The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines. In so doing, it goes beyond the limitations imposed by the first law of thermodynamics.

We can say the principle, which disallows many phenomena consistent with the First Law of Thermodynamics is known as the Second Law of Thermodynamics.

The Second Law of Thermodynamics gives a fundamental limitation to the efficiency of a heat engine and the co-efficient of performance of a refrigerator.

It says that efficiency of a heat engine can never be unity.

The heat released to the cold reservoir can never be made zero.

For a refrigerator, the Second Law says that the co-efficient of performance can never be infinite.

`text(Kelvin-Planck statement)`

No process is possible whose sole result is the absorption of heat from a reservoir and the complete conversion of the heat into work.

`text(Clausius statement)`

No process is possible whose sole result is the transfer of heat from a colder object to a hotter object.

It can be proved that the two statements above are completely equivalent.