Explain the quasi-static process.

(N/A) Suppose the external pressure is suddenly reduced (by lifting the weight on the movable piston in the container). The piston will accelerate outward. During this process,the gas passes through states that are not equilibrium states. These non-equilibrium states do not have well-defined pressure and temperature. 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,and after some time,the gas will settle into an equilibrium state.
An ideal process in which at every stage the system is in an equilibrium state is known as a quasi-static process. Such a process is infinitely slow.
$A$ quasi-static process is obviously a hypothetical construct. In practice,processes that are sufficiently slow and do not involve accelerated motion of the piston or large temperature gradients are approximations 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 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.
To take a gas from the state $(P, T)$ to another state $(P', T')$ via a quasi-static process,we change the external pressure by a very small amount,allow the system to equalize its pressure with that of the surroundings,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. By choosing reservoirs of progressively different temperatures from $T$ 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.