Quasi static process - thermodynamics:

Thermodynamic equilibrium of a system is very difficult to be realized during the occurrence of a thermodynamic process. 'Quasi-static' consideration is a way to consider the real system as if it is behaving in thermodynamic equilibrium and hence permitting the thermodynamic study. In fact system doesn't attain thermodynamic equilibrium, certain assumptions make it akin to a system in equilibrium for the sake of analysis.

Quasi-static refers to almost static and the infinite slowness of the occurrence of the process is considered as basic premise for attaining near equilibrium in the system. Here it is considered as change in state of a system occurs at infinitely slow pace, therefore consuming very large time for completion of process. During dead slow rate of state change magnitude of change in state shall also be infinitely small. This considerably small change in state when is undertaken repeatedly one after the other results in overall state change but the number of processes needed for completion of this state change are considerably large. Quasi static process is presumed to remain in thermodynamic equilibrium due to infinitesimal state change taking place at the time of occurrence of process. Quasi static process can be understood from the example given below.

Let us consider locating of gas in container with certain mass W kept on top lid (lid is so that it does not permit leakage across its interface with the vessel wall) of the vessel as shown in the figure given below. After certain amount of heat being added to gas it is found that lid gets raised up. Thermodynamic state change is shown in the figure. The change in state is significant.

During the change of state as the states could not be considered to be in equilibrium, thus for unsteady state of system, thermodynamic analysis cannot be extended. Difficulty in thermodynamic analysis of unsteady state of system lies in the fact that it is not certain about the state of system as it is changing continually and for analysis one has to start from some specific values.

Assume that the total mass comprises of infinitesimal small masses of 'w' such that all 'w' masses put together become equal to w. Now let us begin heat addition to vessel and the lifting of lid is observed put 1^st fraction mass 'w' over the lid so as to counter the lifting and guess the state change. During this process it is found that state change is negligible. Further add heat to the vessel and again put the 2^nd fraction mass 'w' as soon as the lift is felt to counter it. Again the state change is negligible. Continue with the above process and at the end it is seen that all fraction masses 'w' have been put over the lid, hence amounting to mass 'w' kept over the lid of vessel and the state change occurred is similar to the one which occurred when the mass kept over the lid was 'W'. In this way equilibrium nature of system is maintained and the thermodynamic analysis is carried out. P-V representation for series of infinitesimal state changes occurring between states1 and 2 is also shown in the figure given to us.

Note:

In PV = R₀T, R₀  = 8314 KJ/Kgk

And in PV = mRT; R = R₀/M; here M = Molecular Weight