5. Second principle of thermodynamics

The first principle of thermodynamics was formulated to account for observations concerning energy balances. In particular, it implies that, in this context, a quantity of heat is "equivalent" to work (i.e. of the same nature as the latter). However, this "equivalence" is not sufficient to explain the fact that it is possible to transform work entirely into heat (Joule effect, mechanical friction, etc.) and that it is only possible to transform a quantity of heat partially into work (thermal machines).

It was therefore necessary to formulate a second principle of thermodynamics that would allow a coherent representation of all aspects of the coexistence of heat and work.