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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

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a capacitance of reactance XC=XL is included in the same circuit.
an iron rod is inserted in the coil
frequency of the AC source is decreased
number of turns in the coil is reduced

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detailed solution

Correct option is B

The situation is as shown in the figure.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 inductor, leaving less voltage across the bulb. Therefore, the brightness of the light bulb decreases.

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