The magnetic field energy in an inductor changes from maximum value to minimum value in `5.0 ms` when connected to an AC source. The frequency of the source is

Options

A. `20HZ`
B. `50HZ`
C. `200HZ`
D. `500HZ`

Answer

B

Explanation

`U = (1)/(2) Li^(2)`
U is maximum when I is maximu `= i_(0)`
U is minimum when `I =0`
Time taken by current to change its value from
`I = i_(0) to I = 0 is T//4`
`(T)/(4) = 5 xx 10^(-3) implies T = 0.02 sec`
`f = (1)/(T) = (1)/(0.02) =50` Hz .