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thermodynamic system[¦thər·mō·dī′nam·ik ′sis·təm]
an aggregation of physical bodies that may interact with each other and may exchange energy and mass with other bodies. Thermodynamic systems are the objects of study of thermodynamics.
A thermodynamic system consists of such a large number of particles that the system’s state may be characterized by macroscopic parameters—for example, density, pressure, and the concentrations of the various substances forming the system. If a system’s parameters do not vary with time and if there are no steady fluxes in the system (such as fluxes of heat or mass), the system is said to be in equilibrium (seeEQUILIBRIUM, THERMODYNAMIC). The concept of temperature as a state parameter having the same value for all macroscopic parts of the system is introduced for systems in equilibrium.
The properties of thermodynamic systems in thermodynamic equilibrium are studied by equilibrium thermodynamics, or thermostatics; the properties of nonequilibrium systems are studied by nonequilibrium thermodynamics. Thermodynamics deals with closed, open, adiabatic, and isolated systems. Closed systems do not exchange mass with other systems. Open systems may exchange mass and energy with other systems. Adiabatic systems do not exchange heat with other systems. Isolated systems exchange neither energy nor mass with other systems.