What is the Third Law of Thermodynamics?

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  • Last Modified Date: 14 October 2019
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The laws of thermodynamics express the behavior of energy in natural systems as expressed in our universe. There are three laws of thermodynamics plus a zeroth law. The first law of thermodynamics is called the law of conservation of energy. It says that the energy in the universe remains constant. The second law of thermodynamics says that heat cannot transfer from a colder to a hotter body as its sole result and the entropy of the universe does not decrease. The third law of thermodynamics, simply put, says that it is impossible to reach absolute zero. And the zeroth law says that two bodies in thermal equilibrium with a third body are in thermal equilibrium with each other.

The third law of thermodynamics began with the so-called heat theorem, Wärmetheorem in German, was arrived at in 1906 by Walther Hermann Nernst, who had received an appointment to the University of Berlin’s Second Chemical Institute and a permanent membership in the Prussian Academy of Sciences the previous year. The third law is sometimes known as Nernst’s postulate or Nerst’s theorem as well.


This theorem and Einstein’s 1907 paper showing that quantum mechanics predicts that the specific heats of solids will tend towards absolute zero when they attain temperatures in the neighborhood of absolute zero seemed to reinforce each other. This was important to Nernst because his theorem was not clearly a third law of thermodynamics because it was not able to be deduced from the first two laws of thermodynamics, but he felt that Einstein paper and Max Planck’s quantum mechanics work helped bolster the claims of his theory to actually be a third law of thermodynamics nevertheless.

Nernst won the Nobel Prize in chemistry in 1920 for this work, but there was controversy about it. American chemist Theodore Richards claimed that he, rather than Nernst, had discovered the third law of thermodynamics, as indicated by the possible interpretation of graphs in a paper he wrote in 1902. Nernst’s former friend Svante Arrhenius, already on the outs with Nernst over an earlier dispute, was brought into the discussion by Richards and exerted great efforts in opposing Nernst’s receiving the Nobel Prize for this work.

The third law of thermodynamics is also stated using different terms. For example, “at absolute zero temperature, entropy reaches absolute zero.” Or “a finite number of steps cannot be used to reach absolute zero.” Or “if the thermal motion of molecules were to cease, then the state of absolute zero would occur.” Or “Entropy and system processes cease as a system approached absolute zero.”


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