To produce the magnetic field of the current , a voltage must be consumed in the circuit, which with the current gives the power , which supplies the stored energy of the magnetic field . This voltage is called the inductance voltage, or voltage consumed by self-induction.
Since no power is required to maintain the field, but power is required to produce it, the inductance voltage must be propor- tional to the increase of the magnetic field:
or by (1),
If and therefore decrease, and therefore are negative; that is, becomes negative, and power is returned into the circuit.
The energy supplied by the power is
hence
is the energy of the magnetic field
of the circuit.
9. Exactly analogous relations exist in the dielectric field.
The dielectric field, or dielectric flux, , is proportional to the voltage , with a proportionality factor, , which is called the capacity of the circuit:
The dielectric field represents stored energy, . To produce it, power, , must, therefore, be supplied by the circuit. Since power is current times voltage,