A system is any volumetric region of the universe delineated by a boundary.
In thermodynamics, system refers to the volumetric region under study. The system is characterized by a boundary, across which both matter and energy may pass. A system, in more detail, is the portion of the universe under study, being separated from the remainder of the universe by a boundary, which may be material or not, but which, by convention, delimits a finite volume, across which the possible exchanges of work, heat, or matter may pass. [1]

The term system or "thermodynamic system" are synonyms of "working substance", used by French physicist Sadi Carnot in 1923, "working body", used by German physicist Rudolf Clausius in 1850, and "working medium", used by Russian bioelectrochemist Octavian Ksenzhek in 2007.


The distinctions between “isolated”, “closed”, and “open” seem to have been first laid out explicitly in the Brussels school in the writings of Théophile de Donder, who in 1936 clearly defined closed systems, and his student Ilya Prigogine who in 1961 stated: [3]

“We shall distinguish between isolated systems which can exchange neither energy nor matter, closed systems which can exchange energy but no matter, and open systems which can exchange both energy and matter with the exterior.”

Human system
See main: Human system, Social system, Economic system, etc.
To properly understand the concept of system, for use in the extrapolation of thermodynamic analysis in human systems, e.g. a town, community, or social group, etc., being comprised of reactionary human molecules (people), one must aver to the more detailed original terminology used by Clausius. [2]

1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. Thims, Libb. (2007). Human Chemistry (Volume One), (preview). Morrisville, NC: LuLu.
3. (a) Prigogine, Ilya. (1961). Thermodynamics of Irreversible Processes, (pgs. v, 3). New York: Interscience Publishers.
(b) De Donder, T. (1936). Thermodynamic Theory of Affinity: A Book of Principles, (pgs. 1-2). Oxford: Oxford University Press.

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