The technique used above for generating relations between the partial derivatives
is so useful that one wonders if there are other related state functions to which it
can be applied. There are, but the state functions must each have the feature that
its total differential in terms of a pair of independent variables has, as coefficients,
the other thermodynamic variables. There are three additional functions with the
units of energy that have this property. These functions, together with the internal
energy, are referred to as “Thermodynamic Potentials” and have interesting physical
interpretations which will be studied later. For the moment we will concentrate on
their utility in generating derivative relationships.