2nd Law of Thermodynamics
The entropy of any isolated system never decreases. In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases.
This law indicates the irreversibility of natural processes. Reversible processes are a useful and convenient theoretical fiction, but do not occur in nature. 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. It follows, perpetual motion machines of the second kind are impossible.
The 2nd law of thermodynamics is a general principle, that goes beyond the limitations imposed by the 1st law of thermodynamics. The first law is used to relate and to evaluate the various energies involved in a process. However, no information about the direction of the process can be obtained by the application of the first law. The second law of thermodynamics places constraints upon the direction of heat transfer and sets an upper limit to the efficiency of conversion of heat to work in heat engines. So the second law is directly relevant for many important practical problems.
One of the areas of application of the 2nd law of thermodynamics is the study of energy-conversion systems. For example, it is not possible to convert all the energy obtained from a coal in coal-fired power plant or from a nuclear reactor in a nuclear power plant into electrical energy. There must be losses in the conversion process.