Editing Second law of thermodynamics (section)

=== Carnot's principle ===
The historical origin<ref>[[Nicolas Léonard Sadi Carnot|Carnot, S.]] (1824/1986).</ref> of the second law of thermodynamics was in [[Nicolas Léonard Sadi Carnot|Sadi Carnot]]'s theoretical analysis of the flow of heat in steam engines (1824). The centerpiece of that analysis, now known as a [[Carnot engine]], is an ideal [[heat engine]] fictively operated in the limiting mode of extreme slowness known as quasi-static, so that the heat and work transfers are between subsystems that are always in their own internal states of [[thermodynamic equilibrium]]. It represents the theoretical maximum efficiency of a heat engine operating between any two given thermal or heat reservoirs at different temperatures. Carnot's principle was recognized by Carnot at a time when the [[caloric theory]] represented the dominant understanding of the nature of heat, before the recognition of the [[first law of thermodynamics]], and before the mathematical expression of the concept of entropy. Interpreted in the light of the first law, Carnot's analysis is physically equivalent to the second law of thermodynamics, and remains valid today. Some samples from his book are:
:: ...''wherever there exists a difference of temperature, motive power can be produced.''<ref>Carnot, S. (1824/1986), p. 51.</ref>
:: The production of motive power is then due in [[steam engines]] not to an actual consumption of caloric, but ''to its transportation from a warm body to a cold body ...''<ref>Carnot, S. (1824/1986), p. 46.</ref>
:: ''The motive power of heat is independent of the agents employed to realize it; its quantity is fixed solely by the temperatures of the bodies between which is effected, finally, the transfer of caloric.''<ref>Carnot, S. (1824/1986), p. 68.</ref>

In modern terms, Carnot's principle may be stated more precisely:
: The efficiency of a quasi-static or reversible [[Carnot cycle]] depends only on the temperatures of the two heat reservoirs, and is the same, whatever the working substance. A Carnot engine operated in this way is the most efficient possible heat engine using those two temperatures.<ref>[[Clifford Truesdell|Truesdell, C.]] (1980), Chapter 5.</ref><ref>Adkins, C.J. (1968/1983), pp. 56–58.</ref><ref>Münster, A. (1970), p. 11.</ref><ref>Kondepudi, D., [[Ilya Prigogine|Prigogine, I.]] (1998), pp.67–75.</ref><ref>Lebon, G., Jou, D., Casas-Vázquez, J. (2008), p. 10.</ref><ref>Eu, B.C. (2002), pp. 32–35.</ref>