 WXRT, Assistant Professor, Department of Mechanical Engineering, Vulture Institute of Technology, Solopo. Now, let us move for thermodynamics two sessions. Already, we have studied in the first part what is system, what is surroundings, and how we define heat and work, and so many other parameters like open system, closed system, and isolated system. Then work and heat are the phenomenons, energy is in transit. We have seen the analogy with the cloud and rain. Now, very important calculation part or analytical part is calculation of work done during various processes. Now, before that, we must know what is the thermodynamic process. Again, like previous video, we will just see that what are the outcomes of this particular video. At the end of this session, you will be able to derive the expression for work done for various thermodynamic processes, and you will be able to calculate work done for a cyclic process. Now, before we go into the detail about the processes and their work done, let me ask you a simple question. What is a process? Now, process, if you ask someone what is a process, then one may say that if I am here at point number one, I may give to point number two. Then this is called as a movement from state one to movement to state two. Now, the question is, when I go from one to two, where there is single path available, this is one path, this is another path, this is third path, then this is another path. So there are infinite number of paths. There are infinite number of paths. But do we have infinite number of processes in thermodynamics? Theoretically, yes. Practically, not. So practically, we are considering few processes that I will mention you. And for them, how to evaluate the work done, expression for work done, we'll learn. And I will pose you certain questions so that you can find out the answers for those typical critical problems. So one thing is clear to you that process is a change of state of a system from one to two. In short, if I see the block, for example, suppose there is one Pi, P1, V1, T1, Rho1, then N1, and so, so, so, so many parameters. And there is another change from this to say P2, P3, P4, then V1, V2, V3. See, why I written like this? P1 may change to P2, it may not change. Sometimes it may happen that only P1 changes. And all other parameters remain same. Even then we call as a new system or new state because state represents a thermodynamic system in its entirety, that is very important. In its entirety means it is just like a coordinate system. Suppose on this XY system, if I say that I am at point, say, X is equal to five centimeter. If I say X is equal to five centimeters here, I'm not sure where I'm here, here, here, here, or here. But if I say that I'm at Y is equal to three centimeter, then I can get, look at the point. Similarly, any thermodynamic system, when I say that it has got P1, V1, and T1, it is one point. When I say it is P2, V2, and T2, I've said it is another point. If I say it is P1, V1, T1, and P1, V1, and T2, then also it is different system because only parameter temperature has changed. But I'm saying that out of this N parameters, if any one parameter changes or more than one parameter or almost all the parameters change, we say that system has undergone a process. Now, what is that process? Whether it is having a typical name, that is a typical thermodynamic name, that we are going to discuss in detail. And why we are interested in that? That is a very important thermodynamic question that I will tell you. Now, suppose I say, I see the PV diagram. If I draw a PV diagram. Now, suppose I'm here at point number one, and it has got certain volume, and say this is another volume I'm interested in. So, this is a V1, and this is a V2. Now, suppose I ask you, from this point, if I move to this point, say point number two, point number two. So, what has happened? Pressure remain constant, and volume change from V1 to V2. I go by this method. What has happened? Pressure is P1, volume is V2. If I go from this way, pressure is P1, volume is still V2. Then if I come by this way, and this way, what has happened? In this process, I divided the process into two parts. One is moving downward, other is this one. Or if I go by this process, and then by this process. So, you will wonder that in thermodynamics, why thermodynamics becomes very difficult? Because there is no exact equation that we have. We are always interested in the process, in the process diagram. This is called as the process diagram. So, whenever I have a process, then it is clear to you that, I have a constant pressure process. I have a constant volume process. I have got a constant temperature process. I have got a constant, say zero heat transfer, zero heat supply process, and so many other. So, we list some standard processes which are there that we use in thermodynamics. The first important is constant pressure process. First is P is equal to C. Now, you can see P is equal to C. How to visualize? Suppose I have a cylinder and there is a piston. There is a piston. There is a gas inside. There is a gas inside. If I supply heat, if I supply heat. As a result of this, what will happen? Molecules will get heat. Their kinetic energy will increase. They will impart some pressure on this particular piston. As my pressure is to remain constant, the piston moves in the upward position. So, this piston goes in the upward position. And let us say that this is a small displacement dx. So, what we say? If I put my hand on the piston, then what will happen? As I supply the heat to the gas, the piston goes up. If I don't apply any force, my hand will be moving in the upward direction. But if I apply the force, what happens? I will tell afterwards. So, what has happened? This is the movement. And pressure here is P. And pressure here is also P. So, is there any change in the pressure? No change in pressure. But what has happened to volume? Here, volume is V1. And at the end, it is volume is V2. So, if I want to draw this on a process diagram, that is pressure and volume. Initially, my volume is V1. Finally, my volume is V2. And my pressure is constant. So, naturally, from this, if I see, the net force applied on this piston. Suppose the piston has got cross sectional area A. And P into A is equal to force. And this force into displacement is equal to what I have done. So, what is my force? P into A. What is displacement? dx. Now, I club this. So, it is a P into small change in volume. Now, there comes calculus. When I am infinitesimally increasing the volume, what happens? This is my work done for a small change. If I take integrated from V1 to V2, I will get this as P into V2 minus V1. Now, the question is, whether it is a work obtained or work supplied to the system? That is very, that we can do by common sense. Because if V2 is greater than V1, V2 is greater than V1, what has happened? The term is positive. So, the system is giving positive work. Means, many students have a confusion that expansion means reduction in pressure. It is absolutely wrong. Expansion is defined only with respect to change in volume. Now, see here, here the pressure remaining constant, we have got a expansion. If I say this process in which there is decrease in pressure, but increase in volume, and pressure is not constant. So, in that situation, I have to find out PdV, and P is to be defined in terms of V. From V1 to V2, I have to integrate. So, this is a multivariate calculus. Means, I must define P in terms as a function of V, and then integrate it as a function of V. Or I must define V in terms of P as a function of P, and then integrate in terms of P. Anyway, integral PdV is going to be a burden. So, one thing is clear to you now. Say this is, it can go in this way also. So, there is an increase in pressure, and burden is positive. There is a decrease in pressure, and burden is positive, and pressure remaining constant. There is a positive work. Now my question, simple question to you is, simple question. Out of these three processes, one, two, and three, work done for one, work done for two, and work done by three. You put the less than equal to science in between. That is your task that you have to complete. Now, this is about the constant pressure process, and any relation that you have. Now, another simple thing that you may require is if I go for a isometric process. Many students do not know the term isometric, because isometric, metric is a term for measurement. And for isometric process, when I draw the PV diagram, my volume is constant. So, in this case, work done is equal to 0. For isothermal process, this is PV is equal to constant. You can integrate PV. You know that PV is equal to nRT. Put the value of P is equal to nRT upon V, and integrate. And integrate, you will get the expression, which you already know, that is nRT log of V2 by V1. If V2 is greater than V1, it is a work done. If it is less than, then it is work given to the system. If it is polytropic process or isentropic process, it is PV raise to gamma is equal to constant, same. Now, the task for you, show that work done is equal to P1 V1 minus P2 V2 upon gamma minus 1, or work done is equal to P1 V1 minus P2 V2 upon n minus 1. And simple question, w isothermal and w adiabatic. In this process, which work is greater for the same delta V? For the same change in delta V, which work is greater? So, you do this calculation. And in the third session, we'll see the first law of thermodynamics, its limitations, and why we need to go for the second law of thermodynamics. Now, those who are interested in, say, learning more about these numericals, you can refer to thermodynamics by P. K. Nagg, thermodynamics by Ennis St. Thank you for this particular, say, session. Thank you.