Although the reduction in the number of variables is a useful simplification, the main advantage comes from the fact that the Helmholtz free energy is minimized at equilibrium with respect to any unconstrained internal variables for a closed system at constant temperature and volume.
32.
The historically earlier Helmholtz free energy is defined as, where " U " is the internal energy, " T " is the reversible work done on, or obtainable from, a system at constant " T ".
33.
That is, even when the gas phase ( or solid-gas mixture ) has lower Helmholtz free energy than the liquid phase, which is exactly the case below the triple-point pressure, the boiling may be delayed somewhat, see superheating.
34.
In thermodynamic systems where the temperature and volume are held constant, the measure of " useful " work attainable is the Helmholtz free energy; and in systems where the volume is not held constant, the measure of useful work attainable is the Gibbs free energy.
35.
For systems that obey the generalized mass action law the semi-detailed balance condition is sufficient for the dissipation inequality d F / dt \ geq 0 ( for the Helmholtz free energy under isothermal isochoric conditions and for the dissipation inequalities under other classical conditions for the corresponding thermodynamic potentials ).
36.
When I saw the completely flawed articles Helmholtz free energy, Fundamental thermodynamic relation and a few other thermodynamics articles, I could have nominated them for AFD, because they were so flawed that it was actually damaging for students to read those articles, never mind how many wiki rules were violated.
37.
In traditional use, the term " free " was included in " Gibbs free energy " to mean " available in the form of useful work . " ( An analogous, but slightly different, meaning of " free " applies in conjunction with the Helmholtz free energy, for systems at constant temperature ).
38.
In physics, the letter " F " can also be used to denote the Helmholtz energy, as Helmholtz energy is sometimes referred to as the " Helmholtz function, " " Helmholtz free energy, " or simply " free energy " ( not to be confused with Gibbs free energy or free enthalpy ).
39.
At constant temperature and pressure, one must consider the Gibbs free energy, " G ", while at constant temperature and volume, one must consider the Helmholtz free energy : " A ", for the reaction; and at constant internal energy and volume, one must consider the entropy for the reaction : " S ".
40.
Thus if T, V and number of molecules N are constant, the Helmholtz free energy F \ equiv U-TS ( where U is energy ) is minimized as a system " equilibrates . " If T and P are held constant ( say during processes in your body ), the Gibbs free energy G = U + PV-TS is minimized instead.
