A bulb connected in series with a solenoid is lit by an ac source. If a soft iron core is introduced into the solenoid, then
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → the bulb glows dimmer
The bulb and the solenoid share the source voltage. The field of the solenoid magnetises a soft iron core, and the core makes the magnetic field inside stronger. So the inductance L increases, and with it the inductive reactance \( X_L=\omega L \).
Option (1) there is no change — Incorrect. The core changes the inductance of the solenoid, so its reactance and the current in the series circuit change too.
Option (2) the bulb glows brighter — Incorrect. A brighter glow needs a larger current, but the larger reactance in series reduces the current.
Option (3) the bulb glows dimmer — Correct. With a larger \( X_L \), the solenoid takes a larger share of the applied voltage. The bulb gets less voltage, the current falls, and the bulb glows dimmer.
Option (4) the effect cannot be predicted — Incorrect. The iron core always increases the inductance of the solenoid, so the reactance rises and the effect on the bulb is definite.
NCERT (2026-27):
Example 7.5
