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irreversible process[‚i·ri′vər·sə·bəl ′prä·səs]
a physical process that can occur spontaneously only in one specific direction. Examples of such processes are diffusion, heat conduction, thermal diffusion, viscous flow, and gas expansion into a vacuum. All irreversible processes are nonequilibrium processes. An irreversible process in a closed system is accompanied by an increase in entropy. In open systems, which are capable of exchanging energy or matter with the surrounding medium, during irreversible processes the entropy may remain constant or even decrease owing to entropy exchange with the surrounding medium. In all cases, however, the production of entropy, that is, its increase in the system per unit time, remains positive because of the existence of irreversible processes.
Classical thermodynamics, which studies reversible, equilibrium processes, defines inequalities for irreversible processes; these inequalities determine the possible directions of the irreversible processes.
Irreversible processes are studied through the thermodynamics of nonequilibrium processes and the statistical theory of nonequilibrium processes. The thermodynamics of irreversible processes allows us (1) to obtain the increase of entropy in irreversible processes that occur in various systems as a function of parameters of the nonequilibrium state and (2) to derive equations that describe the time variations of these parameters, for example, diffusion equations, heat-conduction equations, and the Navier-Stokes equation for the hydrodynamics of a viscous fluid. The coefficients in these equations (kinetic coefficients) are regarded as phenomenological constants to be determined experimentally. The statistical theory of irreversible processes makes it possible to obtain expressions for the kinetic coefficients in terms of the molecular constants. Irreversible processes in gases have been studied most completely using the Boltzmann kinetic equation.
D. N. ZUBAREV