The Kelvin and Clausius Formulations

The traditional way to approach the subject is to state the Second Law as it has been deduced on the basis of years of experience, and then show through use of the Carnot cycle the logical consequences (such as the existence of the entropy and an absolute scale of temperature). Two logically equivalent ways of stating the Second Law are 

It is impossible to construct an engine that, working in cycles, shall produce no effect other than the extraction of heat from a reservoir and the production of work. 

For example, you cannot have an engine that will extract heat from the sea and use that energy to power a ship across the ocean, leaving a trail of icebergs in its wake. 

For example, you cannot even heat the cabins on the ship by extracting heat from the sea. This slightly simpler formulation eliminates considerations of heat-to-work transfers. 

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