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Second Law of Thermodynamics

The first law of thermodynamics merely explains the equivalence of work and heat. It does not explain why heat flows from bodies at higher temperatures to those at lower temperatures. It cannot tell us why the converse is possible. The second law of thermodynamics provides answers to these questions. Statement of this law is as follows:

Clausius statement

It is impossible for a self acting machine to transfer heat from a colder body to a hotter one without the aid of an external agency.
From Clausius statement, it is clear that heat cannot flow from a body at low temperature to one at higher temperature unless work is done by an external agency. This statement is in fair agreement with our experiences in different branches of physics.

Kelvin’s statement

It is impossible for a body or system to perform continuous work by cooling it to a temperature lower than the temperature of the coldest one of its surroundings. A Carnot engine cannot work if the source and sink are at the same temperature because work done by the engine will result into cooling the source and heating the surroundings more and more.

Kelvin–Planck’s statement

It is impossible to design an engine that extracts heat and fully utilizes into work without producing any other effect.
From this statement it is clear that any amount of heat can never be converted completely into work. It is essential for an engine to return some amount of heat to the sink.

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