Statements of the Second Law of Thermodynamics

The second law of thermodynamics can be interpreted in different ways.

According to the Clausius statement, heat never transfers from a cold to a hot body without external aid.

A refrigerator cools the food by taking away the heat from the food, and rejecting the heat to the surroundings. It uses electricity to change the spontaneous direction of heat flow.

This statement precludes a perfect refrigerator that operates without a power input.

According to the Kelvin-Planck statement, it is impossible to have a perfect heat engine that takes heat from a single source and completely converts it into work without any losses.

Real heat engines emit heat to the surroundings through exhaust at the end of the cycle.

The Clausius and the Kelvin-Planck statements are equivalent. If one statement is false, it necessarily follows that the other statement is also false.

For example, consider a perfect refrigerator which violates the Clausius statement and a practical heat engine.

When they are merged, the combined engine acts as a perfect heat engine, violating the Kelvin-Planck statement.