http://web.mit.edu/16.unified/www/FA...es/node37.html
"There are several ways in which the second law of thermodynamics can be stated. Listed below are three that are often encountered. As described in class (and as derived in almost every thermodynamics textbook), although the three may not appear to have much connection with each other, they are equivalent. 1.No process is possible whose sole result is the absorption of heat from a reservoir and the conversion of this heat into work. [Kelvin-Planck statement of the second law] 2.No process is possible whose sole result is the transfer of heat from a cooler to a hotter body. [Clausius statement of the second law] 3.There exists for every system in equilibrium a property called entropy..."

"Whose sole result" in the Kelvin-Planck version means that the process is cyclic (the heat engine returns to its initial state) and at the end of the cycle we have e.g. some weight lifted, a corresponding loss of heat in the reservoir (surroundings), and NO OTHER CHANGES.

With this interpretation of "whose sole result" the Kelvin-Planck version (and similarly the Clausius version) is just a trivial truth that has nothing to do with the original idea behind the second law of thermodynamics. In the original idea the heat engine is assisted by an OPERATOR - e.g. the mad bleary-eyed perpetuum mobile constructor standing before the merciless jury of presumably sane scientists. The operator does undergo changes in the process (he presses buttons, his adrenal glands produce adrenaline in response to the stress etc) and these changes are INDISPENSABLE - without them (that is, in the absence of an operator), the heat engine would be unable to perform a cycle and return to its initial state. So the correct statements of the second law of thermodynamics a

1. No process is possible whose sole results are, on the one hand, the absorption of heat from a reservoir and the conversion of this heat into work, and, on the other, indefinite changes in the operator's body.

2. No process is possible whose sole results are, on the one hand, the transfer of heat from a cooler to a hotter body, and, on the other, indefinite changes in the operator's body.

The "indefinite changes in the operator's body" addition is by no means trifling - it converts the Kelvin-Planck and Clausius versions, as presented in the quotation above, from trivial truths into falsifiable, and in the end even false, statements of the second law of thermodynamics.

Pentcho Valev