JEE Main 2025 Jan 29 Shift 1, Physics Q1: LR, CR, LCR Circuits

JEE Main2025Jan 29, Shift 1Physics

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).
D
(A) is true but (R) is false.

Solution

A choke coil placed in series with the tube has impedance $Z = R^{2} + ω^{2} L^{2} .$ The voltage across the resistive tube is

$V_{R} = V_{supply} \cdot \frac{R}{R ^{2} + ω ^{2} L ^{2}},$

which is genuinely less than the applied voltage whenever $ω L > 0.$ Without the choke, the tube would see the full mains voltage and burn out.

So Assertion is correct, Reason is correct, and Reason directly explains why the choke protects the tube.

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JEE Main 2025 Jan 29 Shift 1, Physics Q1: LR, CR, LCR Circuits

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