A quantity called Entropy can be derived from the study of the Carnot Cycle. This quantity can then
be used to formulate mathematically The Second Law of Thermodynamics.
When heat Q is exchanged at temperature T then the entropy changes by amount
Now, in Carnot Cycle
Thus in a reversible thermodynamic process the change in entropy of the system is zero, i.e. the
entropy remains constant whereas in an irreversible process we find that change in entropy is always greater
Hence, the second Law of Thermodynamics states that entropy of the universe never decreases.