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I've already faced this situation several times: given a statement (in area of thermodynamics) I used it to provide an example of some perpetual motion machine (of first or second kind). Therefore, I thought, proving the initial statement wrong. And sometimes I get a response that I haven't actually constructed a perpetual motion machine. Because the temperature reservoirs I use will eventually change their temperatures. Ergo, my construction is not perpetual.

I'm pretty sure that the argument is invalid (not to say -- silly). But I still can be wrong.

Whatever the case. I'd like to have a consistent way of rejecting it (or accepting and using it) starting from first principles and definitions of thermodynamics. References dealing with that argument are also welcome.

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Isn't the essence of a perpetual motion machine that at the end of some cycle, work has been done but the machine has fully returned to it's original state? – RedGrittyBrick Oct 1 '12 at 14:36
@Kostya please could you illustrate the question with a specific example, which will help any answerers give you better answers? – EnergyNumbers Oct 1 '12 at 15:55

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