ABSTRACT
Entropy is a thermodynamic variable which depends only on the state of the system and, because in any arbitrary process the entropy change depends only on the initial and final states, it now no longer matters whether or not the process is reversible. At first this seems paradoxical, but we must bear in mind that in measuring AS we are still effectively confined to reversible changes under well-defined conditions. Only afterwards when we can imagine that all states of the system have been mapped by means of these reversible changes, so that the value of S is known for each state, can we consider irreversible changes. Our situation is indeed quite similar to that encountered in the measurement of the internal energy function, U. To measure changes in U we were restricted to adiabatic paths along which AU = W, the work done on the system. However, once the value of U in all states of the system is known, we can then make use of this knowledge in discussing non-adiabatic processes; in this way Q was defined. In a very similar manner we shall make use of S, which is measured by means of reversible processes, to discuss irreversible processes.
