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?

• A. 320 V
• B. 620 V
• C. 260 V
• D. 230 V

Answer 1

Answer: (A)

320 V

Answer 2

Magnetic energy $\mathrm { U } = \frac { 1 } { 2 } \mathrm { LI } ^ { 2 }$

$\therefore \mathrm { L } = \frac { 2 \mathrm { U } } { \mathrm { I } ^ { 2 } } = \frac { 2 \times 648 } { ( 9 ) ^ { 2 } } = 16 \mathrm { H }$

Induced emf,

$\varepsilon = - \mathrm { L } \frac { \mathrm { dI } } { \mathrm { dt } } = \frac { - ( 16 \mathrm { H } ) ( 0 - 9 \mathrm {~A} ) } { 0.45 \mathrm {~s} } = 320 \mathrm {~V}$