Electric Charges And Fields

Question

An electromagnet has stored 648 J of magnetic energy, when a current of 9 A exists in its coils.
What average emf is induced if the current is reduced to zero in 0.45 s?

Solution

Given,
Magnetic energy U = 648 J
Current in coil, I = 9A
dI = 9 – 0 = 9A
dt = 0.45 s

Using formula,

U = \frac{1}{2} {LI}^{2}

648 = \frac{1}{2} \times L (9)^{2}

i.e.,

L = \frac{648 \times 2}{9 \times 9} = 16 H

Since,

e = \frac{LdI}{dt}

∴

e = \frac{16 (9)}{0.45} = 320 V .