Question

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

Options

• an iron rod is inserted in the coil.

• frequency of a.c. 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.

Answer 1

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

Explanation:

When iron rod is inserted, the coil magnetizes the iron rod increasing the magnetic field.

\[\therefore\] Inductance (L) of coil increases.

V = L\[\frac {di}{dt}\]

\[\therefore\] Voltage drop across the coil increases i.e. voltage drop at bulb decrcascs.

\[\therefore\] Brightness of bulb decreases.