Editing Maxwell's demon (section)

== Original thought experiment ==
The second law of thermodynamics ensures (through statistical probability) that two bodies of different [[temperature]], when brought into contact with each other and isolated from the rest of the Universe, will evolve to a thermodynamic equilibrium in which both bodies have approximately the same temperature.<ref name="Bennett87">{{cite journal|last = Bennett|first = Charles H.|title = Demons, Engines, and the Second Law|journal = Scientific American|volume = 257|issue = 5|pages = 108–116|date = November 1987|url = https://ecee.colorado.edu/~ecen5555/SourceMaterial/DemonsEnginesAndSecondLaw87.pdf|doi = 10.1038/scientificamerican1187-108|access-date = November 13, 2014|bibcode = 1987SciAm.257e.108B|archive-date = December 3, 2020|archive-url = https://web.archive.org/web/20201203173214/https://ecee.colorado.edu/~ecen5555/SourceMaterial/DemonsEnginesAndSecondLaw87.pdf|url-status = dead }}</ref> The second law is also expressed as the assertion that in an [[isolated system]], [[entropy]] never decreases.<ref name="Bennett87" />

Maxwell conceived a thought experiment as a way of furthering the understanding of the second law. His description of the experiment is as follows:<ref name="Bennett87" /><ref name="DdgTn">Maxwell (1871), reprinted in [[#Leff Rex 90|Leff & Rex (1990)]] on p. 4.</ref>

{{quote|... if we conceive of a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are as essentially finite as our own, would be able to do what is impossible to us. For we have seen that molecules in a vessel full of air at uniform temperature are moving with velocities by no means uniform, though the mean velocity of any great number of them, arbitrarily selected, is almost exactly uniform. Now let us suppose that such a vessel is divided into two portions, ''A'' and ''B'', by a division in which there is a small hole, and that a being, who can see the individual molecules, opens and closes this hole, so as to allow only the swifter molecules to pass from ''A'' to ''B'', and only the slower molecules to pass from ''B'' to ''A''. He will thus, without expenditure of work, raise the temperature of ''B'' and lower that of ''A'', in contradiction to the second law of thermodynamics.}}In other words, Maxwell imagines one container divided into two parts, ''A'' and ''B''.<ref name="Bennett87" /><ref name="Sagawa">{{cite book|last1 = Sagawa|first1 = Takahiro|title = Thermodynamics of Information Processing in Small Systems|publisher = Springer Science and Business Media|date = 2012|pages = 9–14|url = https://books.google.com/books?id=oKWi-J6LOsEC&pg=PA13|isbn = 978-4431541677}}</ref> Both parts are filled with the same [[gas]] at equal temperatures and placed next to each other. Observing the [[molecule]]s on both sides, an imaginary [[Demon (thought experiment)|demon]] guards a trapdoor between the two parts. When a faster-than-average molecule from ''A'' flies towards the trapdoor, the demon opens it, and the molecule will fly from ''A'' to ''B''. Likewise, when a slower-than-average molecule from ''B'' flies towards the trapdoor, the demon will let it pass from ''B'' to ''A''. The average [[speed]] of the molecules in ''B'' will have increased while in ''A'' they will have slowed down on average. Since average molecular speed corresponds to temperature, the temperature decreases in ''A'' and increases in ''B'', contrary to the second law of thermodynamics.  A [[heat engine]] operating between the thermal reservoirs ''A'' and ''B'' could extract useful [[work (physics)|work]] from this temperature difference.

The demon must allow molecules to pass in both directions in order to produce only a temperature difference; one-way passage only of faster-than-average molecules from ''A'' to ''B'' will cause higher temperature and pressure to develop on the ''B'' side.