The capacitance C in Equation 1.5 is determined by the geometry of the conductors. If we use Equation 1.1 and the definition that the E field is , it is easy to show that the energy stored in the capacitor is
where V is the volume of space containing the E field and ε is the dielectric constant.
Equation 1.6 is quite general and applies to an E field in a capacitor, in free space, or associated with a transmission line. In most cases the E field varies with position. An integration is needed to calculate the total E field energy in a volume of space.