The theory may be completed by the following statement of the Third Law of Thermodynamics:
On a quasistatic path such that the absolute temperature approaches zero while the external parameters remain bounded, the entropy potential approaches a finite limit S0 which is the same for all such paths.
The Nernst Heat Theorem is a corollary of this law. It may be stated as follows:
In a set of equilibrium states with bounded values of the external parameters the entropy difference between two states at the same temperature approaches zero as the absolute temperature approaches zero.
We now define the absolute entropy of a system to be the entropy potential obtained by putting S0 = 0. The postulates constituting the second part of the second law imply that, for any pair of states 1 and 2 in an isometric set, T2 < T1 if and only if S2 < S1. It follows from the definition of absolute entropy that: All absolute entropies are positive.
The third law of thermodynamics has applications in low temperature physics and chemistry, but it is not needed in engineering.
By R. H. B. Exell, 2001. King Mongkut's University of Technology Thonburi.
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