Noun
Helmholtz free energy
(physics, thermodynamics) A measure of the useful work obtainable from a constant temperature, constant volume thermodynamic system; more precisely, the difference between internal energy (of a system) and the product of its absolute temperature and entropy.
A
=
U
−
T
S
{\displaystyle A=U-TS}
where A is Helmholtz free energy, U is internal energy, T is temperature, and S is entropy.
AB is the "available energy" (now called the Helmholtz free energy ) and AC the "capacity for entropy" (i. Source: Internet