1.21.8: Third Law of Thermodynamics
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The Third Law of Thermodynamics states that the entropy and the heat capacity of a perfect crystal vanish in the limit of zero kelvin [1].
When a system is heated, energy is stored in the form of molecular vibrations leading to an increase in entropy. The isochoric heat capacity is related to the differential dependence of energy on temperature at equilibrium.
A more detailed analysis is required if phase changes and inter-component mixing is involved. Here there are additional contributions to the change in entropy accompanying irreversible processes which Gurney describes as the cratic part of the entropy [2]. In a certain sense an isentropic process is ‘a draw between Maxwell’s demon and a natural process’ [3].
Footnotes.
[1] K. S. Pitzer, Thermodynamics, McGraw-Hill, New York, 3rd. edn.,1965.
[2] R. W. Gurney, Ionic Processes in Solution, McGraw-Hill, New York, 1953.
[3] A. B. Pippard, The Elements of Classical Thermodynamics, University Press, Cambridge ,1957, p.99.