Thermodynamic equilibrium of a system is very difficult to be realised during the occurrence of a thermodynamic process. ‘Quasi–static’ consideration is one of the ways to consider the real system as if it is behaving in thermodynamic equilibri um and thus permitting the thermodynamic study. Actually system does not attain thermodynamic equilibrium, only certain assumptions make it akin to a system in equilibrium for the sake of study and analysis.
Quasi–static literally refers to “almost static” and the infinite slowness of the occurrence of a process is considered as the basic premise for attaining near equilibrium in the system. Here it is considered that the change in state of a system occurs at infinitely slow pace, thus consuming very large time for completion of the process. During the **** slow rate of state change the magnitude of change in a state shall also be infinitely small. This infinitely small change in state when repeatedly undertaken one after the other results in overall state change but the number of processes required for completion of this state change are infinitely large. Quasi–static process is presumed to remain in thermodynamic equilibrium just because of infinitesimal state change taking place during the occurrence of the process. Quasi–static process can be understood from the following example.
Let us consider the locating of gas in a container with certain mass ‘W’ kept on the top lid (lid is such that it does not permit leakage across its interface with vessel wall) of the vessel as shown in Fig. 1.9. After certain amount of heat being added to the gas it is found that the lid gets raised up. Thermodynamic state change is shown in figure. The “change in state”, is significant.
During the “change of state” since the states could not be considered to be in equilibrium, hence for unsteady state of system, thermodynamic analysis could not be extended. Difficulty in thermody-namic analysis of unsteady state of system **** in the fact that it is not sure about the state of system as it is continually changing and for analysis one has to start from some definite values. Let us now 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 start heat addition to vessel and as soon as the lifting of lid is observed put first fraction mass $w’ over the lid so as to counter the lifting and estimate the state change. During this process it is found that the state change is negligible. Let us further add heat to the vessel and again put the second fraction mass ‘w’ as soon as the lift is felt so as to counter it. Again the state change is seen to be negligible. Continue with the above process and at the end it shall be seen that all fraction masses ‘w’ have been put over the lid, thus amounting to mass ‘w’ kept over the lid of vessel and the state change occurred is exactly similar to the one which occurred when the mass kept over the lid was $W’. In this way the equilibrium nature of system can be maintained and the thermodynamic analysis can be carried out. P–V representation for the series of infinitesimal state changes occurring between states 1 & 2 is also shown in
Note:
In PV = R0T, R0 = 8314 KJ/Kgk
And in PV = mRT; R = R0/M; Where M = Molecular Weight
