Editing Second law of thermodynamics (section)

=== Maxwell's demon ===
{{main|Maxwell's demon}}
[[File:James-clerk-maxwell3.jpg|thumb|upright|James Clerk Maxwell]]
[[James Clerk Maxwell]] imagined one container divided into two parts, ''A'' and ''B''. Both parts are filled with the same [[gas]] at equal temperatures and placed next to each other, separated by a wall. Observing the [[molecule]]s on both sides, an imaginary [[demon]] guards a microscopic trapdoor in the wall. 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''. 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.<ref name=":1">{{Cite web |title=Maxwell's demon {{!}} physics {{!}} Britannica |url=https://www.britannica.com/science/Maxwells-demon |access-date=2023-03-14 |website=www.britannica.com |language=en}}</ref>

One response to this question was suggested in 1929 by [[Leó Szilárd]] and later by [[Léon Brillouin]]. Szilárd pointed out that a real-life Maxwell's demon would need to have some means of measuring molecular speed, and that the act of acquiring information would require an expenditure of energy.<ref name=":2">{{Cite journal |last=Norton |first=John |date=3 July 2013 |title=All Shook Up: Fluctuations, Maxwell's Demon and the Thermodynamics of Computation |journal=Entropy |volume=15 |issue=12 |pages=4432–4483 |doi=10.3390/e15104432 |bibcode=2013Entrp..15.4432N |doi-access=free }}</ref> Likewise, Brillouin demonstrated that the decrease in entropy caused by the demon would be less than the entropy produced by choosing molecules based on their speed.<ref name=":1" />

Maxwell's 'demon' repeatedly alters the permeability of the wall between ''A'' and ''B''. It is therefore performing [[thermodynamic operation]]s on a microscopic scale, not just observing ordinary spontaneous or natural macroscopic thermodynamic processes.<ref name=":2" />