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Alternating Current question

2021 · 25 Feb · Shift 1 · Q72
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Alternating Current question

2021 · 25 Feb · Shift 1 · Q72

JEE MainPhysicsAlternating CurrentNumerical+4 / −1
A transmitting station releases waves of wavelength 960 m. A capacitor of 2.56 μ\muμ F is used in the resonant circuit. The self inductance of coil necessary for resonance is ‾×\underline{\hspace{2cm}}\times​× 10 −-− 8 H.
Numerical answer
View written solutionFree

Correct answer: 10

  1. Use the relation between wavelength and frequency

For electromagnetic waves, f=cλf = \frac{c}{\lambda}f=λc​ where c=3×108 m/s,λ=960 mc = 3\times 10^8\ \text{m/s}, \qquad \lambda = 960\ \text{m}c=3×108 m/s,λ=960 m

So, f=3×108960=3.125×105 Hzf = \frac{3\times 10^8}{960} = 3.125\times 10^5\ \text{Hz}f=9603×108​=3.125×105 Hz

  1. Condition for resonance of an LC circuit

At resonance, f=12πLCf = \frac{1}{2\pi\sqrt{LC}}f=2πLC​1​

Hence, L=1(2πf)2CL = \frac{1}{(2\pi f)^2 C}L=(2πf)2C1​

Given capacitance: C=2.56 μF=2.56×10−6 FC = 2.56\ \mu F = 2.56\times 10^{-6}\ \text{F}C=2.56 μF=2.56×10−6 F

  1. Substitute the values

Using L=14π2f2CL = \frac{1}{4\pi^2 f^2 C}L=4π2f2C1​

Now, f2=(3.125×105)2=9.765625×1010f^2 = (3.125\times 10^5)^2 = 9.765625\times 10^{10}f2=(3.125×105)2=9.765625×1010

So, L=14π2×9.765625×1010×2.56×10−6L = \frac{1}{4\pi^2 \times 9.765625\times 10^{10} \times 2.56\times 10^{-6}}L=4π2×9.765625×1010×2.56×10−61​

Using π2≈9.87\pi^2 \approx 9.87π2≈9.87, 4π2≈39.484\pi^2 \approx 39.484π2≈39.48

Also, 9.765625×1010×2.56×10−6=2.5×1059.765625\times 10^{10} \times 2.56\times 10^{-6} = 2.5\times 10^59.765625×1010×2.56×10−6=2.5×105

Therefore, L=139.48×2.5×105L = \frac{1}{39.48\times 2.5\times 10^5}L=39.48×2.5×1051​ L=19.87×106L = \frac{1}{9.87\times 10^6}L=9.87×1061​ L≈1.013×10−7 HL \approx 1.013\times 10^{-7}\ \text{H}L≈1.013×10−7 H

  1. Match with the asked form

The inductance is L≈10.13×10−8 HL \approx 10.13\times 10^{-8}\ \text{H}L≈10.13×10−8 H

So the required integer is 10\boxed{10}10​

  1. Comparison with stored answer

Derived answer = 101010

Stored correct answer = 101010

Hence, they agree.

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