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

NEET 2013 Physics question

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

(1)frequency of the AC source is decreased
(2)number of turns in the coil is reduced
(3)a capacitance of reactance is included in the same circuit
(4)an iron rod is inserted in the coilCorrect

Answer: (4)

Solution

The current in the series - circuit is and the brightness of the bulb increases with the current. (4) Inserting an iron rod increases the permeability, so increases, increases, the impedance rises and the current falls:

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Question ID VK-091326