Question

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

विकल्प

• frequency of the AC source is decreased

• number of turns in the coil is reduced

• a capacitance of reactance X_C = X_L is included in the same circuit

• an iron rod is inserted in the coil

Answer 1

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

Explanation:

`"Impedance is given as, Z" = sqrt("R"^2 + "X"_"L"^2)/("R"^2 + ("L" xx 2 pi "f")^2)`

∴ Impedance lowers as frequency falls. Self inductance and consequently impedance decrease as the number of turns decreases. X_C = X_L at resonance, and impedance falls. Impedance rises when an iron rod is introduced. As a result, the current diminishes. As a result, option (D) is correct.