20. Second Law of Thermodynamics

The second law of thermodynamics indicates the direction of possible energy transformations and by that expresses irreversibility of processes in nature. It was established by direct generalization of experimental facts.

There are several definitions of the second law of thermodynamics, which, despite the external differences, express in essence the same thing and are equal. The German scientist Clausius has formulated this law as follows: it is impossible to transfer heat from a cold system to a hot one if there are no other simultaneous changes in both systems or surrounding bodies. Here is an experimental fact of a certain direction of heat transfer: the heat itself always passes from hot to cold bodies. However, in refrigeration devices the heat transfer is transferred from the cold body to the warmer one, but this transfer is connected with "other changes" - cooling is achieved by work.

Another definition belongs to the English scientist Kelvin: it is impossible to do a cyclical process, the only result of which would be to get the work by heat taken from one source.

Here summarizes an experimental fact: if, for example, a steam engine performs the work by the heat taken from the steam boiler, then the work performed is not the "only result" of the process, because part of the heat is allways released into the atmosphere with the exhaust steam. The same applies to internal combustion engines and all heat engines in general. In other words, no one heat engine can have a efficiency equal to one (\( 0 \leqslant \eta < 1\)). The efficiency (\( \eta \)) of a heat engine is defined as the ratio of the work done by the machine (\( \Delta W' \)) to the quantity of heat (\( Q \)) received for this purpose

\( \eta = \frac{\Delta W'}{Q} = \frac{benefit}{cost} \) (1-22)

If the first law of thermodynamics (the law of energy conservation) can be expressed in the form of the statement: it is impossible to construct the perpetual motion machine of the first kind, the definition of the second law given by Kelvin, allows to express this law in the form of the statement: it is impossible to construct the perpetual motion machine of the second kind, i.e. the machine performing work by cooling of any one body.

Perpetuum mobile of the 2nd generation does not contradict the law of energy conservation, but if it were possible, we would have an almost unlimited source of work, taking it from the oceans for example and by cooling them. However, cooling the ocean as soon as it becomes lower than its ambient temperature would mean transferring heat from a cooler body to a hotter body, and this process cannot go by itself. This example shows that the two definitions of the second law are the same.

The deep physical reason of irreversibility of processes in nature can be understood only on the basis of molecular-kinetic theory.