Law of statistical mechanics stating that, in a system in thermal equilibrium, on average, an equal amount of energy is associated with each independent energy state. It states specifically that a system of particles in equilibrium at absolute temperature math.T will have an average energy of 12math.kmath.T, where math.k is the Boltzmann constant, associated with each degree of freedom. For example, an atom of a gas has three degrees of freedom (its three position coordinates); therefore, it will have an average total energy of 32math.kmath.T.

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