The magnetic field energy in an inductor is given by … (i), where L = inductor, i = current.

The graph of alternating current is given by:

From (i), we can see that the magnetic field energy reached its maximum and minimum value when the current is maximum and 0 respectively.

Also, from the graph we can see that the time taken by the current to change from its maximum to zero value is T/4, where T = time period.

Now, from the given problem, = 5 ms = 0.005 s

Hence, time period T = (0.005 x 4) s = 0.02 s

Therefore, frequency of oscillation is equal to = = 50 Hz. (Ans)