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A coil of self-inductance L is connected in series with a bulb B and an AC source. Brightness of the bulb decreases when
Frequency of the AC source is decreased
Number of turns in the coil is reduced
A capacitance of reactance $X_{C} = X_{L}$ is included in the same circuit
An iron rod is inserted in the coil

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

Frequency of the AC source is decreased

D

An iron rod is inserted in the coil

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Solution

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The correct option is D An iron rod is inserted in the coil
As the iron rod is inserted, the magnetic field inside the coil magnetizes the iron increasing the magnetic field inside it. Hence. the inductance of the coil increases. Consequently, the inductive reactance of the coil increases. As a result, a larger fraction of the applied AC voltage appears across the lnductor, leaving less voltage across the bulb. Therefore' the brightness of the light bulb decreases.

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