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JEE Advanced 2020 Paper 1 · Physics · Electromagnetic Induction and Alternating Current · medium

A battery of emf 12 V and negligible internal resistance is connected in series with a resistor R1=2 kΩ and a parallel combination of a resistor R2=4…

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A battery of emf 12 V12\ \mathrm{V} and negligible internal resistance is connected in series with a resistor R1=2 kΩR_1=2\ \mathrm{k\Omega} and a parallel combination of a resistor R2=4 kΩR_2=4\ \mathrm{k\Omega} and an initially uncharged capacitor C=10 μFC=10\ \mu\mathrm{F}. The circuit is closed at t=0t=0. Which of the following statements is/are correct?
  1. A. The battery current immediately after closing the circuit is 6 mA6\ \mathrm{mA}.
  2. B. The final charge on the capacitor is 80 μC80\ \mu\mathrm{C}.
  3. C. The time constant of the charging is 60 ms60\ \mathrm{ms}.
  4. D. The final energy stored in the capacitor is 0.72 mJ0.72\ \mathrm{mJ}.
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Answer: A, B

At t=0t=0 the uncharged capacitor behaves like a wire across R2R_2, so I0=12/R1=6I_0=12/R_1=6 mA, and (A) is true. In the steady state no current enters the capacitor; the current is 12/(2+4) kΩ=212/(2+4)\ \mathrm{k\Omega}=2 mA and the capacitor voltage equals the drop across R2R_2, 2 mA×4 kΩ=82\ \mathrm{mA}\times4\ \mathrm{k\Omega}=8 V, so Q=CV=80 μCQ=CV=80\ \mu\mathrm{C} and (B) is true. The time constant uses the Thevenin resistance seen by the capacitor, R1∥R2=43 kΩR_1\parallel R_2=\frac43\ \mathrm{k\Omega}, giving τ=13.3\tau=13.3 ms, not 6060 ms (C is false). Final energy is 12CV2=12×10×10−6×64=0.32\frac12CV^2=\frac12\times10\times10^{-6}\times64=0.32 mJ, and 0.720.72 mJ would correspond to the full 1212 V (D is false).
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