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Alternating Current

Question
CBSEENPH12047939

In an LCR circuit as shown in figure, both switches are open initially. Now switch S1 is closed, S2 kept open. (q is charge on the capacitor and τ = RC is capacitive time constant). Which of the following statement is correct?       

                                

  • At t = τ2, q = CV ( 1 - e-1 )

  • Work done by the battery is half of the energy dissipated in the resistor

  • At t = τ, q = CV/2

  • At t = τ, q = CV ( 1 - e-2 )

Solution

D.

At t = τ, q = CV ( 1 - e-2 )

As switch S1 is closed and switch S2 is kept open. Now, capacitor is charging through a resistor R. Charge on a capacitor at any time t is

        q = q0 ( 1 - e-t/τ

And  q0 = CV

 At  t = τ2,

      q = CV ( 1 - e-t/2τ )

          = CV ( 1 - e-1/2 )

At t = τ

      q = CV ( 1 - e-τ/τ )

          = CV ( 1 - e-1 )

At t = 2τ

      q = CV ( 1 - e-2t /τ )

          = CV ( 1 - e-2 )