Two identical capacitors A and B, charged to the same potential 5V… | JEE Main 2020 (9 Jan, Shift 2)

Q85JEE Main 2020 · 9 JAN · SHIFT 2Difficult

P-N Junction Diode

Two identical capacitors A and B, charged to the same potential 5V are connected in two different circuits as shown below at time t = 0. If the charge on capacitors A and B at time t = CR is QA and QB respectively, then (Here e is the base of natural logarithm)

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Answer: (c)

The capacitor discharges by sending current out of its $+$ plate, through $R$ , and back through the diode to the $-$ plate (upward through the diode).
In circuit A the diode points downward, so this current reverse biases it: no discharge, $Q_A = CV$ .
In circuit B the diode points upward, so it is forward biased and $C$ discharges through $R$ : $Q = CV e^{-t/RC}$ .
At $t = RC$ : $Q_B = \frac{CV}{e}$ . So the answer is (c).

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P-N Junction Diode question from JEE Main 2020 (9 Jan, Shift 2)

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