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Alternating Current question

2017 · 9 Apr · Shift 1 · Q58
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Alternating Current question

2017 · 9 Apr · Shift 1 · Q58

JEE MainPhysicsAlternating CurrentMCQ+4 / −1
A sinusoidal voltage of peak value 283 V and angular frequency 320/s is applied to a series LCR circuit. Given that R=5 Ω\OmegaΩ, L=25 mH and C=1000 μ\muμ F. The total impedance, and phase difference between the voltage across the source and the current will respectively be :
  1. A
    10 Ω\OmegaΩ and tan −-− 1 (53)\left( {{5 \over 3}} \right)(35​)
  2. B
    7 Ω7\,\Omega7Ω and 45o
  3. C
    10 Ω10\,\Omega10Ω and tan −-− 1 (83)\left( {{8 \over 3}} \right)(38​)
  4. D
    7 Ω7\,\Omega7Ω and tan −-− 1 (53)\left( {{5 \over 3}} \right)(35​)
View written solutionFree

Correct answer: B

  1. Given data
  • Resistance: R=5 ΩR = 5\,\OmegaR=5Ω
  • Inductance: L=25 mH=25×10−3 HL = 25\text{ mH} = 25 \times 10^{-3}\text{ H}L=25 mH=25×10−3 H
  • Capacitance: C=1000 μF=1000×10−6 F=10−3 FC = 1000\,\mu\text{F} = 1000 \times 10^{-6}\text{ F} = 10^{-3}\text{ F}C=1000μF=1000×10−6 F=10−3 F
  • Angular frequency: ω=320 s−1\omega = 320\,\text{s}^{-1}ω=320s−1

We need the total impedance and phase difference for a series LCR circuit.


  1. Inductive reactance

XL=ωL=320×25×10−3=8 ΩX_L = \omega L = 320 \times 25 \times 10^{-3} = 8\,\OmegaXL​=ωL=320×25×10−3=8Ω


  1. Capacitive reactance

XC=1ωC=1320×10−3=10.32=3.125 ΩX_C = \frac{1}{\omega C} = \frac{1}{320 \times 10^{-3}} = \frac{1}{0.32} = 3.125\,\OmegaXC​=ωC1​=320×10−31​=0.321​=3.125Ω


  1. Net reactance

For a series LCR circuit,

X=XL−XC=8−3.125=4.875 ΩX = X_L - X_C = 8 - 3.125 = 4.875\,\OmegaX=XL​−XC​=8−3.125=4.875Ω

Since XL>XCX_L > X_CXL​>XC​, the circuit is overall inductive.


  1. Total impedance

Z=R2+(XL−XC)2Z = \sqrt{R^2 + (X_L - X_C)^2}Z=R2+(XL​−XC​)2​

Z=52+4.8752=25+23.765625=48.765625≈6.98 ΩZ = \sqrt{5^2 + 4.875^2} = \sqrt{25 + 23.765625} = \sqrt{48.765625} \approx 6.98\,\OmegaZ=52+4.8752​=25+23.765625​=48.765625​≈6.98Ω

So,

Z≈7 ΩZ \approx 7\,\OmegaZ≈7Ω


  1. Phase difference

For a series LCR circuit,

tan⁡ϕ=XL−XCR=4.8755=0.975\tan \phi = \frac{X_L - X_C}{R} = \frac{4.875}{5} = 0.975tanϕ=RXL​−XC​​=54.875​=0.975

Thus,

ϕ=tan⁡−1(0.975)≈44.3∘≈45∘\phi = \tan^{-1}(0.975) \approx 44.3^\circ \approx 45^\circϕ=tan−1(0.975)≈44.3∘≈45∘

So the voltage leads the current by about 45∘45^\circ45∘.


  1. Check options
  • A: 10 Ω10\,\Omega10Ω and tan⁡−1(5/3)\tan^{-1}(5/3)tan−1(5/3) → incorrect impedance
  • B: 7 Ω7\,\Omega7Ω and 45∘45^\circ45∘ → matches
  • C: 10 Ω10\,\Omega10Ω and tan⁡−1(8/3)\tan^{-1}(8/3)tan−1(8/3) → incorrect
  • D: 7 Ω7\,\Omega7Ω and tan⁡−1(5/3)\tan^{-1}(5/3)tan−1(5/3) → incorrect phase angle

  1. Final answer

The total impedance and phase difference are:

7 Ω and 45∘\boxed{7\,\Omega \text{ and } 45^\circ}7Ω and 45∘​

So the correct option is B.

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