Question

The magnetic field energy in an inductor changes from maximum value to minimum value in 5 ms when connected to an AC source. The frequency of the source is

• A. $20\text{Hz}$
• B. $50\text{Hz}$
• C. $20\text{Hz}$
• D. $500\text{Hz}$

Answer 1

The correct option is B $50\text{Hz}$

From given
$E_L=\frac{1}{2}L{I}^2$
$I=I_0\sin \omega t$
$E_L=\frac{1}{2}L{I}_0^2{\sin }^2\omega t$

Time period
$T=4\times 5\times {10}^{-3}s$
$T=20\times {10}^{-3}s$
$f=\frac{1}{T}=\frac{1}{20\times {10}^{-3}}$
$f=50Hz$