,
) is the vector momentum per unit volume of the field. The last of equations (47b) expresses the energy principle;
is the vector flow of energy, and
the energy per unit volume of the field. In fact, we get from (48) by introducing the well-known expressions for the components of the field intensity from electrodynamics,
We conclude from (48) that the energy tensor of the electromagnetic field is symmetrical; with this is connected the fact that the momentum per unit volume and the how of energy are equal to each other (relation between energy and inertia).