Now, let us pay attention to the quasi-static process and understand this in you know, in great detail. So, before I talk to you about the quasi-static process, we need to understand what is equilibrium. Now, there are three kinds of equilibrium we need to know. The first kind of equilibrium is thermal equilibrium. The second is chemical equilibrium and the third is mechanical equilibrium. Now, what do you understand by chemical equilibrium? Equilibrium in a natural means balance. Now, what do we understand by thermal equilibrium? Thermal equilibrium means that the temperature difference between two bodies is the, both the bodies or you know, you can have any number of bodies. They are all at same temperature. So, the delta T is 0. There is no temperature difference between the bottoms or between the systems. Now, chemical equilibrium means now, for a chemical reaction to take place, you have a set of reactants and from these reactants, you get a set of products. Now, from reactants to products, this is the forward reaction. So, the rate at which this forward reaction takes place, you have the backward reaction taking place at the same rate. That is from product to reactant. That is the backward reaction. So, the rate of forward reaction, when it becomes equal to the rate of backward reaction, then it becomes a case of chemical equilibrium. Now, we come to the third kind of equilibrium. That is mechanical equilibrium. In the mechanical equilibrium, there are no unbalanced forces in any direction. So, sigma f x is 0, sigma f y is 0 and even if you are having a three dimensional, you can say study of forces, then even sigma f z is 0. There are no unbalanced forces in the system. Now, when you have all these three equilibriars, co-existing simultaneously, when you have in the system, the co-existence of thermal equilibrium, chemical equilibrium and mechanical equilibrium, then the system is set to be under thermodynamic equilibrium. So, this is a very, very important point to understand over here. So, when you have thermal plus chemical plus mechanical, it gives rise to thermodynamic equilibrium. Thermodynamic equilibrium. So, if somebody asks you to define a thermodynamic equilibrium, then it is a combination of or co-existence of thermal, chemical and mechanical equilibrium in a system. Now, after having understood the concept of equilibrium, now let us move on to understand the co-existence process. So, for that let us have a piston cylinder arrangement over here. So, this is the cylinder and this is the piston. On top of the piston, you have some small, small weights placed on this piston. And you have some gas which is trapped inside this piston cylinder space. Now, what happens is, you remove one weight or before I remove that, let us assume that at this particular you can say condition, the state of the system is at state 1. So, at this particular state, the pressure is p 1 and the volume is v 1. Now, I will remove the first weight. When you remove the first weight, what will happen? The pressure will decrease and the volume will increase. So, let us redraw this position a bit. So, let me move it up. So, you will have a pressure. So, this is high pressure, pressure 1. So, when you remove the top weight, the pressure would reduce a bit and the volume would increase a bit. So, this piston will have a new position and let us say that position is not very far. It does not move a very long distance, it is just this much. So, this is the new position of the piston. This is not very considerable. This is a very, very small distance through which the piston is moved. So, this is not even this much, this is very, very small. So, let us say the pressure has been removed this much and the volume is also this is the new condition. This is the new state. When you remove another weight on top, at little bit more upward travel is there. So, you come to this particular state. So, when you keep on removing these weights, your piston will keep on moving upwards. So, you will get a process which is of this sort. Let us say at 2. Now, the great thing about this is that every intermediate state that you achieve. So, at the beginning that a state 1, your system is under thermodynamic equilibrium. There is nothing happening in the system. Now, when you remove the first weight on the top, you have an intermediate state which is this. At this intermediate state also, your system is under thermodynamic equilibrium. So, every intermediate state that your system achieves is under thermodynamic equilibrium. So, this is a property or a characteristic of a quasi static process that every intermediate state that has achieved is under equilibrium. Now, how do you achieve equilibrium? If you want to have equilibrium attained at each and every intermediate state, then the process has to be carried out very, very, very slowly. I would say infinite is slowly. So, this brings us to the definition of a quasi static process. So, quasi static process is that process in which all the intermediate states are at equilibrium and the process is carried out infinite is slowly. So, I hope you understood the concept and the understanding of the quasi static process and more importantly, you understood what is the equilibrium and the concept of thermodynamic equilibrium. That is the existence of these three equilibrium in a particular system. Now, let us move on to the next topic altogether. Now, this brings us to the end of the introductory part of the subject of thermodynamics. In the next video onwards, we go to the very first of the laws of the subject of thermodynamics that is called the zeroth law of thermodynamics in which we will start talking about temperature and that measurement. Because, zeroth law of thermodynamics basically forms the basis of temperature measurements. So, now, let us move on to the next series of videos in which we start discussing about the zeroth law of thermodynamics.