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A series LCR circuit with L = 0.12 H, C = 480 nF, R = 23 Ω is connected to a 230 V variable frequency supply. (a) What is the source frequency for which current amplitude is maximum? Obtain this maximum value. (b) What is the source frequency for which average power absorbed by the circuit is maximum? Obtain die value of this maximum power. (c) For which frequencies of the source is the power transferred to the circuit half the power at resonant frequency? What is the current amplitude at these frequencies? (d) What is the Q-factor of the given circuit?

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Class 12 Physics
CBSE and UP Board
Alternating Current
Chapter-7 Exercise 7.20
Additional Exercise

NCERT Solutions for Class 12 Physics Chapter 7 Question 20

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1 Answer

  1. Inductance, L = 0.12 H

    Capacitance, C = 480 nF = 480 x 10⁻9 F

    Resistance, R = 23 Ω

    Supply voltage, V = 230 V

    Peak voltage is given as: Vo =√2 x 230 = 325.22 V

    Ans (a).

    Current flowing in the circuit is given by the relation,

    I0=  V0/√[(R2 + (ωL- 1/ωC)2]

    Where, Io = maximum at resonance

    At resonance, we have

    ωR– 1/ωRC =0

    Where, ω= Resonance angular frequency

    Where , ωR = Resonance angular frequency

    Therefore ωR = 1/√(LC) = 1/ √ (0.12 x 480 x 10⁻9 ) = 4166.67 rad/s

    Resonant frequency , νR = ωR/2π = 4166.67 /(2 x 3.14) = 663.48 Hz

    And ,maximum current,   (I0)Max  =   V0/R = 325.22/23 = 14.14 A

    Ans (b).

    Maximum average power absorbed by the circuit is given as:

     (Pav)Max= 1/2 x  (I0MaxR

    = 1/2 x (14.14)² x 23 = 2299.3 W

    Hence, resonant frequency is 663.48 Hz.

    Ans (c).

    The power transferred to the circuit is half the power at resonant frequency.

    Frequencies at which power transferred is half,

    =  ωR ± Δω

    = 2π (νR ± Δν)

    Where,

    Δω=R/2L= 23/(2 x 0.12)= 95.83 rad/s

    Hence, change in frequency,

    Δν = 1/2π    x    Δω  =95.83 /2π = 15.26 Hz

    Therefore

    νR +Δν  = 663.48 + 15.26 = 678.74 Hz

    and νR – Δν  = 663.48 – 15.26 =648.22 Hz

    Hence, at 648.22 Hz and 678.74 Hz frequencies, the power transferred is half. At these frequencies, current amplitude can be given as:

    I’ = 1/√ 2 x (I0)Max

    = 14.14/√ 2 = 10A

    Ans (d).

    Q-factor of the given circuit can be obtained using the relation,

    Q = ωR L/R  = 4166.67 x 0.12 /23 = 21.74

    Hence, the Q-factor of the given circuit is 21.74.

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