of things if you had a weight hanging from a spring and you just remove half the weight it wouldn't just go very you know nice and smoothly to another state what's going to happen is and let me see if I can do this using their cut and paste tool it'll essentially right when I evaporate half of it the gas is going to expand a bunch and then this weight is going to come back down it's going to spring and go down so let me do it again it's going to expand because that gas is going to push up and then it's going to come back down and then you know it's just going to oscillate a little bit and then eventually it'll come back to some stable and maybe it'll go back it'll be like right about there and let me fill this in this this is this shouldn't be white this should be black let me put some walls on it on the container some walls on the container so if we wait long enough eventually we'll get to another equilibrium state where this thing is in the piston on top isn't or the ceiling isn't moving anymore and now the gases has filled this container now at this point in time we were in equilibrium the pressure throughout the gas was the same the temperature throughout the gas was the same the volume was in a stable situation it wasn't changing from second to second so because of that our macro states were well defined macro States well defined well defined now when we wait long enough this thing will get to some stability where this thing stops moving when this thing stops moving our volume stops changing and hopefully our pressure will start become uniform throughout the container and our temperature will become uniform I'll be a higher volume or lower pressure probably a lower temperature if we assume that there's no other heat being added to the system and then we'll be well defined again we'll be well defined again so we could say what the pressure in the volume of the temperature's going temperature is going to be but what about right when I remove this rock and this thing flew up and it oscillated and for a while the pressure at the top was lower than the pressure down here maybe the temperature at the top was lower than the temperature down here the whole thing was in a state of flux it was not an equilibrium and at that point when were let me let me draw that really so you know we were in that state where everything was just crazy right when we evaporated the rock you know we have a little rock up here everything's good it's going up and down maybe the pressure up here was lower than the pressure was than the lower than the pressure down here the temperature was lower than the temperature everything did not have a chance to reach an equilibrium at this date and this is important especially as we go into talking about things like reversible reactions and reversible processes and and quasi static processes at this point in the reaction when we just did this none of these macro states were well defined you couldn't tell me what the volume of the system is because it's changing for every second two second it's go or microseconds of microsecond it's fluctuating you couldn't tell me what the pressure of this system is because it's changing every second you couldn't tell me what the temperature is maybe the temperature could be you know if I did this as the temperature could be something there it could be something there all sorts of crazy things are happening so when the system is in a state of flux your macro states are not well defined and I really want to hit that point home so let me just draw that in the diagram let me draw that in a in a PV diagram and we're going to use these fairly heavily so on my y-axis I'm going to put pressure and my x-axis I'm going to put volume so our initial state here when we had the rock sitting on top of this ceiling this movable ceiling or this piston maybe we had some well-defined pressure and volume so my Y so this is pressure and this is volume so this is where we started off so it was well-defined this is state one let me label it right there this is state one now when we evaporated half the rock we got we eventually and we waited long enough and this got to an equilibrium we got to state two and our pressure volume and our temperature was well-defined and I'll just put it on this pressure volume so maybe this is state two we got down here and this is aside why you know I could maybe put temperatures an extra dimension but temperature is completely determined by pressure and volume especially if we're dealing with an ideal gas remember and we did this in multiple videos you know you have PV is equal to and while we could eat well well right and our t these are cut these are constants the number of moles isn't changing this is the universal gas constant not changing so if you know P and V you know T so that's the only two things we have to plot but I'll talk a lot more about that in future videos but the important thing to realize is I started off at this state where a pressure and volume were well-defined I finished in this state where pressure and volume were well-defined but how did I get there and because this reaction I did all of a sudden it happened super fast and it was essentially thrown out of equilibrium out of equilibrium equilibrium I don't know how I got here this this the pressure and volume were not well-defined from going from that state to this state pressure and volume and temperature are only well-defined if every if every every intermediate step is still almost in equilibrium and we'll talk a lot more about that in the next video but I want to really make this point home it would be nice if we could draw some you know paths we could say oh you know ever as every we moved from some pressure and volume to some other pressure and volume and we moved along a well-defined path but we cannot say that because when we went from there there our definitions just disappeared for pressure and volume you cannot define those those macro states and these intermediate non equilibrium states now this is a little aside we could have defined the microstates the microstates never change at any given snapshot in time I could have listed every particle that's in this thing and I could have give it's kinetic energy I could have given you its position I could have given you its momentum and there's no reason why I couldn't have done that so I could have actually made it you know I could have made a plot of one particular particle and I could have said what its kinetic energy and it's over a course of time is it any given moment in time and this is really important so microstates are always well-defined the microstates what's exactly happening to the atom in terms of its force in its velocity in its and its momentum while macro states are only defined are only I should say well-defined when the system in this case is the balloon in this case is this piston on top of the cylinder this movable ceiling the macro states are only well-defined when the system is in equilibrium or where you can essentially say when you say the pressure is X the pressure is the same throughout or the volume isn't changing from moment to moment or the temperature is the same thing throughout anyway I'll leave you there and we'll talk more about why I went through all of this pain in the next video