= Entropic force
{title2=$F_S=T(\partial S/\partial x)_T$}
{wiki}
An <entropic force> is the part of a generalized <force> arising from the dependence of <entropy> on a coordinate. At fixed <temperature>, the system's force is $-\partial A/\partial x=-\partial U/\partial x+T\partial S/\partial x$, where $A=U-TS$ is the <Helmholtz free energy>. If the <internal energy> is independent of the coordinate, the force is entirely entropic. For an <ideal chain> whose configurations become less numerous on extension, it acts toward shorter extension. The external <tension> holding the chain extended has the opposite sign. Thus the force depends on the statistical number of available configurations, even when the energy of each configuration is unchanged.
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