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Alternating Current question

2016 · Shift 0 · Q43
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Alternating Current question

2016 · Shift 0 · Q43

JEE MainPhysicsAlternating CurrentMCQ+4 / −1
An arc lamp requires a direct current of 10 A at 80 V to function. If it is connected to a 220 V (rms), 50 Hz AC supply, the series inductor needed for it to work is close to :
  1. A
    0.044 H
  2. B
    0.065 H
  3. C
    80 H
  4. D
    0.08 H
View written solutionFree

Correct answer: B

  1. Lamp rating and required operating conditions

The arc lamp needs:

  • Current: I=10 AI = 10\,\text{A}I=10A
  • Voltage across lamp: VR=80 VV_R = 80\,\text{V}VR​=80V

Since it works on direct current at 80 V80\,\text{V}80V and 10 A10\,\text{A}10A, we treat the lamp as an effective resistance:

R=VRI=8010=8 ΩR = \frac{V_R}{I} = \frac{80}{10} = 8\,\OmegaR=IVR​​=1080​=8Ω

  1. AC supply and series inductor

The lamp is connected to:

V=220 V (rms),f=50 HzV = 220\,\text{V (rms)}, \quad f = 50\,\text{Hz}V=220V (rms),f=50Hz

A pure inductor is connected in series so that the current remains 10 A10\,\text{A}10A.

For a series RRR-LLL circuit, the impedance is:

Z=VI=22010=22 ΩZ = \frac{V}{I} = \frac{220}{10} = 22\,\OmegaZ=IV​=10220​=22Ω

Also,

Z=R2+XL2Z = \sqrt{R^2 + X_L^2}Z=R2+XL2​​

where XL=ωLX_L = \omega LXL​=ωL.

So,

222=82+XL222^2 = 8^2 + X_L^2222=82+XL2​

484=64+XL2484 = 64 + X_L^2484=64+XL2​

XL2=420X_L^2 = 420XL2​=420

XL=420≈20.49 ΩX_L = \sqrt{420} \approx 20.49\,\OmegaXL​=420​≈20.49Ω

  1. Find inductance

Angular frequency:

ω=2πf=2π×50=100π rad/s\omega = 2\pi f = 2\pi \times 50 = 100\pi\,\text{rad/s}ω=2πf=2π×50=100πrad/s

Thus,

L=XLω=20.49100πL = \frac{X_L}{\omega} = \frac{20.49}{100\pi}L=ωXL​​=100π20.49​

L≈20.49314≈0.065 HL \approx \frac{20.49}{314} \approx 0.065\,\text{H}L≈31420.49​≈0.065H

  1. Match with options

The closest option is:

0.065 H\boxed{0.065\,\text{H}}0.065H​

So the correct option is B.

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