Page 79 - Basic Electrical Engineering

We know,

or,

Substituting values,

1.18 ENERGY STORED IN A MAGNETIC FIELD

Let us consider a coil supplied with an alternating voltage v due to which an
alternating current flows through the coil. When current increases from its

zero value, the magnetic field starts increasing and reaches its maximum
value when current reaches its maximum value. When current starts

decreasing, the field goes on decreasing and gradually becomes zero. Then,
in the negative cycle if the current flows, the field gets established in the
opposite direction, which collapses when current again reaches zero. This

way the field is established and then collapses in every consecutive half cycle
of current flow. When the field is established, energy in the form of a

magnetic field is stored and when the field collapses, the same energy is
returned to the supply source. As such, no energy is consumed by the purely

inductive coil. Therefore, energy stored is equal to the energy supplied.

Energy stored,

W = νi dt

Induced EMF in the coil,