 Make sure I don't irreversible and irreversible processes in the case. What is irreversible process? And what is irreversible? I am not asking you to say that. Reversible is basic English. Sister, it is spontaneous. Tell me. Reversible is like you can trace every step backwards. Reversible is the same part. Reversible is the same part. Walk done. What is the formula of walk done? F is lost. But minus p extended dv. This is the basic formula of walk done. We want to be. Because we are considering external pressure of gas here. But here the thing is what? If you have this system. Here we are considering the pressure of gas actually. P of this gas which is there in the system. And this is the external pressure. So in physics we will write down p external dv. We will consider external pressure over there. And this p external is equals to minus p of gas at equilibrium. In chemistry we will consider pressure of gas dv. Let us call this term as equal to minus p. So here we are considering the pressure of gas. But in physics we will consider the external pressure. That is not my problem. Because we are considering the walk done by the system actually. The system to work and the system to consider. Mostly. Maybe that is the reason. But why plus minus p m? Because here we are considering pressure of gas. And physics we will consider the external pressure. We are considering the system actually. The system which is working that is what we are considering. Okay. Now this is not my question. You must have done the derivation. So what we are writing? Somewhere like write this as the external pressure. And integrate this dv. That is the expression. And then in some of the cases, we will don't take this p external out. When we can take this p external out. But it is constant. So when it is constant and when it is variable that you have to understand first. Right? So the irreversible process will be the external pressure constant. Okay. So when you do not understand these two. Why we will discuss that? For that you need to understand what is the reversible process and what is the irreversible process. But we will discuss that. Why is it reversible and irreversible? The point is whenever you derive walk done in irreversible process. You can take this p external out. So we will write p external into delta v is the walk done in irreversible process. Because p external is constant. Why it is constant that we will see. In two different situations we are taking here. We have a piston-cylinder system. This is very important. They ask questions on this conceptual theoretical question. They also ask in irreversible and irreversible process. Suppose we have a piston-cylinder system. Right? And here we have gaseous molecules present. Okay. Now what you do? Obviously here we have some equilibrium. P external is equals to what? The pressure of gas? This equilibrium? Then only it will be a static. Now what you do? We are disturbing this equilibrium. So when you are here, I will take some sand. Like one bag will have in that order. What I will do? We will put some weight onto this piston. Which was at equilibrium initially. So we will take some sand into this bag. Or a stupor. Okay. Here is sand particles. So when you put some weight onto this, this piston will come down. Right? Compression takes place. And when it compresses, it exerts more pressure onto the piston. Till we will get the equilibrium here. Right? So here we have gaseous molecules. Compression takes place. And at this point what happens? This point, this pressure of gas is equals to the external pressure. Okay. Right? Yes. So when you put this bag over here, all of a sudden this piston comes down. Because we will have some pressure. This will have some weight component. Because of that we will have some pressure. So basically we are applying some pressure into this. And all of a sudden it comes down. And when equilibrium is maintained, it stops. Right? Now what you do here? You take one particles out from this sand. Continuously. Continuously. You are decreasing the pressure now. So this will slightly go up. Step by step you go up. So this process, when it is going very slow in upward direction, this process is reversible process. Okay? It is reversible process. But when you put this sand bag here, all of a sudden it comes down. Right? It is very fast. Right? So this is irreversible process. So you take this sand bag off. Yes, you can do that. But then you won't understand what is reversible and reversible process. I am trying to make you understand what is irreversible and reversible process. When you remove this weight, very small particles if you are removing, so indirectly what you are doing, you are decreasing the pressure by very slight amount. By D P amount. Right? So this piston very slowly goes up. Even you cannot even observe that piston is actually going up. But it is going up. Right? So keep on removing this sand particles. It slightly goes up. So this process by which you can achieve this state again. This process is what? It is a step-by-step process. And this is a reversible process. So why it is, why we can achieve the previous step into this, the particle that you have removed, pull it again, you will get the same state. Right? So this process is reversible process. All of a sudden it comes down. It is irreversible process. So they ask difference between irreversible and reversible process into the exam, board exam. Like, even if I take it off, it will get back to that point. Yes, yes. So in the entire service, the pressure was constant. That is what we will discuss that. Actually irreversible process also maintains equilibrium. Irreversible process also maintains equilibrium but only at initial and final stages. In irreversible is the external pressure constant. Yes. See actually the second point is what? When you put some weight on to it, so it comes down because of this pressure, right? So this is the external pressure now and this is constant throughout when it comes down from here to this point. That is why the external pressure is constant in irreversible process. Yeah. And since you are continuously removing sand particles, so pressure you are changing continuously. So external pressure is not constant here. You understood this point, right? You are continuously removing the particles so pressure you are changing continuously. So hence the external pressure is not constant in irreversible process. The pressure isn't a function. Yes, it is. That is why we will get different expression of work done in reversible and irreversible process. No, we are actually this directly you understood this is some pressure you are applying and you are decreasing the pressure slowly. That is it. Okay, is it clear? Right? So there are four or five important points into this. Okay? The first point, first you write down the irreversible process. Heading right to irreversible process and one by one you write down these points. Irreversible process. It is unidirectional. Irreversible process, unidirectional means it cannot be reversed. It cannot be reversed along the same path easily. Same path easily. Next line, write down in this only. Write down work has to be done. Work has to be done on the system. Work has to be done on the system to restore its original position. Means to restore the position of system. It has finite number of steps. It has finite number of steps. One particle you remove, that is one step. Another you remove, second step. Like that, okay? So it has finite number of steps. It is a very fast process. It is a very fast process. System, next point. System at thermodynamic equilibrium. Only initial and final step. System is at thermodynamic equilibrium. At only initial and final step. External pressure is constant. External pressure is constant. External pressure is constant. So since we have equilibrium only at initial and final position. So we can apply pv is equals to nrt at only initial and final position. Right? pv is equals to nrt we can apply at only initial and final position. Next you write down reversible process. Irreversible it is constant. Next is reversible process. It is constant only. You see, you put this back. It comes down all of a sudden. So this is a reversible process. Very fast process. So it comes down according to because of this pressure that you applied. Apply pressure applied by this back. External pressure that you applied. Okay, so it is constant. Whatever this value it is constant throughout. Right? Next point you write down reversible. Reversible is what it is bi-directional. It can be reversed bi-directional and can be reversed along the same path easily. There is no external work required. There is no external work required to restore the system external work required to restore the system to its original position. It contains infinite number of steps. It contains infinite number of steps. Very slow process very slow process. The system is in thermodynamic equilibrium at every step. The system is in thermodynamic equilibrium at every step. The external pressure is not constant. Okay? So you see the graph of reversible and irreversible process. Okay? Okay, see here. The pressure is Vi. Initial pressure is this. Final pressure is this. Right? So we are decreasing the pressure. Right? Pressure decreases means what? Expansion takes place. Right? So this one is Vi and this one is Vf. Expansion means what? Volume increases. Right? So the graph is this. This is the area we have here. Okay? Given this graph we have, we have two different graphs here. Which one is reversible? Which one is irreversible? First one is reversible. First one is reversible. Right? First one is Vf. Okay? Second one, third one is Vf. You don't know these two. This one is reversible or irreversible? Irreversible. And this one? Irreversible. So this graph is for irreversible. Expansion. Because the volume is increasing. Okay? So this graph is for irreversible. Expansion. Because the volume is increasing. So it is irreversible expansion. And this is irreversible and this is irreversible compression. Wait a minute. See, here what happens, you see the initial first of all you tell me, this is the case of expansion or compression? Expansion. Expansion. So we should keep the arrow like this, right? The expansion. Otherwise we have to give it decrease. The initial pressure and final pressure. Okay? So it is decreasing so expansion. Okay? So here what happens, you see PI since it is a graph like this means at every point you are decreasing the pressure, right? You are decreasing the pressure. So suppose if PI is initially here, where you decrease it up to this value, right? So obviously we will have some amount of expansion. And what is the expansion we have? This is the expansion when the pressure is this. Further you decrease the pressure. Again we will have expansion till here. Further you decrease the pressure. Again we will have expansion. Right? So you see all these steps are involved into this. So number of steps involved is 1, 2 and 3 if you take for these three instead. Okay? So obviously we are decreasing the pressure and expansion takes place. So this graph represents what? This graph represents reversible expansion. Reversible expansion. Okay? Reversible expansion. Just opposite of this. PI will be here, PF will be here. So but if you break it down into steps then it becomes irreversible. Like instead of the curve if you had those steps then it would be irreversible. Because of the steps it is irreversible. See this change if you are considering delta P it just tends to 0. Right? This is for a very small change. I have given this to make you understand. No, no, no. But if it was actually like that delta P wasn't tending to 0 then it is irreversible. If you are considering this process. Infinite number of irreversible process that you get. Okay? But the process is very slow and we have infinite number of steps in them. And each step will be included. Here if you increase the pressure goes here it will have the same volume corresponding to this point. Because in this the graph is like this this graph means what? This graph means what? At this pressure the volume is this at this pressure the volume is this at this pressure the volume is this So pressure and volume has the relation to this. Pressure and volume has the relation. It is not constant. This pressure was here. It was all of the degrees. This pressure and volume is independent of the volume here. independent of the volume here because the process is irreversible if you draw like this it means pressure and volume is related to each other which is not the case what we do we just put the pressure put the bag over there all of a sudden it comes down this is constant pressure yes yes yes so when you put the pressure over there the sand bag all of a sudden it comes down like this right here we have initial pressure put the bag it comes down on this and then next one next one got it now from this stage if you remove the bag it goes up this comes over here it goes up all of a sudden this is the volume right so work done if you calculate for a reversible process first of all you write down this the formula of work done is always integral of minus p external dv this is the always work done formula depending upon the process will apply the condition okay so work done for reversible process is what reversible process of pressure is not constant this p external we cannot take out of the integral side so that will be what that will be p external dv this is for reversible process we'll do one derivation of based on this also right work done in irreversible process is what since p external is constant here so we can take this p external out and this becomes what v2 minus p1 so in general if I write work done in irreversible process it is always minus p external into delta because p external is constant of it of this