Entropy is a specific process in thermodynamic system, where it is used for representing a lack in the thermal energy of the concerned system while it is being converted to mechanical energy. In this case, the whole system functions within a certain sense of randomness, and therefore the associated process has to function in that level.
The entropy changes and becomes of the higher levels when it is placed in a secluded system and it is taken to be maximum at the equilibrium point. When there is a total change in entropy, there is enclosing adiabatic surroundings which is zero or at the higher level.
For (a) & (c) cycle
(for a & c irreversible cycle) … (8.14)
From equation (8.13) & (8.14) we can write
(as b is reversible)
(as entropy change is same in band c)
Thus it is proved that the entropy of an isolated system always increases, it can never decrease.
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