Hot body cold body (diagram)
Modified diagram of the original 1824 Carnot heat engine showing the hot body, working body (system), and cold body, the letters labeled according to the stopping points in the various steps of the seven-step Carnot cycle.
In thermodynamics, a thermodynamic system or "system" is any material body of the universe made to expand or contract, according to Boerhaave's law, by the action of heat, and thus made to do mechanical work, according to the cycle of operation steps of the Papin engine (1690).

The original thermodynamic system, shown adjacent, drawn by French physicist Sadi Carnot in 1823. It is important to note, that the "system", in the original steam engine sense of the word, is one of three bodies that comprise the basic heat engine. The working body (system) is typically steam, although it can be any physical body of the universe; the other two being the hot body (typically a boiler, heated by fire) and the cold body (typically a spray of cold water, from a nearby stream). This is the basic model in thermodynamics, whose operation is defined by the seven step Carnot cycle, added to which other factors, such as electrical work, chemical work, elongation work can be quantified by their modification to the change in the internal energy of the the working body or "system".

The modern term "thermodynamic system" was introduced in 1923, a clarification-type derivative of the older terms working substance (Carnot, 1824), working body (Clausius, 1850), or system (Maxwell, 1871), and of the newer terms, such as working medium (Ksenzhek, 2007).

The double word term "thermodynamic system" verse "system" seems to have arisen out of the unwritten practice of placing the classifier “thermodynamics” in front of whatever term was being used, such as thermodynamic function, thermodynamic arrow, thermodynamic evolution, thermodynamic temperature, etc., in unintentional efforts to help distinguish the term in some sense or degree.

In a modern use, when diagrams are made, such as shown below, in which the center region is titled thermodynamic system, the connection to the operation and depiction of the hot body and cold body, often called "reservoirs", often becomes lost in translation. [2]

The term thermodynamic system seems to have come into use in the popular the 1923 textbook Thermodynamics by American physical chemists Gilbert Lewis and Merle Randall. [1] They state that what is customarily called a “system”, which can be enclosed either by physical walls or imaginary mathematical surfaces, is whatever part of the objective world that is the subject of thermodynamic discourse, e.g. a crystal or a cubic centimeter portion of crystal. On this definition that surmise:

“A thermodynamic system may contain no substance at all, in the ordinary sense, and consist of radiant energy, or an electric or magnetic field … [but] usually comprises a substance, which may be homogeneous or heterogeneous.”
Thermodynamic system (two views)
Left: Modern generic version of the thermodynamics system, showing that both radiation, heat, and work can pass the boundary, often defined as an "closed system", and that also mass can pass across the boundary, often defined as an "open system". Mass can pass the boundary as either inert transport mass, in which a certain amount of energy is assigned to the work of moving the mass; or in the case of chemically reactive mass, the effect is quantified by the chemical potential terms, taking into affect the nature of the change of the internal energy in a chemical reaction sense. Right: a close-up of the original Carnot engine (1824).

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Difficulties on terminology
As mentioned above, the new student of thermodynamics, when introduced to the the term "system" or "thermodynamic system" (such as diagrammed above) may easily fall into the assumption that the “thermodynamic system” is a two-component system, comprising both the system and the surroundings, or a three-component system, comprising the boiler (hot body) A, condenser (cold body) B, and substance (working body), below (left).

This thermodynamic "system" model, in turn, stems from the 1850 terminology of German physicist Rudolf Clausius, who referred to the "working body" as the system, such as shown generically below (middle). In this sense, where the term "thermodynamic system", as given to the newby thermodynamics student, has little visual connection to the 1824 working substance “steam body” description of heat engine operation by French physicist Sadi Carnot, as depicted below (right), in which the "system" is the body of water molecule contained in the piston volume region acdb.

This diagram model by Carnot of his generic heat engine, in turn, was mainly based on the steam engine designed by Scottish engineer James Watt, shown below (left), having a separate condenser (1765), sun and planet gear (1781), centrifugal governor (1788), where the "system", "working substance", or "working body" is the "body" of an amount of water, heated in a boiler (hot body, A) and fed as steam, through conduit d, into the piston C, later to be condensed into water again via contact with the condenser (cold body, B), diagrammed below as section D, is the steam engine:

Watt enginePapin engine (small)Papin digesterVacuum pump

Watt engine (c. 1780)
Papin engine (1690)
Papin digester (1679)Vacuum pump (1850)

Watt's design, in the the course of the timeline of engine development, in turn, was modeled on the Papin engine design of a "heated" piston-and-cylinder, shown above (center left), configured with the ability to do work, via certain steps of operation, as detailed in the 1690 memoir "A New Method to Obtain Very Great Motive Powers at Small Cost" by French physicist Denis Papin.[4] In terse summary, after decades of experimental work in attempts to create a "gunpowder engine", one that lowers the piston (doing work) following a discharge of gunpowder in the cylinder, after watching a steam release valve move up and down on one of his digestors (pressure cooker), he conceived of the idea that to make a steam engine, one could put a body of water (system) in a sealed piston and cylinder and then operate the engine in two steps.
Working body (Guericke)

The two steps of Papin engine operation are: (a) have someone hold a fire under the piston, making the water boil and expand pushing the piston upward and (b) then have some one cool the piston by holding a tub or stream of cool water under the hot piston, thus causing the steam to condense, pulling the piston back down. This was one engine cycle. This is the basic model of operation of all steam engines and was used by English engineer Thomas Necomen in 1698 to make the first steam engine, called the Savery sump pump (or Miner's friend). The Watt steam engine evolved from the Savery engine.

Papin's heat engine design, which contains the first verbal description of what later came to be called the "Carnot cycle", in turn, was based on the very-popularized 1647 piston-and-cylinder vacuum pump weight-lifting ideas initiated in 1647 by German inventor and physicist Otto Guericke, such as the ones shown below:

To explain further, as to the operation of the Guericke engine, in one experiment shown to the left, 20+ men pulled on the piston till it reached halfway up position, then a large glass receiver, which had been mad perfectly vacuous by Guericke’s pump, was then applied to the stop-****; and when the men were exerting their utmost force, a communication was opened between the receiver and the cylinder, and the piston was suddenly forced down to the bottom of the cylinder in spite of the efforts of the men to keep it up.

In the experiment shown to the right, conducted in 1654, Guericke arranged the piston at the top of the cylinder, having a scale loaded with 2,686 lbs attached to it. In this configuration, a little boy, by means of a small syringe applied at the stop-**** x to pump out the air, was able to bring down the piston and raise the weight. [2]

In sum, these early piston and cylinder experiments, in later connective coordination, via the caloric theory, to Dutch physician and chemist Herman Boerhaave’s 1724 heat augmentation axiom, are to which the conception of a thermodynamic "system" stem. In this sense, the modern joint term "thermodynamic system" may not be an advisable one, and may lead to confusion, particularly in regards to the “theory of transformation-equivalents”, introduced in the period 1850-65 by German physicist Rudolf Clausius, otherwise known as entropy. [3]

Earth-bound thermodynamic systems
See main: Earth-bound thermodynamic systems
In subjects such as life thermodynamics, evolutionary thermodynamics, biological thermodynamics, ecological thermodynamics, economic thermodynamics, sociological thermodynamics, or human thermodynamics in general, the diagramming of the boundary of the thermodynamic system becomes a paramount issue. The basic model delineates a cylinder type imaginary volumetric region on the surface (substrate) of the earth, viewed such, while remaining in contact with the surface of the earth, it rotates in a 24-hour, two-part heat cycle, being put in contact with a hot body (the sun) for approximately 12-hours, the expansion phase, and then put in contact with a cold body (the night sky) for approximately 12-hours, the contraction phase, whereby after the body, if it is considered to be reversible, returns to its original condition.

1. Lewis, Gilbert N. and Randall, Merle. (1923). Thermodynamics and the Free Energy of Chemical Substances, (pgs. 8-9). New York: McGraw-Hill Book Co., Inc.
2. Zdunkowski, Wilford and Bott, Andreas. (2004). Thermodynamics of the Atmosphere - A Course in Theoretical Meteorology. (pg. 2). Cambridge: Cambridge University Press.
3. (a) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (I)”, (pgs. 257-59). Nature, Jan. 31.
(b) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (II)”, (pgs. 278-81). Nature, Feb. 07.
(c) Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies. (Google Books). London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
4. (a) Papin, Denis. (1690). “A New Way to Obtain Very Great Motive Powers at Small Cost” (Nova Methodus ad Vires Motrices Validissimas levi Pretio Comparandas). Acta Eruditorum, anno, Aug., pgs. 410-14.
(b) Muirhead, James. (1859). The Life of James Watt, (English translation: Ch. XI, Denys Pain: His memoir of 1690, Section: A New Way to Obtain Very Great Motive Powers at Small Cost”, pgs. 131-42). London: John Murray.

External links
‚óŹ Thermodynamic system – Wikipedia.

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