Reversible process

A reversible process is an idealized change that can be undone so the system and its surroundings return to their initial states with no net change. It proceeds through equilibrium states and produces no entropy.

Thermodynamic equilibrium: A state in which macroscopic properties remain stable because no unbalanced driving forces persist. A reversible process advances through equilibrium states, requiring changes slow enough to preserve equilibrium.
Second law of thermodynamics: A fundamental law stating that the total entropy of an isolated system cannot decrease. The law constrains possible processes; reversibility is the limiting case with no total entropy increase.
Carnot theorem: The theorem that no engine operating between two thermal reservoirs can outperform a reversible engine. It uses reversibility to establish the greatest possible efficiency for a specified pair of reservoir temperatures.
Irreversible process: A process that cannot be reversed without leaving net changes in the system or its surroundings. It is the direct thermodynamic opposite of a reversible process and includes nearly all natural processes.
Entropy production: The generation of entropy within a system or across its boundary during an irreversible process. A reversible process has zero entropy production, distinguishing it from real dissipative change.
Entropy: A thermodynamic state function that measures energy dispersal and helps determine the direction of spontaneous change. The entropy change of the system and surroundings together is zero for a reversible process.
Carnot efficiency: The maximum efficiency of a heat engine operating between hot and cold reservoirs, given by one minus their absolute-temperature ratio. This efficiency is attained only by an ideal reversible engine.
Quasistatic process: A process that proceeds through states arbitrarily close to thermodynamic equilibrium. A quasistatic process may still be irreversible if friction, viscosity, or heat flow across finite temperature differences occurs.
Thermodynamic irreversibility: The inability to restore a process and its surroundings exactly to their initial states. Friction, finite temperature differences, and mixing make ordinary processes irreversible.
Heat: Energy transferred between bodies because of a temperature difference. Reversible heat transfer requires an infinitesimal temperature difference between the bodies exchanging energy.

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