It gives the available useful energy from a closed system, so it is usually symbolized A from the German for "work." The p-V Legendre transform of a similar quantity named for Gibbs, it is given by the internal energy minus the absolute temperature times the entropy.
The differential of this quantity, which is given in general by minus Boltzmann's constant times the thermodynamic temperature times the natural logarithm of the partition function, is non-positive for isothermal, isometric processes, so it is minimized for equilibria in constant-temperature, constant-volume systems. It gives the available useful energy from a closed system, so it is usually symbolized A from the German for "work." The p-V Legendre transform of a similar quantity named for Gibbs, it is given by the internal energy minus the absolute temperature times the entropy. For 10 points, name this thermochemical quantity, a thermodynamic potential named for a German physicist.
The differential of this quantity, which is given in general by minus Boltzmann's constant times the thermodynamic temperature times the natural logarithm of the partition function, is non-positive for isothermal, isometric processes, so it is minimized for equilibria in constant-temperature, constant-volume systems.
{Helmholtz} free {energy} [or {Helmholtz function}; prompt on {free energy} or {Helmholtz} or {A}]