System (thermodynamics)

System (thermodynamics)

A thermodynamic system is a precisely specified macroscopic region of the universe, defined by boundaries or walls of particular natures, together with the physical surroundings of that region, which determine processes that are allowed to affect the interior of the region, studied using the principles of thermodynamics.

All space in the universe outside the thermodynamic system is known as the surroundings, the environment, or a reservoir. A system is separated from its surroundings by a boundary, which may be notional or real but, by convention, delimits a finite volume. Exchanges of work, heat, or matter and energy between the system and the surroundings may take place across this boundary. A thermodynamic system is classified by the nature of the exchanges that are allowed to occur across its boundary.

A thermodynamic system has a characteristic set of thermodynamic parameters, or state variables. They are experimentally measurable macroscopic properties, such as volume, pressure, temperature, electric field, and others. The set of values of the thermodynamic parameters necessary to uniquely define a state of a system is called the thermodynamic state of a system. The state variables of a system are normally related by one or more functional relationships, the equations of state.

An isolated system is in thermodynamic equilibrium when its state does not change with time. Systems that are not isolated can be in thermodynamic equilibrium, or in a state constant or precisely cyclically changing in time - a steady state - that is far from equilibrium, or they can be in a changing state that is not in thermodynamic equilibrium. Classical thermodynamics considers only states of thermodynamic equilibrium or states constant or precisely cycling in time.

Originally, in 1824, Sadi Carnot described a thermodynamic system as the working substance of a heat engine under study.