1.

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

Answer» Correct Answer - B
`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 .


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