Question

A coil has an inductance of $5H$ and resistance of $20 \Omega$. An emf of $100V$ is applied to it. What is the energy stored in magnetic form, when current has reached it final value?

• A. 62.5J
• B. 14.8 cal
• C. Both (a) and (b)
• D. None

Answer 1

Answer: (C)

Here $L=5 H , R =20 \Omega, V =100$ volt
When current reaches its final value, role of $L$ vanishes.
maximum current $I_{0}=\frac{V}{R}=\frac{100}{20}=5 A$
$\therefore $ Energy stored in magnetic field $=\frac{1}{2} L I^{2}$
$E=\frac{1}{2} \times 5 \times(5)^{2}=62.5 J$
$E =\frac{62.5}{4.2}$ cal
$E =14.8$ calorie