Editing Entropy (section)

{{Short description|Property of a thermodynamic system}}
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{{Other uses}}
{{For introduction}}
{{Distinguish|Enthalpy}}
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{{Infobox physical quantity
| name =Entropy
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| unit =joules per kelvin (J⋅K<sup>−1</sup>)
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| symbols = ''S''
| baseunits = kg⋅m<sup>2</sup>⋅s<sup>−2</sup>⋅K<sup>−1</sup>
| dimension =
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{{Thermodynamics sidebar|expanded=sysprop}}
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{{Modern physics}}
{{Complex systems}}
'''Entropy''' is a [[Science|scientific]] concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from [[classical thermodynamics]], where it was first recognized, to the microscopic description of nature in [[statistical physics]], and to the principles of [[information theory]]. It has found far-ranging applications in [[chemistry]] and [[physics]], in biological systems and their relation to life, in [[cosmology]], [[economics]], [[sociology]], [[Atmospheric science|weather science]], [[climate change]] and [[information system]]s including the transmission of information in [[Telecommunications|telecommunication]].<ref>{{Cite journal|last=Wehrl|first=Alfred|date=1 April 1978|title=General properties of entropy|url=https://link.aps.org/doi/10.1103/RevModPhys.50.221|journal=Reviews of Modern Physics|volume=50|issue=2|pages=221–260|doi=10.1103/RevModPhys.50.221|bibcode=1978RvMP...50..221W}}</ref>

Entropy is central to the [[second law of thermodynamics]], which states that the entropy of an isolated system left to spontaneous evolution cannot decrease with time. As a result, isolated systems evolve toward [[thermodynamic equilibrium]], where the entropy is highest. A consequence of the second law of thermodynamics is that certain processes are [[Irreversible process|irreversible]].

The thermodynamic concept was referred to by Scottish scientist and engineer [[William Rankine]] in 1850 with the names ''thermodynamic function'' and ''heat-potential''.<ref>{{cite book |last=Truesdell |first=C. |author-link=Clifford Truesdell |date=1980 |title=The Tragicomical History of Thermodynamics, 1822–1854 |url=https://archive.org/details/tragicomicalhist0000unse |url-access=registration |location=New York |publisher=Springer-Verlag |isbn=0387904034 |page=215 |via=[[Internet Archive]]}}</ref> In 1865, German physicist [[Rudolf Clausius]], one of the leading founders of the field of thermodynamics, defined it as the quotient of an infinitesimal amount of [[heat]] to the instantaneous [[temperature]]. He initially described it as ''transformation-content'', in German ''Verwandlungsinhalt'', and later coined the term ''entropy'' from a Greek word for ''transformation''.<ref name=brush1976>[[Stephen G. Brush|Brush, S.G.]] (1976). ''The Kind of Motion We Call Heat: a History of the Kinetic Theory of Gases in the 19th Century, Book 2, Statistical Physics and Irreversible Processes'', Elsevier, Amsterdam, {{ISBN|0-444-87009-1}}, pp. 576–577.</ref>

Austrian physicist [[Ludwig Boltzmann]] explained entropy as the measure of the number of possible microscopic arrangements or states of individual atoms and molecules of a system that comply with the macroscopic condition of the system. He thereby introduced the concept of statistical disorder and [[probability distribution]]s into a new field of thermodynamics, called [[statistical mechanics]], and found the link between the microscopic interactions, which fluctuate about an average configuration, to the macroscopically observable behaviour, in form of a simple [[logarithm]]ic law, with a [[proportionality (mathematics)|proportionality constant]], the [[Boltzmann constant]], which has become one of the defining universal constants for the modern [[International System of Units]].<ref>{{cite journal |last=Jagannathan |first=Kannan |year=2019 |title=Anxiety and the Equation: Understanding Boltzmann's Entropy |journal=American Journal of Physics |volume=87 |issue=9 |pages=765 |doi=10.1119/1.5116583|bibcode=2019AmJPh..87..765J }}</ref>