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JEE Main 2013 (22 Apr, Shift 1) · LC Oscillations

LC Oscillations question from JEE Main 2013 (22 Apr, Shift 1)

Q21JEE Main 2013 · 22 APR · SHIFT 1Difficult

LC Oscillations

In a series L - C - R circuit, C=10−11C = 10^{-11} Farad, L=10−5L = 10^{-5} Henry and R=100R = 100 Ohm, when a constant D.C. voltage E is applied to the circuit, the capacitor acquires a charge 10−910^{-9} C. The D.C. source is replaced by a sinusoidal voltage source in which the peak voltage E0E_0 is equal to the constant D.C. voltage E. At resonance the peak value of the charge acquired by the capacitor will be:

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Answer: (d)

  1. DC case: Q=CE⇒E=10−910−11=100 VQ = CE \Rightarrow E = \frac{10^{-9}}{10^{-11}} = 100\text{ V}
  2. At resonance Z=RZ = R: I0=E0R=100100=1 AI_0 = \frac{E_0}{R} = \frac{100}{100} = 1\text{ A}
  3. ω0=1LC=110−16=108 rad/s\omega_0 = \frac{1}{\sqrt{LC}} = \frac{1}{\sqrt{10^{-16}}} = 10^{8}\text{ rad/s}
  4. Q0=I0ω0=10−8 CQ_0 = \frac{I_0}{\omega_0} = 10^{-8}\text{ C}
  5. So the answer is (d) 10−8 C10^{-8}\text{ C}.
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