Thermodynamic system

A thermodynamic system is a body of matter and/or radiation, confined in space by walls, with defined permeabilities, which separate it from its surroundings. The surroundings may include other thermodynamic systems, or physical systems that are not thermodynamic systems. A wall of a thermodynamic system may be purely notional, when it is described as being 'permeable' to all matter, all radiation, and all forces. A state of a thermodynamic system can be fully described in several different ways, by several different sets of thermodynamic state variables.

A widely used distinction is between isolated, closed, and open thermodynamic systems.

An isolated thermodynamic system has walls that are non-conductive of heat and perfectly reflective of all radiation, that are rigid and immovable, and that are impermeable to all forms of matter and all forces. (Some writers use the word 'closed' when here the word 'isolated' is being used.)

A closed thermodynamic system is confined by walls that are impermeable to matter, but, by thermodynamic operations, alternately can be made permeable (described as 'diathermal') or impermeable ('adiabatic') to heat, and that, for thermodynamic processes (initiated and terminated by thermodynamic operations), alternately can be allowed or not allowed to move, with system volume change or agitation with internal friction in system contents, as in Joule's original demonstration of the mechanical equivalent of heat, and alternately can be made rough or smooth, so as to allow or not allow heating of the system by friction on its surface.

An open thermodynamic system has at least one wall that separates it from another thermodynamic system, which for this purpose is counted as part of the surroundings of the open system, the wall being permeable to at least one chemical substance, as well as to radiation; such a wall, when the open system is in thermodynamic equilibrium, does not sustain a temperature difference across itself.

A thermodynamic system is subject to external interventions called thermodynamic operations; these alter the system's walls or its surroundings; as a result, the system undergoes transient thermodynamic processes according to the principles of thermodynamics. Such operations and processes effect changes in the thermodynamic state of the system.

When the intensive state variables of its content vary in space, a thermodynamic system can be considered as many systems contiguous with each other, each being a different thermodynamical system.

A thermodynamic system may comprise several phases, such as ice, liquid water, and water vapour, in mutual thermodynamic equilibrium, mutually unseparated by any wall; or it may be homogeneous. Such systems may be regarded as 'simple'.

A 'compound' thermodynamic system may comprise several simple thermodynamic sub-systems, mutually separated by one or several walls of definite respective permeabilities. It is often convenient to consider such a compound system initially isolated in a state of thermodynamic equilibrium, then affected by a thermodynamic operation of increase of some inter-sub-system wall permeability, to initiate a transient thermodynamic process, so as to generate a final new state of thermodynamic equilibrium. This idea was used, and perhaps introduced, by Carathéodory. In a compound system, initially isolated in a state of thermodynamic equilibrium, a reduction of a wall permeability does not effect a thermodynamic process, nor a change of thermodynamic state. This difference expresses the Second Law of thermodynamics. It illustrates that increase in entropy measures increase in dispersal of energy, due to increase of accessibility of microstates.

In equilibrium thermodynamics, the state of a thermodynamic system is a state of thermodynamic equilibrium, as opposed to a non-equilibrium state.

According to the permeabilities of the walls of a system, transfers of energy and matter occur between it and its surroundings, which are assumed to be unchanging over time, until a state of thermodynamic equilibrium is attained. The only states considered in equilibrium thermodynamics are equilibrium states. Classical thermodynamics includes (a) equilibrium thermodynamics; (b) systems considered in terms of cyclic sequences of processes rather than of states of the system; such were historically important in the conceptual development of the subject. Systems considered in terms of continuously persisting processes described by steady flows are important in engineering.

The very existence of thermodynamic equilibrium, defining states of thermodynamic systems, is the essential, characteristic, and most fundamental postulate of thermodynamics, though it is only rarely cited as a numbered law. According to Bailyn, the commonly rehearsed statement of the zeroth law of thermodynamics is a consequence of this fundamental postulate. In reality, practically nothing in nature is in strict thermodynamic equilibrium, but the postulate of thermodynamic equilibrium often provides very useful idealizations or approximations, both theoretically and experimentally; experiments can provide scenarios of practical thermodynamic equilibrium.

In equilibrium thermodynamics the state variables do not include fluxes because in a state of thermodynamic equilibrium all fluxes have zero values by definition. Equilibrium thermodynamic processes may involve fluxes but these must have ceased by the time a thermodynamic process or operation is complete bringing a system to its eventual thermodynamic state. Non-equilibrium thermodynamics allows its state variables to include non-zero fluxes, that describe transfers of mass or energy or entropy between a system and its surroundings.

In 1824 Sadi Carnot described a thermodynamic system as the working substance (such as the volume of steam) of any heat engine under study.