Third law of thermodynamics
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The third law of thermodynamics states that:
As a system approaches absolute zero of temperature all processes cease and the entropy of the system approaches a minimum value or zero for the case of a perfect crystalline substance.
Statements
Succinctly, the third law of thermodynamics states:
- All processes cease as temperature approaches zero; or,
- As temperature goes to 0, the entropy of a system approaches a constant.
Description
The third law was developed by Walther Nernst, during the years 1906-1912, and is thus sometimes referred to as Nernst's theorem. The third law of thermodynamics states that the entropy of a system at zero absolute temperature is a well-defined constant. This is because a system at zero temperature 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 by any procedure, no matter how idealised, to reduce any system to the absolute zero of temperature in a finite number of operations".
In simple terms, the Third Law states that the entropy of a pure substance at absolute zero temperature is zero. This law provides an absolute reference point for the determination of entropy. The entropy determined relative to this point is the absolute entropy.
A special case of this 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 ln(1) = 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 is with respect to the magnetic moments of a material. Paramagnetic materials (moments random) will "order" as the T approaches 0 K. 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 0 K no solid solutions should exist. Phases in equilibrium at 0 K should either be pure elements or atomically ordered phases. See J.P. Abriata and D.E. Laughlin, “The Third Law of Thermodynamics and low temperature phase stability,” Progress in Materials Science 49, 367-387, 2004.
- Adiabatic process
- Ground state
- Laws of thermodynamics
- Thermodynamic entropy
- Thermodynamics
- Timeline of thermodynamics, statistical mechanics, and random processes
Further reading
- Goldstein, Martin, and Inge F., 1993. The Refrigerator and the Universe. Harvard Univ. Press. Chpt. 14 discusses the Third Law. Overall, a gentle introduction to thermodynamics.