(also work function), one of the characteristic functions of a thermodynamic system. Denoted by A, it is defined in terms of the internal energy U, the entropy S, and the temperature T as
A = U - TS
The Helmholtz free energy is a thermodynamic potential; in its properties it is analogous to the Gibbs free energy, but in contrast to it, leads to simple relationships for processes occurring at constant temperature and volume, limiting its range of use. In an isothermal, equilibrium process at a constant volume, the decrease in the Helmholtz free energy of a given system is equal to the total work performed by the system during this process.
The Helmholtz free energy, like the Gibbs free energy, used to be called a free energy and used to be denoted by F. To distinguish it from the Gibbs free energy, it was sometimes called the free energy at constant volume. The Soviet literature also uses the term izokhorno-izotermicheskii potentsial (shortened to izokhornyi potentsial). The term “the Helmholtz free energy” and the symbol A were accepted at the 18th Congress of the International Union of Pure and Applied Chemistry in 1961.
V. A. KIREEV