A coil of self-inductance L is connected in series with a bulb B and an AC source. Brightness of the bulb decreases when (a) a capacitance of reactance XC = XL is included in the same citcuit. (b) an iron rod is inserted in the coil. (c) frequency of the AC source is decreased. (d) number of turns in the coil is reduced.

Answer: (b). Iron rod increases L, so XL and impedance increase and current falls. A capacitor with XC = XL gives resonance (maximum current); lower frequency or fewer turns reduce XL. These brighten the bulb. So the answer is (b) an iron rod is inserted in the coil.

A coil of self-inductance L is connected in series with a bulb B and… | NEET 2013

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RL and RC Series Circuits

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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Answer: (b)

Iron rod increases $L$ , so $X_L$ and impedance increase and current falls.
A capacitor with $X_C = X_L$ gives resonance (maximum current); lower frequency or fewer turns reduce $X_L$ . These brighten the bulb.
So the answer is (b) an iron rod is inserted in the coil.

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