Let us consider the heating of gas in a container by sure mass ‘W’ kept on top of lid (lid is such that it does not allow leak across its edge by vessel wall) of vessel as given away in Fig.
After definite amount of heat being additional to gas it is found that the lid gets raised up. Thermodynamic state transform is given away in figure. The adjust in state is major. During the change of condition since states could not be consider to be in equilibrium, therefore for unsteady condition of system, thermodynamic analysis could not be extensive. Let us now suppose that total mass comprise of infinitesimal small masses of ‘w’ such that all ‘w’ masses put jointly become equivalent to W. Now let us start heat adding to vessel and as soon as lifting of lid is experiential put first fraction mass ‘w’ over lid so as to counter exciting and estimate state change. During this method it is found that state change is insignificant. Let us additional add heat to vessel and again put second fraction mass ‘w’ as soon as lift is felt so as to counter it. Once more state change is see to be slight. Continue by the over process and at the end it shall be see that all fraction masses ‘w’ have been place over the lid, therefore amounting to mass ‘W’ reserved over the lid of vessel and state change occur is precisely comparable to the one which occur while mass kept over the lid was ‘W’. In this method the equilibrium nature of system can be maintain and thermodynamic analysis can be carried out. p-v representation for the sequence of insignificant state changes occurring among states 1 and 2 is given away.