Le Chatelier's principle

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Le Chatelier's principle describes the stability of a system in thermodynamic equilibrium[1][2]:

In response to small deviations away from equilibrium, the system will change in a manner that restores equilibrium.

This translates to conditions on the second derivatives of thermodynamic potentials such as entropy, S(U,\ldots). For instance, the entropy is a concave function of its arguments such as internal energy. Thus, one has

\frac{\partial^2 S}{\partial U^2} \geq0\ .

Similarly, specific heats can be shown to be positive definite.


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