A series LCR circuit with L = 0.12 H, C = 480 nF, R = 23Ω 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 the 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?

(a) Given: L = 0.12 H


C = 480 nF


Converting into Farad, we get 480 × 10-9F


R = 23Ω


Voltage = 230 V


Peak voltage can be calculated as follows:


V0 = √2V


Substituting values we get


V0 = 230√2


V0 = 325.22 V


Current flowing through the circuit is given by the following:


I0 =


At resonating frequency, ωR L - 1/ωRC = 0


ωR = 1/√LC


= 1/ √0.12 (H) × 480 × 10-9 (F) = 4166.67 rad/s


Resonant frequency can be calculated using the formula:


vR = ωR/2π


vR = 4166.67(Hz)/2 × 3.14


vR = 663.48 Hz


Maximum current is calculated as follows:


(I0)max = V0/R = 325.22(Hz)/23 = 14.14 A


(b) The maximum power absorbed by the circuit can be calculated as follows:


Pav = 1/2(I0)2maxR


Substituting the values, we get


Pav = 1/2 × (14.14)2(A) × 23 (Ω)


Pav = 2299.33 W


Therefore, the resonating frequency is 663.48 Hz.


(c) The power transferred is equal to the half of the power at the resonating frequency.


Frequency at which power is half = ωR �∆ω


Or = 2π (vR �∆v)


Where ∆ω = R/2L


Substituting the values we get


∆ω = 23/2(Ω) × 0.12(H)


∆ω = 95.83 rad/s


Therefore, change in frequency is written as follows:


∆v = 1/2π × ∆ω


Substituting the values, we get


∆v = 95.83(rad/s)/2π = 15.26 Hz


vR + ∆v is calculated as follows:


663.48(Hz) + 15.26(Hz) = 678.74 Hz


vR-∆v is calculated as follows:


663.48(Hz) – 15.26(Hz) = 648.22 Hz


The current amplitude is calculated as follows:


I’ = 1/√2 × (I0)max


I’ = 14.14(A)/ √2


I’ = 10 A


(d) Q factor can be calculated as follows:


Q = ωRL/R


Q = (4166.67 rad/s) × 0.12H/23Ω


Q = 21.74


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