THE CONCEPTS OF THERMODYNAMICS
Although thermodynamics unquestionably belongs to classical physics, it is a theory which shows some strikingly novel features.
The same phenomena are described by two different theories. In one interpretation, the thermal behaviour of bodies is described in terms of macroscopic quantities, and the processes are cycles of operations performed by heat engines. This is the so-called phenomenological theory because it is assumed that the terms used directly represent the phenomena. In the other interpretation, as statistical theory, the same phenomena are described in terms of microscopic quantities, as the motion of molecules governed both by the laws of mechanics and of probability. In both formulations the laws are of the widest possible generality.
The laws of phenomenological thermodynamics are expressed, mathematically, as simple functional relations between a few variables. Though of course they are based upon the outcome of certain experiments, the laws are normally used not as empirical but as analytic statements. They have therefore been called 'principles'--which name usually indicates that we take such sentences as a guide in constructing theories rather than as expressing experience.
The laws of thermodynamics, moreover, are often given in a negative form; and they state an impossibility rather than the possibility of carrying out certain experiments. This is done, partly, because physicists feel that in this way the laws retain their empirical character which, in a positive and universal formulation, they tend to lose. To take the first law as example: it refers to the experiment that a given quantity of heat is equivalent to a certain amount of mechanical work. This may also be expressed in a universal sentence as the conservation of energy: The total energy in all processes is conserved. In other words, we cannot gain energy from nothing, and all physical processes represent a