ABSTRACT
The entropy is a function of state. This can be easily proved by means of a reversible cycle performed between the states i and j, illustrated in Figure 8.2 (a), where
rev dSdSdS T Qð0
SSdSdS −== ∫∫ ′
Figure 8.2. Cyclic processes performed between two states i and j: (a) reversibly (b) irreversibly
(the dotted line) Since the entropy change takes the same value for any reversible process from i to j, it is clear from Eq.(8.9) that, up to an integration constant, S is defined in a unique way for every state i or j of the system. It is therefore a function of state. Consequently, dS is an exact differential, whereas is not: 1/T is said to be an integrating factor for . Every reversible adiabatic process
revQð revQð( )0ð =Q follows a path included in a surface of constant entropy (an isentropic process).
