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

Answer: (a). Brightness falls when the net reactance, and hence impedance, increases. Iron rod increases L, so XL = ω L increases and current decreases. Lower frequency or fewer turns reduce XL; adding a capacitor cancels part of XL. All these brighten the bulb. So the answer is (a) an iron rod is inserted in the coil.

A coil of self-inductance L is connected in series with a bulb and an… | MHT CET 2025 (20 Apr, Shift 1)

Q18MHT CET 2025 · 20 APR · SHIFT 1Easy

RL and RC Series Circuits

A coil of self-inductance $L$ is connected in series with a bulb and an a. c. source. Brightness of the bulb decreases when

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Answer: (a)

Brightness falls when the net reactance, and hence impedance, increases.
Iron rod increases $L$ , so $X_L = \omega L$ increases and current decreases.
Lower frequency or fewer turns reduce $X_L$ ; adding a capacitor cancels part of $X_L$ . All these brighten the bulb.
So the answer is (a) an iron rod is inserted in the coil.

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