From this law follows that it is impossible to construct a device that operates on a cycle and whose sole effect is the transfer of heat from a cooler body to a hotter body. Reversible processes are a useful and convenient theoretical fiction, but do not occur in nature. This law indicates the irreversibility of natural processes.
In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases. The entropy of any isolated system never decreases.
