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 $L=5H,R=20\Omega ,V=100$ volt
When current reaches its final value, role of $L$ vanishes.
maximum current $I_0=\frac{V}{R}=\frac{100}{20}=5A$
$\therefore$ Energy stored in magnetic field $=\frac{1}{2}L{I}^2$
$E=\frac{1}{2}\times 5\times (5{)}^2=62.5J$
$E=\frac{62.5}{4.2}$ cal
$E=14.8$ calorie