A coil of self-inductance `L` is connected in series with a bulb `B` and an `AC` source. Brightness of the bulb decreases when

Options

A. Frequency of the `AC` source is decreased
B. Number of turns in the coil is reduced
C. A capacitance of reactance `X_(C)=X_(L)` is included in the same circuit
D. An iron rod is inserted in the coil

Answer

D

Explanation

`Z=sqrt(R^(2)+X_(L)^(2)) =sqrt(R^(2)+(2pifL)^(2))`
`I=V/R, P=I^(2)R, f uarr Luarr Zuarr Idarr Pdarr`