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← IIT-JEE 2012 · Paper 2
PhysicsSingle correct+3 / -1Question 4 of 60

IIT-JEE 2012 · Paper 2, Physics question 4

In the given circuit, a charge of +80 μC is given to the upper plate of the 4 μF capacitor. Then in the steady state, the charge on the upper plate of the 3 μF capacitor is

A 4 μF capacitor whose upper plate (an isolated terminal) carries +80 μC; its lower plate connects to a parallel combination of 2 μF and 3 μF capacitors whose other ends are earthed.
Choose one option
Answer and worked solution

Official answer: C

Charge distribution. The 4 μF capacitor's upper plate is isolated with +80 μC, so in the steady state its lower plate carries −80 μC. The node joining it to the 2 μF and 3 μF capacitors is isolated and neutral, so the upper plates of those two carry +80 μC in total.

Split. The 2 μF and 3 μF capacitors are in parallel between that node and earth, so the charge divides in proportion to capacitance:
Q3=32+3×80=48 μC.Q_3=\frac{3}{2+3}\times80=48\ \mu\text{C}.

(C)