Reversible and Irreversible Process

A thermodynamic process is reversible only if it is possible to exactly retrace its path in the opposite direction, so that both the system and its surroundings pass back through precisely the same sequence of intermediate states as in the original process. This requires three conditions: (1) the process proceeds at an extremely (infinitely) slow rate; (2) the system remains in mechanical, thermal and chemical equilibrium with its surroundings throughout; (3) no dissipative forces (friction, viscosity, electrical resistance) are present anywhere. A quasi-static isothermal expansion, or the slow compression/expansion of an ideal spring, are the standard near-reversible examples. Every reversible process is necessarily quasi-static, but the converse is not true -- pushing a piston extremely slowly can still be irreversible if friction between piston and cylinder wall dissipates energy that can never be recovered. All naturally occurring processes are irreversible (Example 8.23: gas escaping a bottle, ink diffusing in water, a falling object coming to rest -- none …