A coil and a bulb are connected in series with a dc source. When a soft iron piece is inserted in the coil. Then after some time,
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Intensity of the bulb remains the same
Inserting soft iron increases the self inductance of the coil. By Lenz’s law this inductance opposes any change in the current, so the current is held back for a short while as it settles to its new value, and the bulb dims for a moment.
In the steady state the current is no longer changing, so the induced emf \( L\,dI/dt \) is zero and the inductance has no effect. The current is then fixed only by the resistance of the circuit.
\( \displaystyle I = \frac{V}{R_{\mathrm{coil}} + R_{\mathrm{bulb}}} \)
The iron changes L but not the resistance of the wire, so after some time the current, and hence the brightness of the bulb, is the same as before the iron was inserted.