Working body or "thermodynamic system" diagramBoundary surface
Left: 2D view of a working body. Right: 3D view of a working body.
In thermodynamics, working body, aka "working substance" (Carnot, 1824), "system" or "thermodynamic system", is the a collection of partitioned-off atoms, molecules, matter, or particles confined to a region or boundary that does work (or consumes work) due to the contact action at the boundary surface of a either a hot body (or cold body).

The first to develop the concept of a "thermodynamic system" was the French physicist Sadi Carnot who in his 1824 Reflections on the Motive Power of Fire studied what he called the working substance (system), i.e. typically a body of water vapor, in steam engines, in regards to the system's ability to do work when heat is applied to it. [1] The working substance could be put in contact with either a heat reservoir (a boiler), a cold reservoir (a stream of cold water), or a piston (to which the working body could do work by pushing on it). In 1850, the German physicist Rudolf Clausius generalized this picture to include the concept of the surroundings and began to use the term "working body" when referring to the system. From his 1850 manuscript On the Motive Power of Fire Clausius states: [2]

"With every change of volume (to the working body) a certain amount work must be done by the gas or upon it, since by its expansion it overcomes an external pressure, and since its compression can be brought about only by an exertion of external pressure. To this excess of work done by the gas or upon it there must correspond, by our principle, a proportional excess of heat consumed or produced, and the gas cannot give up to the surrounding medium the same amount of heat as it receives."

In the following diagram, is shown the Carnot engine as is typically modeled in current use, where heat flows from a high temperature TH furnace through the fluid of the "working body" (working substance) and into the cold sink TC, thus forcing the working substance to do mechanical work W on the surroundings, via cycles of contractions and expansions.

In the diagram shown, the “working body” (system), a term introduced by Clausius in 1850, can be any fluid or vapor body through which heat Q can be introduced or transmitted through to produce work. In 1824, Sadi Carnot, in his famous paper Reflections on the Motive Power of Fire, had postulated that the fluid body could be any substance capable of expansion, such as vapor of water, vapor of alcohol, vapor of mercury, a permanent gas, or air, etc. Although, in these early years, engines came in a number of configurations, typically QH was supplied by a boiler, wherein water was boiled over a furnace; QC was typically a stream of cold flowing water in the form of a condenser located on a separate part of the engine. The output work W here is the movement of the piston as it is used to turn a crank-arm, which was then typically used to turn a pulley so to lift water out of flooded salt mines. Carnot defined work as “weight lifted through a height”.
human system  (diagram)
Carnot heat engine
A boundaried social system of seven people (human molecules) defined as a "working body".

Human working bodies
In human thermodynamics, the working body is defined such that instead of water molecules, confined to the internal regions of a steam engine, put in alternating contact with a hot body (a fire) and a cold body (cool water), driven to do mechanical work, we have human molecules, confined to the internal regions of a social system, put in alternating contact with a hot body (the day sun) and a cold body (the cool night sky), driven to do the daily work of life, e.g. economic work, social work, volunteer work, household work, parenting work, territorial expansion work, interpersonal work, relationship work, etc. [3]

1. Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power.” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
2. Clausius, Rudolf. (1850). "On the Motive Power of Heat, and on the Laws Which can be Deduced From it for the Theory of Heat." Poggendorff's Annalen der Physik, LXXIX, 368, 500.
3. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007).
Human Chemistry (Volume Two), (preview
). Morrisville, NC: LuLu.

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