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

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

Electromagnetic Induction

Solution:

Magnetic field energy of a coil is given by $\frac{1}{2} LI^2$ In this case I changes from maximum to zero value in 5 ms $i.e.$ in T/4 ms $\therefore $ T/4 = 5 ms or T = 5 $\times$ 4 = 20 ms = $20 \times 10^{-3}$ s
Then, frequency $\nu = \frac{1}{T} = \frac{1}{20 \times 10^{-3}}$ = 50 Hz