Explain Quasi-static process.

Explanation

Suppose, the external pressure is suddenly reduced (by lifting the weight on the movable piston in the container).
So, 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.
If the 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 and after some time the gas will settle to an equilibrium state.
An idealist process in which at every stage the system is an equilibrium state such process is infinitely slow. This process is known as Quasi-static process.
A quasi-static process is obviously a hypothetical construct. In practice, processes that are sufficiently slow and do not involve accelerated motion of the piston, large temperature gradient are approximation to an ideal quasi-static process.
The system changes its variables (P, T, V) so slowly that it remains in thermal and mechanical equilibrium with its surrounding throughout.
In a quasi-static process, at every stage the difference in the pressure of the system and the external pressure is infinitesimally small.
To take a gas from the state (P, T) to another state (P., T.) via a quasi-static process we change the external pressure change by a very small amount allow the system to equalise its pressure with that of the surrounding and continue the process infinitely slowly until the system achieves the pressure P..
Similarly to change the temperature we introduce an infinitesimal temperature difference between the system and the surrounding reservoirs and by choosing reservoirs of progressively different temperatures T to T., the system achieves the temperature T".

In a quasi-static process, the temperature of the surrounding reservoir and the external pressure differ only infinitesimally from the temperature and pressure of the system.