The Postulates of Kelvin and Clausius

The first law does not address the limitations of heat conversion into work or heat transfer between systems. The following two postulates based on experimental experience do just this. They are the foundation of what is called the second law of thermodynamics.  

A complete transformation of heat (extracted from a uniform source) into work is impossible.  

It is impossible to transfer heat from a body at a given temperature to a body at higher temperature as the only result of a transformation. 

Remark At this point we use the temperature 9 to characterize a reservoir as hotter or colder than another. The precise meaning of temperature is discussed in the following section. 

These two postulates are equivalent. A way to prove this is by assuming that the first postulate is wrong. This is then shown to contradict the second postulate. Subsequently the same reasoning is applied starting with the second postulate, i.e. the assumption that the second postulate is wrong is shown to contradict the first. 

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