A
system is said to be at equilibrium (with respect to
some set of feasible transformations) if it has minimal free energy. A system
containing objects at different temperatures
is in disequilibrium, because heat
flow can reduce the free energy.
Springs have equilibrium lengths, reactants and products in solution have
equilibrium concentrations, thermally excited systems
have equilibrium probabilities of occupying various states,
and so forth.
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