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JEE Advanced · Physics · 22. AC Circuits

Paragraph :
The capacitor of capacitance \(C\) can be charged (with the help of a resistance \(R\) ) by a voltage source \(V\), by closing switch \(S_1\) while keeping switch \(S_2\) open. The capacitor can be connected in series with an inductor \(L\) by closing switch \(S_2\) and opening \(S_1\).

Question :
Initially, the capacitor was uncharged. Now, switch \(S_1\) is closed and \(S_2\) is kept open. If time constant of this circuit is \(\tau\), then

  1. A after time interval \(\tau\), charge on the capacitor is \(C V / 2\)
  2. B after time interval \(2 \tau\), charge on the capacitor is \(C V /\left(1-e^{-2}\right)\)
  3. C the work done by the voltage source will be half on the heat dissipated when the capacitor is fully charged
  4. D after time interval \(2 \tau\), charge on the capacitor is \(C V\left(1-e^{-1}\right)\)
Verified Solution

Answer & Solution

Correct Answer

(B) after time interval \(2 \tau\), charge on the capacitor is \(C V /\left(1-e^{-2}\right)\)

Step-by-step Solution

Detailed explanation

Charge on capacitor at time \(t\) is \(q=q_0\left(1-e^{-t / \tau}\right)\)
\(\begin{array}{ll}\text { Here, } & q_0=C V \text { and } t=2 \tau \\ \therefore & q=C V\left(1-e^{-2 \tau / \tau}\right)=C V\left(1-e^{-2}\right)\end{array}\)
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