Irreversible process

An irreversible process is a thermodynamic process that cannot be reversed so as to restore both the system and the surroundings to their initial conditions. Equivalently, an irreversible process involves dissipation and produces positive entropy in the combined system-plus-environment, in contrast to a reversible process .

Physical interpretation

Irreversibility is the generic case in nature. It arises whenever the process is driven by finite gradients (temperature differences, pressure differences, chemical potential differences) or includes dissipative mechanisms such as friction, viscosity, diffusion, mixing, inelastic deformation, or chemical reaction. These mechanisms degrade organized energy transfers into more “disordered” microscopic energy distributions, making exact retracing impossible without additional net changes in the environment.

A process can be quasistatic and still irreversible—for example, a very slow compression with friction: the system remains near equilibrium (so $P$ and $T$ are well-defined), yet mechanical energy is dissipated as heat.

Key entropy inequalities

Irreversibility is quantified by the second law through the Clausius inequality . For any process connecting equilibrium states,

where $T_{\mathrm{b}}$ is the temperature at the boundary where heat $\delta Q$ crosses the boundary . Equality holds only for a reversible process.

A common entropy balance form is

with entropy generation $S_{\mathrm{gen}} \ge 0$, and $S_{\mathrm{gen}} > 0$ for irreversible processes.

For an isolated system ($\delta Q = 0$ and no matter transfer), this reduces to

capturing the tendency toward equilibrium .