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Second Law of Thermodynamics

The second law of thermodynamics prohibits the construction of a perpetual motion machine "of the second kind." There are two usual statements of this law. Kelvin's formulation states that it is impossible for a system operating in a cycle and in contact with one thermal reservoir to perform positive work in the surroundings. Clausius's formulation states that it is impossible for a system operating in a cycle to produce positive heat flow from a colder body to a hotter body.