Nonequilibrium Process

in thermodynamics and statistical physics, a physical process in which a nonequilibrium state exists. Examples of nonequilibrium processes are (1) a process in which (thermodynamic or statistical) equilibrium is established in a system that was previously in a nonequilibrium state and (2) the transition of a system from an equilibrium state into a nonequilibrium state or from one nonequilibrium state into another under the influence of external perturbations. In open systems, nonequilibrium processes may occur as steady-state processes (processes in which the physical state of the system remains constant), for example, heat transfer by heat conduction at a constant temperature difference. Nonequilibrium processes are always irreversible processes, and such processes lead to an increase in entropy.