A light bulb and an air core inductor are connected to an AC source. The glow of the light bulb increases if
(A) the frequency of an AC source increases
(B) a copper rod is inserted inside the inductor
(C) the frequency of an AC source decreases
(D) an iron rod is inserted inside the inductor
(E) a capacitor of suitable capacitance is connected in series with an inductor such that $X_L = X_C$
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B), (C) and (E) only
Consider a light bulb in series with an air-core inductor connected to an AC source. The glow of the bulb depends on the current in the circuit.
The inductive reactance is $X_L = 2 \pi f L$.
(A) Increasing frequency increases $X_L$, reducing current → bulb glow decreases.
(B) Inserting a copper rod slightly reduces the effective reactance due to eddy currents, allowing more current → bulb glow slightly increases.
(C) Decreasing frequency decreases $X_L$, increasing current → bulb glow increases.
(D) Inserting an iron rod increases $L$, $X_L$ increases → current decreases → bulb glow decreases.
(E) Connecting a capacitor such that $X_L = X_C$ → series resonance → net reactance zero → current maximum → bulb glow maximum.