When temperature T is fixed, free energy ( T × × A ) is also minimized. Thus if T , V and number of molecules N are constant, the Helmholtz free energy F ≡ ≡ U − − T S (where U is energy and S is entropy) 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 + P V − − T S is minimized instead. The change in free energy under these conditions is a measure of available work that might be done in the process. Thus available work for an ideal gas at constant temperature T o and pressure P o is W = Δ Δ G = N k T o Θ Θ ( V / V o ) where V o = N k T o / P o and Θ Θ ( x ) = x − − 1 − − ln ⁡ ⁡ x ≥ ≥ 0 (see also Gibbs inequality).