The term system has a variety of meanings in different contexts, e.g., redox system, electron transfer system, loosely or completely coupled system, biological or mechanical system, instrumental system, data management system, MKSA system. In thermodynamics, the system is considered as an experimental system (experimental chamber), separated from the environment as an isolated, adiabatic, closed, or open system. Quote Gnaiger 1993 Pure Appl Chem: The internal domain of any system is separated from the external domain (the surroundings) by a boundary. In theory, energy transformations outside the system can be ignored when describing the system. The surroundings are merely considered as a source or sink for quantities transferred across the system boundary. According to the transfer properties of the boundary, three types of thermodynamic systems are distinguished. (1) The boundaries of isolated systems are impermeable for all forms of energy and matter. Isolated systems do not interact with the surroundings. Strictly, therefore, internal changes of isolated systems cannot be observed from outside since any observation requires interaction. (2) The boundaries of closed systems are permeable for heat and work, but impermeable for matter. A limiting case is electrons which cross the system boundary when work is exchanged in the form of electric energy [added: and light]. The volume of a closed system may be variable. (3) The boundaries of open systems allow for the transfer of heat, work and matter. Changes of isolated systems have exclusively internal origins, whereas changes of closed and open systems can be partitioned according to internal and external sources. Production and destruction of a quantity within the system are internal changes, whereas changes of heat, work and matter due to transfer across the system boundaries are labelled extenal. (External) transfer is thus contrasted with (internal) production or destruction.
Communicated by Gnaiger E: 2019-01-01; last update 2019-01-05
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