quantities on both sides are extensive * . (The product of an intensive variable like P and an extensive quantity ∆ V is extensive.) . THERMODYNAMIC PROCESSES . .
Quasi-static process Consider a gas in thermal and mechanical equilibrium with its surroundings. The pressure of the gas in that case equals the external pressure and its temperature is the same as that of its surroundings. Suppose that the external pressure is suddenly reduced (say by lifting the weight on the movable piston in the container). The piston will accelerate outward.
During the process, the gas passes through states that are not equilibrium states. The non- equilibrium states do not have well-defined pressure and temperature. In the same way, if a finite temperature difference exists between the gas and its surroundings, there will be a rapid exchange of heat during which the gas will pass through non-equilibrium states. In due course, the gas will settle to an equilibrium state with well-defined temperature and pressure equal to those of the surroundings.
The free expansion of a gas in vacuum and a mixture of gases undergoing an explosive chemical reaction, mentioned in section . are also examples where the system goes through non- equilibrium states. Non-equilibrium states of a system are difficult to deal with. It is, therefore, convenient to imagine an idealised process in which at every stage the system is an equilibrium state.
Such a * As emphasised earlier, Q is not a state variable. However, ∆ Q is clearly proportional to the total mass of system and hence is extensive. process is, in principle, infinitely slow, hence the name quasi-static (meaning nearly static). The system changes its variables ( P , T , V ) so slowly that it remains in thermal and mechanical equilibrium with its surroundings throughout.
In a quasi-static process, at every stage, the difference in the pressure of the system and the external pressure is infinitesimally small. The same is true of the temperature difference between the system and its surroundings (Fig. . ).
To take a gas from the state ( P , T ) to another