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. number of turns in the coil is reduced.
B. a capacitance of reactance `X_(C) = X_(L)` is included in the same circuit.
C. an iron rod is inserted in the coil.
D. frequency of the AC source is decreased.

Answer

C

Explanation

(c )
`Z= sqrt(X_(L)^(2)+R^(2))=sqrt((2pinuL)^(2)+R^(2))`
Brightness of bulb decreases when power consumption decreases .
`P = I_("rms")^(2)R `
`= V_("rms")^(2) ((R )/((2pinuL)^(2))+R^(2))`
Inductance `L prop` n ( number of turns of coil)
If number of turns in the coil is reduced power consumption increases.
If `X_(C)= X_(L) ` then power will be maximum.
If an iron is inserted in the coil L is increased so effective impedance increases. From equation (i) power consumption decreases as `I_("rms)` decreases and as a result brightness of bulb decreases .
If frequency of AC source ios decreased effective impedance will decreases and as a result power consumption increases .So brightness of bulb increases.