Irreversible process

An irreversible process cannot be reversed without leaving net changes in the system or its surroundings. It generates entropy and occurs spontaneously in only one direction.

Entropy production: The rate at which entropy is generated within a system during an irreversible process. Positive entropy production distinguishes irreversible change from an ideal reversible path.
Thermodynamic system: A specified quantity of matter or region of space chosen for thermodynamic analysis. Irreversibility is assessed by tracking the system together with its surroundings.
Heat engine: A device that converts heat drawn from a hot reservoir partly into work while rejecting heat elsewhere. Irreversible effects reduce the work an engine can extract from heat.
Thermodynamic arrow of time: The direction of time associated with increasing entropy in macroscopic processes. Irreversible processes give physical meaning to a preferred direction of time.
Second law of thermodynamics: A thermodynamic law stating that total entropy does not decrease in an isolated system. It constrains every irreversible process to increase total entropy.
Thermodynamic state: A condition of a system specified by macroscopic properties such as pressure, volume, and temperature. Comparing initial and final states reveals changes left by a proposed reversal.
Carnot theorem: A theorem stating that no engine between two heat reservoirs outperforms a reversible engine operating between them. It quantifies the performance limit imposed by irreversible operation.
Exergy: The maximum useful work obtainable as a system comes to equilibrium with its environment. Irreversibility destroys exergy, reducing the work available from energy.
Thermodynamic equilibrium: A state in which macroscopic flows and driving forces have vanished. At equilibrium, no spontaneous irreversible change proceeds macroscopically.
Quasistatic process: An idealized process slow enough that a system remains arbitrarily close to equilibrium throughout. Quasistatic behavior alone does not guarantee reversibility, despite its frequent association with it.

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