Thermodynamic equilibrium

Consider a gas enclosed in a cylinder fitted with a movable piston, shown in Fig. 4.1. The gas has temperature $T_1$, pressure $P_1$ and volume $V_1$. These state functions continue to be constant as long as the piston is motionless and no heat exchange takes place. This is an equilibrium state.

Now move the piston in the upward direction so that the gas expands. It passes through states for which pressure, volume and temperature are not specified, and vary continuously during the movement of the piston. The gas would then be in a nonequilibrium state.

Stop the movement of the piston. Suppose at this stage the pressure and volume of the gas are respectively $P_2$ and $V_2$, and the temperature is constant at $T_1$. The state functions are constant since the piston is motionless. The gas is then in another equilibrium state. …