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==[[Second Law of Thermodynamics]]==

==[[Second Law of Thermodynamics]]==

James Watt's steam engine drew heat from a specific source and converted some of it to useful work; the remainder of the heat was transferred to a cooler reservoir.  In 1824 a French engineer, N.L. Sadi Carnot, proposed the Carnot cycle<ref>[https://en.wikiversity.org/wiki/Carnot_engine] Carnot Engine</ref>, consisting of two isothermal processes (involving a constant temperature) and two adiabatic processes (no heat is gained or lost).  The result was, in theory, the most efficient-working heat engine cycle of any kind, one involving processes that must be reversible and involve no change in entropy.  What was discovered in practice was the second law of thermodynamics, which states (in one of its various formulations) that [[entropy]] in an isolated system cannot decrease, and that ''irreversible processes'' can only make it increase.<ref>Gregory H. Wannier, ''Statistical Physics'', John Wiley & Sons, New York, 1966</ref> An equivalent formulation states that heat cannot spontaneously flow from a cooler body to a hotter body.  Clausius stated in what is known as the Clausius statement that <ref>S. J. Blundell & K. M. Blundell, ''Concepts in Thermal Physics'', Oxford University Press, New York, 2016</ref>

James Watt's steam engine drew heat from a specific source and converted some of it to useful work; the remainder of the heat was transferred to a cooler reservoir.  In 1824 a French engineer, N.L. Sadi Carnot, proposed the Carnot cycle<ref>[https://en.wikiversity.org/wiki/Carnot_engine Carnot Engine]</ref>, consisting of two isothermal processes (involving a constant temperature) and two adiabatic processes (no heat is gained or lost).  The result was, in theory, the most efficient-working heat engine cycle of any kind, one involving processes that must be reversible and involve no change in entropy.  What was discovered in practice was the second law of thermodynamics, which states (in one of its various formulations) that [[entropy]] in an isolated system cannot decrease, and that ''irreversible processes'' can only make it increase.<ref>Gregory H. Wannier, ''Statistical Physics'', John Wiley & Sons, New York, 1966</ref> An equivalent formulation states that heat cannot spontaneously flow from a cooler body to a hotter body.  Clausius stated in what is known as the Clausius statement that <ref>S. J. Blundell & K. M. Blundell, ''Concepts in Thermal Physics'', Oxford University Press, New York, 2016</ref>

{{cquote|No process is possible whose sole result is the transfer of heat from a colder to a hotter body.}}

{{cquote|No process is possible whose sole result is the transfer of heat from a colder to a hotter body.}}