Quasi-static process

Consider a system of gas contained in a cylinder-piston assembly (Fig 1.6). The system is initially in an equilibrium state represented by a set of properties (x₁, y₁). The weight of the piston just balances the upward force exerted by the gas. If the weight is removed there will be an unbalanced upward force between the system and the surroundings. The piston will move up till the stop because of the unbalance forces created by the gas. In this final position, the system will be again under equilibrium state with properties say, (x₂, y₂). Both the initial and final states can be located in a generalized 2 dimensional diagram. However, as the system was not in equilibrium between these two end states, they can not be presented in the same diagram. Such non equilibrium states are represented with dotted lines.

Fig: 1.6

Now let us replace the single weight with number of infinitesimally smaller weights as shown in Fig: 1.7.

Fig: 1.7

In this case also the gas is in initial equilibrium state 'i' with set of properties (x_i, y_i). Let us take out weight very slowly which is infinitesimally small. The system will come to equilibrium with new set of properties. Now remove the next weight slowly as before and this time also the piston will move up infinitesimally slow to another equilibrium state (x₁, y₁). State points and being in equilibrium can now be located in the coordinate. Experiment is repeated in similar manner till the last wieght is removed when we will attain the final state 'f' with properties at equilibrium condition (x_f, y_f).

From this experiment we find that each and every state points are under equilibrium state and they can be indicated in the coordinate. Now the initial, intermediate and final states, all being in equilibrium states, can be joined with a solid line. Such a process in which each of the state is in equilibrium state is known as quasi-static equilibrium state. A quasi-static process is thus a sucession of equilibrium states. The character of such a process is that it is infinitely slow process. Later on we will recognize the processes as reversible processes after learning the second law of thermodynamics. A reversible state is a quasi-static process without any dissipation. Till we learn about the reversible process, let us be content with the quasi static process.