Question

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

• A. 50 Hz
• B. 20 Hz
• C. 200 Hz
• D. 500 Hz

Answer 1

Answer: (A)

50 Hz

Answer 2

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