http://johncarlosbaez.wordpress.com/2012/01/23/classical-mechanics-versus-thermodynamics-part-2/
http://en.wikipedia.org/wiki/Thermodynamic_potential
http://en.wikipedia.org/wiki/Maxwell_relations
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thepot.html
[*7.80]
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