Equilibration

Equilibration is the process by which a system approaches a state of equilibrium, in which macroscopic properties are time-invariant and no net flows of matter or energy occur. In thermodynamics, equilibrium is defined by uniform intensive variables (such as temperature, pressure, and chemical potential) throughout the system and by the maximization of entropy subject to conservation laws.

Different domains describe equilibration in specific terms: Thermal equilibration refers to heat transfer between bodies at different temperatures until a common temperature is reached. Chemical equilibration occurs when chemical reactions reach a balance where the forward and reverse reaction rates are equal, giving constant concentrations at a given temperature. Mechanical equilibration arises when internal and external forces balance, eliminating net accelerations or deformations. Concentration or diffusive equilibration describes the spread of substances to uniform concentration via diffusion. Electrical equilibration involves redistribution of charge to minimize electrical potential differences.

In statistical mechanics and physics, equilibrium often corresponds to a stationary distribution, such as the Boltzmann distribution for thermal systems. The approach to equilibrium is characterized by relaxation times, and the dynamics may be described by rate equations or diffusion equations. In open systems, true equilibrium may be unattainable, giving way to non-equilibrium steady states.

Equilibration is distinct from relaxation, which refers to the transient process by which a system approaches equilibrium after a perturbation; equilibration is the end state. Practical examples include mixing in chemical reactors, thermal contact in calorimetry, and charge redistribution in electrical circuits.