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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

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

Correct answer: an iron rod is inserted in the coil

Solution

Inserting an iron rod into the coil increases its inductance (L), which increases the inductive reactance (XL = 2πfL). This reduces the current in the circuit, thereby decreasing the brightness of the bulb.

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