Classical thermodynamics deals only with reversible changes. The meaning of this restriction has been demonstrated in Sect. 6 by the example of a simple Carnot engine. Reversible processes have to be performed “infinitely slowly”. Any real process occurs with a finite velocity and, therefore, is necessarily irreversible. For instance, an exchange of heat between two bodies A and B is possible only if A is warmer than B, or a piston between two gas containers moves only if the pressure in the two containers differs. In both cases the actual process is associated with an increase of entropy. It is a quite strange situation that thermodynamics deals only with reversible processes which conserve the entropy of a closed system, whereas the entropy increases in all actual processes.
KeywordsEntropy Production Thermoelectric Power Irreversible Process Heat Bath Total Entropy
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