Third law of thermodynamics

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Template:Laws of thermodynamics The third law of thermodynamics (hereinafter "Third Law") states that "as a system approaches the zero absolute temperature (hereinafter "ZAT"), all processes cease and the entropy of the system approaches a minimum value. That minimum value is zero in the case of a perfect crystalline substance. Succinct statements of the Third Law include:

All the temperature of a system approaches ZAT, all processes cease;
As temperature goes to ZAT, the entropy of a system approaches a constant.

1 Description
2 See also
3 Further reading
4 External links

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Description

The Third Law states that the entropy of a system at ZAT is a well-defined constant. This is because a system at ZAT exists in its ground state, so that its entropy is determined only by the degeneracy of the ground state; or, it states that "it is impossible to reduce any system to ZAT in a finite number of operations, no matter how idealised."

In simple terms, the Third Law states that the entropy of a pure substance at ZAT is zero. This Law establishes an absolute reference point for the calculation of entropy. The entropy relative to this reference point is the absolute entropy.

A special case is systems with a unique ground state, such as crystal lattices. The entropy of a perfect crystal lattice as defined by Nernst's theorem is zero (since ln1 = 0). However this disregards the fact that real crystals must be grown at finite temperature and possess an equilibrium defect concentration. When cooled down, they are generally unable to achieve complete perfection. This, of course, is in line with the observation that entropy must always increase, since no real process is reversible.

Another application of the Third Law concerns the magnetic moments of a material. Paramagnetic materials (moments random) will "order" as the temperature approaches ZAT. They may order in a ferromagnetic sense (all moments parallel to each other) or they may order in an antiferromagnetic manner.

Yet another application of the Third Law is the fact that at ZAT, no solid solutions should exist. Phases in equilibrium at ZAT should either be pure elements or atomically ordered phases. See J.P. Abriata and D.E. Laughlin, 2004, "The Third Law of Thermodynamics and low temperature phase stability," Progress in Materials Science 49: 367-387.

The Third Law was developed by Walther Nernst, during the years 1906-1912, and is thus sometimes referred to as Nernst's theorem.

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