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 , where 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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Entropic force is a type of emergent force that arises from the tendency of a system to maximize its entropy, or disorder. In statistical mechanics, entropy is a measure of the number of microscopic configurations that correspond to a thermodynamic system's macroscopic state. Entropic forces are not fundamental forces like gravity or electromagnetism; instead, they emerge from the statistical behavior of particles in a system due to microscopic interactions.