Reversible and irreversible processes

A process from state i to state f is reversible if it can be run backwards so that both the system and its surroundings return to their original states, with no change anywhere else in the universe.

The spontaneous processes of nature are irreversible. A pot taken off the stove has a hotter base; heat spreads until it is uniform, and the pot never cools on one side to reheat its base. That would violate the second law.

More irreversible processes: free expansion of a gas; the burning of a petrol–air mixture ignited by a spark; cooking gas leaking from a cylinder and spreading through a kitchen, which never gathers itself back into the cylinder.

Stirring a liquid in contact with a reservoir turns work into heat in the reservoir. Undoing it exactly would mean turning that heat wholly into work, against the second law.

Two main causes of irreversibility: the process passes through non-equilibrium states (free expansion, explosive reactions), or it involves friction, viscosity and other dissipative effects, such as a sliding body coming to rest or a spinning blade in a liquid being stopped by viscosity.

Dissipative effects can be reduced but never removed completely, so irreversibility is the rule in nature rather than the exception.

Reversibility needs two things together: the process must be quasi-static and free of dissipation. Example: slow isothermal expansion of an ideal gas in a cylinder with a frictionless piston.

Reversibility matters because an engine built only from reversible processes reaches the highest efficiency possible between two temperatures; any engine with irreversibility, as every real engine has, does worse.