Editing Energy (section)

== History ==
{{Main|History of energy|timeline of thermodynamics, statistical mechanics, and random processes|}}
[[File:Thomas Young (scientist).jpg|thumb|upright|[[Thomas Young (scientist)|Thomas Young]], the first person to use the term "energy" in the modern sense]]
The word ''energy'' derives from the {{langx|grc|ἐνέργεια|[[energeia]]|activity, operation}},<ref>{{cite web |url=http://www.etymonline.com/index.php?term=energy |title=Energy |work=Online Etymology Dictionary |last=Harper |first=Douglas |access-date=May 1, 2007 |url-status=live |archive-url=https://web.archive.org/web/20071011122441/http://etymonline.com/index.php?term=energy |archive-date=October 11, 2007 }}</ref> which possibly appears for the first time in the work of [[Aristotle]] in the 4th century BC. In contrast to the modern definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness and pleasure.

In the late 17th century, [[Gottfried Leibniz]] proposed the idea of the {{langx|la|[[vis viva]]}}, or living force, which defined as the product of the mass of an object and its velocity squared; he believed that total ''vis viva'' was conserved. To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the motions of the constituent parts of matter, although it would be more than a century until this was generally accepted. The modern analog of this property, [[kinetic energy]], differs from ''vis viva'' only by a factor of two. Writing in the early 18th century, [[Émilie du Châtelet]] proposed the concept of [[conservation of energy]] in the marginalia of her French language translation of Newton's ''[[Philosophiæ Naturalis Principia Mathematica|Principia Mathematica]]'', which represented the first formulation of a conserved measurable quantity that was distinct from [[momentum]], and which would later be called "energy".

In 1807, [[Thomas Young (scientist)|Thomas Young]] was possibly the first to use the term "energy" instead of ''vis viva'', in its modern sense.<ref>{{Cite book| last = Smith | first = Crosbie | title = The Science of Energy – a Cultural History of Energy Physics in Victorian Britain | publisher = The University of Chicago Press | year = 1998 | isbn = 978-0-226-76420-7}}</ref> [[Gustave-Gaspard Coriolis]] described "[[kinetic energy]]" in 1829 in its modern sense, and in 1853, [[William John Macquorn Rankine|William Rankine]] coined the term "[[potential energy]]". The law of [[conservation of energy]] was also first postulated in the early 19th century, and applies to any [[isolated system]]. It was argued for some years whether heat was a physical substance, dubbed the [[caloric theory|caloric]], or merely a physical quantity, such as [[momentum]]. In 1845 [[James Prescott Joule]] discovered the link between mechanical work and the generation of heat.

These developments led to the theory of conservation of energy, formalized largely by William Thomson ([[Lord Kelvin]]) as the field of [[thermodynamics]]. Thermodynamics aided the rapid development of explanations of chemical processes by [[Rudolf Clausius]], [[Josiah Willard Gibbs]], and [[Walther Nernst]]. It also led to a mathematical formulation of the concept of [[entropy]] by Clausius and to the introduction of laws of [[radiant energy]] by [[Jožef Stefan]]. According to [[Noether's theorem]], the conservation of energy is a consequence of the fact that the laws of physics do not change over time.<ref name="jphysics">{{Cite book |last1=Lofts |first1=G. |title=Jacaranda Physics 1 |last2=O'Keeffe |first2=D. |publisher=John Wiley & Sons Australia Limited |year=2004 |isbn=978-0-7016-3777-4 |edition=2 |location=Milton, Queensland, Australia |page=286 |chapter=11 – Mechanical Interactions |display-authors=etal}}</ref> Thus, since 1918, theorists have understood that the law of [[conservation of energy]] is the direct mathematical consequence of the [[translational symmetry]] of the quantity [[conjugate variables|conjugate]] to energy, namely time.