Changes

==[[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, 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 that ''"It is impossible for a self-acting machine, unaided by external agency, to convey heat from a body at one temperature to another body at a higher temperature."''

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, 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>

What it meant was heat transfers to cooler temperatures, and not the other way around, which led to the concept of entropy.  An idealized formula for an increase in entropy when heat is applied reads:

<center><big><math> \Delta S = q/T </math></big></center>

 

Where <math>S</math> is the state of the system or entropy, <math>q</math> is the added heat, and <math>T</math> is the absolute temperature.  This increase in entropy must be at least <math>q/T</math>, but the entropy change itself (<math> S</math> is always greater.

 

 

What it meant was heat transfers to cooler temperatures, and not the other way around, which led to the concept of entropy.  An idealised formula for an increase in entropy when heat is applied reads:

<center><big><math> \delta S_{rev} = Q/T </math></big></center>

Where <math>\delta S_{rev}</math> is the small change in entropy caused by a reversible process, <math>Q</math> is the added heat, and <math>T</math> is the absolute temperature.  This increase in entropy must be at least <math>q/T</math>, but the entropy change itself <math>\delta S</math> is always greater.

==[[The Third Law of Thermodynamics]]==

==[[The Third Law of Thermodynamics]]==