IIT-JEE 2011 · Paper 2 · Physics Q30

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PhysicsMultiple correct+4Question 30 of 60

A series R-C circuit is connected to AC voltage source. Consider two cases; (A) when C is without a dielectric medium and (B) when C is filled with dielectric of constant 4. The current $I_R$ through the resistor and voltage $V_C$ across the capacitor are compared in the two cases. Which of the following is/are true?

I_R^A>I_R^B

I_R^A<I_R^B

V_C^A>V_C^B

V_C^A<V_C^B

Answer and worked solution

Official answer: B, C

Effect of the dielectric. $C$ becomes $4C$ , so $X_C=\frac1{\omega C}$ falls to a quarter.

Current. $I=\dfrac V{\sqrt{R^2+X_C^2}}$ increases, so $I_R^A<I_R^B$ (B).

Capacitor voltage. $V_C=\dfrac{VX_C}{\sqrt{R^2+X_C^2}}=\dfrac V{\sqrt{R^2/X_C^2+1}}$ decreases as $X_C$ falls, so $V_C^A>V_C^B$ (C).

(B), (C)

Topics:Reactance and impedance Dielectrics

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IIT-JEE 2011 · Paper 2, Physics question 30