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Alternating Current question

2010 · Shift 0 · Q65
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Alternating Current question

2010 · Shift 0 · Q65

JEE MainPhysicsAlternating CurrentMCQ+4 / −1
In a series LCRLCRLCR circuit R=200ΩR = 200\OmegaR=200Ω and the voltage and the frequency of the main supply is 220V220V220V and 50Hz50Hz50Hz respectively. On taking out the capacitance from the circuit the current lags behind the voltage by 30∘.{30^ \circ }.30∘. On taking out the inductor from the circuit the current leads the voltage by 30∘.{30^ \circ }.30∘. The power dissipated in the LCRLCRLCR circuit is
  1. A
    305W305W305W
  2. B
    210W210W210W
  3. C
    zeroWzeroWzeroW
  4. D
    242W242W242W
View written solutionFree

Correct answer: D

  1. Given data
  • Resistance: R=200 ΩR = 200\,\OmegaR=200Ω
  • Supply voltage: V=220 VV = 220\,\text{V}V=220V
  • Frequency: 50 Hz50\,\text{Hz}50Hz

We need the power dissipated in the series LCRLCRLCR circuit.

For a series LCRLCRLCR circuit, P=I2R=VIcos⁡ϕP = I^2 R = VI\cos\phiP=I2R=VIcosϕ where ϕ\phiϕ is the phase angle of the complete circuit.


  1. Case 1: Capacitor removed

Then the circuit becomes a series LRLRLR circuit.

Given: current lags voltage by 30∘30^\circ30∘. For an LRLRLR circuit, tan⁡ϕL=XLR\tan \phi_L = \frac{X_L}{R}tanϕL​=RXL​​ So, tan⁡30∘=XLR\tan 30^\circ = \frac{X_L}{R}tan30∘=RXL​​ 13=XL200\frac{1}{\sqrt{3}} = \frac{X_L}{200}3​1​=200XL​​ Hence, XL=2003 ΩX_L = \frac{200}{\sqrt{3}}\,\OmegaXL​=3​200​Ω


  1. Case 2: Inductor removed

Then the circuit becomes a series CRCRCR circuit.

Given: current leads voltage by 30∘30^\circ30∘. For an RCRCRC circuit, tan⁡ϕC=XCR\tan \phi_C = \frac{X_C}{R}tanϕC​=RXC​​ (using magnitude of phase angle) Thus, tan⁡30∘=XCR\tan 30^\circ = \frac{X_C}{R}tan30∘=RXC​​ 13=XC200\frac{1}{\sqrt{3}} = \frac{X_C}{200}3​1​=200XC​​ Hence, XC=2003 ΩX_C = \frac{200}{\sqrt{3}}\,\OmegaXC​=3​200​Ω


  1. Now consider the complete LCRLCRLCR circuit

Net reactance in series LCRLCRLCR circuit is X=XL−XCX = X_L - X_CX=XL​−XC​ But here, XL=XC=2003X_L = X_C = \frac{200}{\sqrt{3}}XL​=XC​=3​200​ So, X=0X = 0X=0

Therefore the circuit is in resonance, and the impedance is purely resistive: Z=R=200 ΩZ = R = 200\,\OmegaZ=R=200Ω

Hence current is I=VR=220200=1.1 AI = \frac{V}{R} = \frac{220}{200} = 1.1\,\text{A}I=RV​=200220​=1.1A


  1. Power dissipated

Since the circuit is purely resistive at resonance, cos⁡ϕ=1\cos\phi = 1cosϕ=1 Therefore, P=I2R=(1.1)2×200P = I^2R = (1.1)^2 \times 200P=I2R=(1.1)2×200 P=1.21×200=242 WP = 1.21 \times 200 = 242\,\text{W}P=1.21×200=242W


  1. Option check
  • A: 305 W305\,\text{W}305W — incorrect
  • B: 210 W210\,\text{W}210W — incorrect
  • C: 0 W0\,\text{W}0W — incorrect
  • D: 242 W242\,\text{W}242W — correct

Final Answer: 242 W\boxed{242\,\text{W}}242W​

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