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I've been searching for this for a while. There is a principle of equivalence in general relativity: http://en.wikipedia.org/wiki/Equivalence_principle

But I need the principle of equivalence in thermodynamics. Is it the same as the second law of thermodynamics? http://en.wikipedia.org/wiki/Second_law_of_thermodynamics

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Without a doubt, it is the zeroth law of thermodynamics, as it defines an equivalence relation. It states that

If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

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    $\begingroup$ So, basically, transitivity of the equilibrium property (which, of course, makes the set of all systems in mutual equilibrium an equivalence class in the set of all systems because reflexivity and symmetry are also satisfied)? $\endgroup$
    – Danu
    Apr 13, 2014 at 10:46
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    $\begingroup$ Yes, that's it. $\endgroup$ Apr 13, 2014 at 11:08
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I disagree with @Raskolnikov I think the principle of equivalence is described by the carnot engine:carnot

So basically $$Q_H =Q_C + W$$

This is equivalence as I understand, it doesn't have to be a law.

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