SSC JE Electrical Engineering 2014 Set 2 Paper

Magnetic field energy in an inductor changes from Maximum value to minimum value in 5 m sec.In an Ac circuit, Current goes from the maximum value to zero in T/4 sec where T is the time period of As.
$\frac{\mathit{T}}{4}=5\Rightarrow \mathit{T}=20$ms
$\mathrm{f}=\frac{1}{\mathit{T}}=\frac{1}{20\times {10}^{-3}}=50$ Hz