Quasi-Static Process

A quasi-static process is an infinitely slow process in which a system's variables ($P$, $V$, $T$) change so gradually that the system remains, at every single instant, in thermal, mechanical and chemical equilibrium with its surroundings -- so its pressure, temperature and internal energy are always well-defined, and the equation of state can be applied throughout. By contrast, a sudden change (e.g. abruptly dropping a mass onto a piston) drives the gas through non-equilibrium intermediate states, where pressure is not even uniform throughout the gas, and the ideal gas equation cannot describe those intermediate stages at all -- only the initial and final equilibrium states remain describable. Example 8.14 gives the standard illustration: adding sand grains onto a piston one at a time, very slowly, so the piston creeps inward smoothly rather t …