Alternating Current — JEE Main · 2025 · 29 Jan 2025 · Shift 1 Question with Solution

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Choke coil is simply a coil having a large inductance but a small resistance. Choke coils are used with fluorescent mercury-tube fittings. If household electric power is directly connected to a mercury tube, the tube will be damaged.
Reason (R) : By using the choke coil, the voltage across the tube is reduced by a factor

(R / R^{2} + ω^{2} L^{2})

, where

ω

is frequency of the supply across resistor R and inductor L . If the choke coil were not used, the voltage across the resistor would be the same as the applied voltage.
In the light of the above statements, choose the most appropriate answer from the options given below:

(A)Both (A) and (R) are true but (R) is not the correct explanation of (A).
(B)(A) is false but (R) is true.
(C)Both (A) and (R) are true and (R) is the correct explanation of (A).✓ Correct
(D)(A) is true but (R) is false.

Answer: (C)

Solution

A choke coil has large inductance

L

and small resistance

R

, and is connected in series with a fluorescent tube (which behaves resistively). At supply frequency

ω

, the coil's impedance

R^{2} + ω^{2} L^{2}

is large, so most of the applied voltage drops across the inductor and only a small fraction appears across the tube:

V_{tube} = V \cdot \frac{R}{R ^{2} + ω ^{2} L ^{2}}

If the choke were absent, the tube (resistance

R

) would be connected directly across the supply, and the entire applied voltage would appear across it — which can damage it. Both statements are true, and the reason correctly explains why the choke coil protects the tube.

JEE Main · 2025 · 29 Jan 2025 · Shift 1 · Single correct option

JEE Main · 2025 · 29 Jan 2025 · Shift 1 · Alternating Current

Solution

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