Stored Energy

If we are to neglect the resistance of the circuit wire, then there would be no energy loss in maintaining a magnetic field. However, energy is required to establish the field, and it can then be recovered when the field is destroyed.

For a toroid, the induced voltage at any instant is:

where is the number of turns, and the magnetic flux.

If the current at any instant is , then the instantaneous power () is:

The energy () released from the coil in a time is:

or, by considering (1):

The total energy stored in the coil then becomes:

In order to further define the energy stored in a magnetic field, consider a magnetic circuit of length and cross-sectional area , as diagramed in Figure 1.

We know that the magnetic flux density can be defined as:

which leads to:

from which:

Taking into account equations (8) and (5), we obtain the energy stored in the magnetic circuit:

which can also be written as:

We know that if the magnetic field strength is uniform, then:

Taking into account (11), equation (10) becomes:

where () is the volume. Although this equation was proved for a toroid, it can in fact be demonstrated for all magnetic circuits.

For a curve as the one diagramed in Figure 2, is the blue shaded area:

It can be noted that, if there is no magnetic saturation (i.e. the curve is straight), then:

We also know that the magnetic field strength is related to the magnetic flux density with the equation:

or:

where is the magnetic permeability of free space, and the relative magnetic permeability.

Taking into account equations (15), (13), and (12), the energy stored in this particular case becomes: