Water does not flow up a hill, heat does not flow from a low temperature body to a high temperature body. Our experiences suggest that processes have a definite direction.

A proposed thermodynamic system that does not violate the First law of thermodynamics does not ensure that the thermodynamic system will actually occur.

The First law of thermodynamics does not give any information to the direction of the process. There is a need to place restrictions on the direction of flow of a process.

The limited amount of energy that can be transformed from one form to another form and the direction of flow of heat and work have not been discussed. The Second law of thermodynamics addresses these areas.

The principle underlying the directionality of spontaneous change and inefficiency of a heat engine is called the second law of thermodynamics. This law may be stated in various equivalent ways.

Among the best known statements are the following two statements. Kelvin-Planck statement: It is impossible to construct a heat engine that produces work with heat interaction only from a single thermal reservoir.

This implies that a heat engine requires at least two thermal reservoirs with different temperatures, and that it is impossible to build a heat engine that has a thermal efficiency of 100%.

Clausius statement: It is impossible to construct a heat pump or a refrigerator which moves heat from a low temperature thermal reservoir to a high temperature thermal reservoir without adding work.

This implies that the coefficient of performance of a heat pump or a refrigerator is always less than infinity. Every relevant experiment that has been conducted verifies the Second law of thermodynamics.

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