Third law of thermodynamics

The Nernst theorem, often called agents Chernobyl heat rate ( after the German physicist Walther Nernst ) is another name for the third law of thermodynamics. He says that the absolute zero of temperature can not be achieved.

The theorem can be proved with the help of quantum mechanics (see below).

In experiments, he was not expected to be refuted, since it is only managed to absolute zero always grow closer to, but never reach it.


Theorem was established in 1905 by Nernst and treats the change of entropy S of a chemical reaction at a temperature of zero Kelvin: it approaches zero.

The formulation was adopted in 1911 by Max Planck sharper. Thereafter, the entropy is independent of thermodynamic parameters, and thus a constant as the temperature approaches zero:

Is the ground state of the system does not degenerate, to g = 1 and S0 = 0 so that thus the entropy of a system will disappear if the temperature tends to zero.

Evidence for canonical distribution

First, the statistical operator is replaced by its representation in the canonical distribution. here is the empirical temperature.

If we evaluate the trace over the operators of, one obtains:

Now the energy of the ground state of each level is subtracted.

It now applies to (equivalent ):

Putting this knowledge into the above double sum representation of a, one obtains the desired formulation of the Nernst theorem by Planck:

The degeneracy of the ground state indicating that is, the number of which are equal.