 Please write down thermodynamic process. So you might have heard about the quasi static process. Anybody knows the meaning of quasi? Quasi means what? Meaning of quasi? Nearly or almost? In a slow process. Static means what you're saying. Quasi means what? Like it seems to be apparently correct. So it is not real thing. Okay. It's not really, but it appears to be. Okay. So basically it is a quasi static process, which means that it appears to be static, but it is not static. But for mathematical derivation or any expression you want to derive, you can treat as if it is static process, but in reality it is not. Now why such thing is important? Because you know what happens is, suppose you take a system in which you have a piston which can move. Okay. This piston can move this side. The gas is there. So if you allow this piston to move very fast this way, then the gas inside it will not be in equilibrium and then you cannot study it. Fine. So that is the reason why if you're moving the piston, you should move the piston extremely slowly so that gas gets sufficient time to come in equilibrium. Okay. So that is why this is quasi static process. So if you are moving the piston extremely slowly, it means that by the way you are not moving, the surrounding is moving and surrounding pressure is let us say P1 and pressure of the gas is P2, then if the piston is moving extremely slowly, then P1 is nearly equal to P2. Okay. Now pressure of the gas is only slightly more than P1 so that it pushes forward. Delta P tends to zero. So this is what happens. So whatever is the pressure outside, you can say that is the pressure of the gas also in case of quasi static process. Okay. Similarly, when the piston is not moving, let us say this is the scenario. This is the partition which is fixed. Okay. This is gas one and that is gas two. And suppose temperature of gas one is T1, temperature of gas two is T2. Because of this partition, they will start exchanging heat. Okay. Now you cannot permit the rapid exchange of the heat. If the heat is allowed to exchange rapidly, then temperature of the nearby gas molecules here, gas one, temperature of this will be lesser than the temperature which is here. Because these molecules, these one, will get first chance to exchange heat. Because of that, temperature of the entire system of gas one will be different at different points. Okay. And hence the heat transfer should happen slowly. So heat transfer expansion, these two things should happen as if they are not happening. Or you can say that it is happening extremely slowly. Then only you can say that, you know, the systems are in equilibrium and then only you can study them. Okay. So if heat is getting exchanged between T1 and T2, then T1 should be nearly equal to T2. Okay. And that is how the temperature, that is how the heat will, it should be exchanged. Otherwise system will not be in equilibrium. Okay. So going forward, our focus is only quasi static process. Okay. We are going to graphically analyze everything. Because if it is a quasi static process, I can define the state of the system at every point on the process. What I mean to say that when the system is going from, let's say point number one to point number two. Okay. And let's say it is following this path. It is going like this at every point on this path, the system is in equilibrium. And hence you can define the macro variables of the system. You can define its pressure, you can define its volume. So that is the reason why you can make a curve by connecting all the points. If you only know the initial and final point and system was not following quasi static process, then you'd have no idea what process the system has followed because you can't define pressure and volume for that process. Fine. So if there's a plot between P and V, it automatically means it is a quasi static process. Otherwise you can't draw the plot itself. Is it clear to all of you any doubts with respect to what is equilibrium state and what is quasi static process? What does it mean? Okay. I think Paul will be a better idea. Okay. You have any doubts? If you have any doubts, please click yes. Okay. No doubts. Okay. Someone is teaching so well. Anyways, we'll move on to the next. Now, since we are talking about the processes, please write down first the definition of process. When I say process, I mean to say thermodynamic process. Okay. Process is the path followed by the system to go from one state to another is described by micro variables like pressure, volume, temperature and other things. Okay. So you should know what is pressure, volume and temperature. We must know pressure, volume, temperature and other macro variables to describe the path can also be graphically visualized, 2D or 3D charts. Okay. Since I know the pressure, volume and temperature, so I can plot, you know, something like this. For example, I can have pressure on the y-axis and volume on the x-axis. And at every moment, I just find out what is the pressure and the volume. So once I get all the points, pressure and volume, I connect all the points with line like this. And I'll say that this is the path. This is the path followed. It has gone from 0.1 to 0.2. Fine. So the kind of path followed by the system. Okay. The kind of path followed by the system can tell us how much is the heat absorbed and how much is the work done. Okay. Can the path tell us what is the change in internal energy? Anyone? Can path tell us what is the change in internal energy? Path will tell us what is the work done and what is the heat absorbed. For example, work done at constant volume will be 0. Okay. Heat absorbed at constant volume will be Ncv delta T. Okay. So depending on what is the process, what is the path, different values of work done and heat will be there. What about change in internal energy? The rate is independent of path taken sir. It is independent. It does not matter. You don't need to know the path. Delta U will be always equal to Ncv delta T if ideal gas is your system. Now since path is very important to determine the work done and the heat absorbed or released, that is the reason why we are going to talk about the few basic processes. Okay. The name of the process will be taken by what path they are following. Okay. So please write down the basic processes. See, I have few basic things, right? Pressure is there, volume is there, temperature is there and heat. So depending on these variables, the path is named. For example, if pressure is constant from initial to final point everywhere, the pressure is constant, then what path name is this? Anyone knows it already? Isobaric. Isobaric. The name of the path is isobaric. If volume is constant, isochoric, if temperature is constant, isothermal. Okay. And if heat is not exchanged between system and surrounding, then what I can say, adiabatic. Okay. Are these only four types of processes possible? Only four types of processes are possible or there can be many more? Cyclic processes. What? Cyclic processes. Cyclic process is like collection of multiple processes. It could be a combination of isobaric, isochoric, isothermal. So that is different. There can be many, many types of, many different types of processes. Okay. So we are going to learn about how to visualize all the different kinds of processes, but we'll start our study with just understanding these four basic types of processes initially. Okay. We are not going to rush. So that's why we are going a little slow. So first, write down the isobaric process. Let's study them one by one. Since depending on the path, heat absorbed and the work done will change. So we are going to find out the heat absorbed and the work done for different, different kinds of processes. So we have four different kinds of processes. We are going to talk about heat absorbed and work done for all the four kinds. Let's say this is pressure and this is volume. So suppose the process is isobaric. How the path will look like? Let's say point number one is this. This is the initial point, where we'd be the final point parallel to x axis, parallel to x axis because volume is constant. We have done many such kind of graph in kinematics study also. So don't hesitate point one to let's say point two. Okay. So this is the isobaric process how it looks in PV plot. Okay. Now what is the difference between this process and let's say if I'm coming from point two to one, what is the difference? One to two and two to one. Compression expansion. The volume is increasing and volume is decreasing. That is clear cut. But what is happening physically? So the work done. What? The work done, the sign will change. It will be positive and negative and another. No, you're not understanding. I'm talking about physically what it implies. Compression and expansion. In one to two work done is by the system. Okay. Two to one work is done on the system. Okay. You can see that in one to two the system is expanding. In two to one surrounding is expanding. The surrounding is doing the work. But system is compressing. So system plus surrounding is entire universe. If system expands, surrounding has to contract because some of the system and surrounding is universe. And if surrounding expand, system has to contract. Depending on who is expanding, the work is done by that. Okay. So let's say the state variables are given to us 0.1, 0.2. The work volumes are given V1 and V2. And at 0.1 pressure is P. Okay. So can I get the value of delta Q and W for isobaric? Tell me. Delta Q will be what? CP is also given. CV is also there. Delta Q, since it is a constant pressure process, it should be N, CP, delta T, isn't it? And the work done should be what? All of you are on mute actually. Work done will be what? Tell everyone what will be the work done? P delta V. It will be P into delta V, which is V2 minus V1. Okay. The good thing about the formula is that automatically it will take into account the positive and negative sign. So in case of the process 2 to 1, V will be V1 minus V2, which will be a negative quantity. Okay. So when you are using this formula, work done is equal to integral of PDV. You don't need to worry about whether work is done by the system on the system. Automatically, this will come out with the sign. Okay. So this is the work done and that is the delta Q. And you can also write it as PV2 minus PV1. PV2 can be written as NRT2. PV1 can be written as NRT1. So this can be modified into NRT2 minus T1. Okay. So remember that you can find the work done in isobaric process if you know the temperature also. Okay. So this is isobaric process.