We now return to the general action principle of electrodynamics, (9.13), before the specialization to electrostatics. Recall at the beginning of Chapter 11 we isolated the terms referring to the electromagnetic field, and omitted time derivative terms. In this way we obtained a separation of static electric and magnetic energies, given by (11.2) and (11.3). The former was the basis for our ensuing discussion of electrostatics. The second describes magnetostatics, which is the subject of our investigation here. Notice that this separation is possible only because of the condition