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Annotated color version of the original 1824 Carnot heat engine showing the hot body (boiler), working body (system, steam), and cold body (water), the letters labeled according to the stopping points in Carnot cycle

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Note: Conjugate variables in italics
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Specific heat capacity  <math>c=</math>
<math> T </math><math>\partial S</math>
<math> N </math><math>\partial T</math>
Compressibility  <math>\beta=- </math>
<math> 1 </math><math>\partial V</math>
<math> V </math><math>\partial p</math>
Thermal expansion  <math>\alpha=</math>
<math> 1 </math><math>\partial V</math>
<math> V </math><math>\partial T</math>

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Thermodynamics is a branch of physics concerned with heat and temperature and their relation to energy and work. It defines macroscopic variables, such as internal energy, entropy, and pressure, that partly describe a body of matter or radiation. It states that the behavior of those variables is subject to general constraints, that are common to all materials, not the peculiar properties of particular materials. These general constraints are expressed in the four laws of thermodynamics. Thermodynamics describes the bulk behavior of the body, not the microscopic behaviors of the very large numbers of its microscopic constituents, such as molecules. The basic results of thermodynamics rely on the existence of idealized states of thermodynamic equilibrium. Its laws are explained by statistical mechanics, in terms of the microscopic constituents.

Thermodynamics applies to a wide variety of topics in science and engineering, especially physical chemistry, chemical engineering and mechanical engineering.

Historically, the distinction between heat and temperature was studied in the 1750s by Joseph Black. Characteristically thermodynamic thinking began in the work of Carnot (1824) who believed that the efficiency of heat engines was the key that could help France win the Napoleonic Wars.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> The Irish-born British physicist Lord Kelvin was the first to formulate a concise definition of thermodynamics in 1854:<ref name=kelvin1854>{{#invoke:Citation/CS1|citation |CitationClass=journal }} reprinted in {{#invoke:citation/CS1|citation |CitationClass=book }} Hence Thermo-dynamics falls naturally into two divisions, of which the subjects are respectively, the relation of heat to the forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.</ref>

"Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency."

Initially, thermodynamics, as applied to heat engines, was concerned with the thermal properties of their 'working materials', such as steam, in an effort to increase the efficiency and power output of engines. Thermodynamics was later expanded to the study of energy transfers in chemical processes, such as the investigation, published in 1840, of the heats of chemical reactions<ref>Hess, H. (1840). Thermochemische Untersuchungen, Annalen der Physik und Chemie (Poggendorff, Leipzig) 126(6): 385–404.</ref> by Germain Hess, which was not originally explicitly concerned with the relation between energy exchanges by heat and work. From this evolved the study of Chemical thermodynamics and the role of entropy in chemical reactions.<ref name="Gibbs 1876">Gibbs, Willard, J. (1876). Transactions of the Connecticut Academy, III, pp. 108–248, Oct. 1875 – May 1876, and pp. 343–524, May 1877 – July 1878.</ref><ref name="Duhem 1886">Duhem, P.M.M. (1886). Le Potential Thermodynamique et ses Applications, Hermann, Paris.</ref><ref name="Lewis Randall 1923">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref name="Guggenheim 1933">Guggenheim, E.A. (1933). Modern Thermodynamics by the Methods of J.W. Gibbs, Methuen, London.</ref><ref name="Guggenheim 1949/1967">Guggenheim, E.A. (1949/1967)</ref><ref name="Prigogine and Defay 1954">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref name=Fermi>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref name="Perrot" >{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>


Thermodynamics sections
Intro  Introduction  History  Branches of description  Thermodynamic equilibrium  Non-equilibrium thermodynamics  Laws of thermodynamics  System models  States and processes   Instrumentation    Conjugate variables    Potentials   Axiomatics  Scope of thermodynamics   Applied fields    See also   References  Cited bibliography  Further reading   External links   

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