Third principle of thermodynamics

The third principle of thermodynamics , also called principle of Nernst (1904), states that:

To the limit of the absolute zero, Temperature which could not be reached , the Entropie of balance of a system tends towards a constant independent of the other intensive parameters, constant which is taken null, if possible .

That gives a direction to a given value of the Entropie (and not “with an additive constant near”). This principle is irréductiblement related to the quantum indiscernibility of the identical particles.

A simple example: rare gases

Argon is almost a Perfect gas monoatomic. Its Entropie was calculated in the article Second principle of thermodynamics, approaches statistical: a concrete example.

Another simple, but exotic example

Helium III: it is here about a quantum liquid, which, for a temperature much lower than the temperature of Fermi, has practically one possible state and thus its entropy is null. On the other hand, the crystal helium III has an entropy Nk Ln2 = R Ln2 = 5.74 J/K/mol thus larger than that of the liquid. So to liquefy the solid, it is necessary to extract from heat: it is the only known case where the latent heat of liquefaction is negative.

In the presence of a very strong magnetic field, the solid is entirely polarized: there is practically only one possible state; the preceding phenomenon disappears: it is the Pomerantchuk effect.

Consequences of the third principle

The heat-storage capacities must tend towards zero, when T tends towards zero. It is thus of the heat-storage capacity of the crystals (law of Debye) $C_P$ = 3R. (T/T0) ^3. When the temperature becomes very low, there remains a residue, the electronic capacity, which it also tends towards zero 3R /2. (T/Te).

One cannot reach the absolute zero. One is closer to physics, if it is considered that the good variable of temperature is -1/kT: then to say that T tends towards zero, wants to say that this variable tends towards less the infinite one, which obviously is never atteignable.

Nevertheless, one can speak (but with prudence), of negative temperatures.

History

Nernst imagined the third principle well before the quantum theory, for reasons related to measurements at low temperature. Giauque (1895-1982) made many measurements which confirmed the theory of Nernst (for example the residual value of entropy of the ice due to hydrogen bond O-H….O). Its method of adiabatic demagnetization also enabled him to reach very low temperatures (less than 1 K).

the Constante of Sackur-Tetrode made it possible to find an approximate value of the Planck's constant, which of this fact was placed at the row of universal constant for all the bodies, and thus deeply anchored in a theory of the matter. It is known that in 1925, that was concretized with the creation of quantum mechanics.

The production of cold atoms makes it possible today (since 1995) to reach of so low temperatures which one can highlight the concept of perfect gas quantum of Bump-Einstein (with of course of the corrective measures because the gas is real). The concept of gas of Fermions is obviously more common, since it goes in work into the theory of the surface of Fermi of the electrons, in metals.

See too

With the First principle of thermodynamics and the Second principle of thermodynamics, this third principle is enough to find all thermodynamics, science deductive, relating to the thermal and calorific phenomena.

Thermodynamic