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Alternating Current question

2022 · 28 Jul · Shift 1 · Q59
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Alternating Current question

2022 · 28 Jul · Shift 1 · Q59

JEE MainPhysicsAlternating CurrentMCQ+4 / −1
The equation of current in a purely inductive circuit is 5sin⁡(49 πt−30∘)5 \sin \left(49\, \pi t-30^{\circ}\right)5sin(49πt−30∘). If the inductance is 30 mH30 \,\mathrm{mH}30mH then the equation for the voltage across the inductor, will be : {\left\{\right.{ Let π=227}\left.\pi=\frac{22}{7}\right\}π=722​}
  1. A
    1.47sin⁡(49πt−30∘)1.47 \sin \left(49 \pi t-30^{\circ}\right)1.47sin(49πt−30∘)
  2. B
    1.47sin⁡(49πt+60∘)1.47 \sin \left(49 \pi t+60^{\circ}\right)1.47sin(49πt+60∘)
  3. C
    23.1sin⁡(49πt−30∘)23.1 \sin \left(49 \pi t-30^{\circ}\right)23.1sin(49πt−30∘)
  4. D
    23.1sin⁡(49πt+60∘)23.1 \sin \left(49 \pi t+60^{\circ}\right)23.1sin(49πt+60∘)
View written solutionFree

Correct answer: D

  1. Given current equation

    i(t)=5sin⁡(49πt−30∘)i(t)=5\sin(49\pi t-30^\circ)i(t)=5sin(49πt−30∘)

    For a pure inductor,

    v=Ldidtv=L\frac{di}{dt}v=Ldtdi​

  2. Differentiate the current

    didt=5⋅49πcos⁡(49πt−30∘)\frac{di}{dt}=5\cdot 49\pi \cos(49\pi t-30^\circ)dtdi​=5⋅49πcos(49πt−30∘)

    Therefore,

    v=L⋅5⋅49πcos⁡(49πt−30∘)v= L\cdot 5\cdot 49\pi \cos(49\pi t-30^\circ)v=L⋅5⋅49πcos(49πt−30∘)

  3. Substitute L=30 mH=30×10−3 HL=30\text{ mH}=30\times 10^{-3}\text{ H}L=30 mH=30×10−3 H

    v=30×10−3×5×49πcos⁡(49πt−30∘)v=30\times 10^{-3}\times 5\times 49\pi \cos(49\pi t-30^\circ)v=30×10−3×5×49πcos(49πt−30∘)

  4. Compute the amplitude

    Using π=227\pi=\dfrac{22}{7}π=722​,

    49π=49⋅227=15449\pi=49\cdot \frac{22}{7}=15449π=49⋅722​=154

    So,

    V0=30×10−3×5×154V_0=30\times 10^{-3}\times 5\times 154V0​=30×10−3×5×154

    V0=0.03×770=23.1V_0=0.03\times 770=23.1V0​=0.03×770=23.1

  5. Convert cosine to sine form

    Since

    cos⁡θ=sin⁡(θ+90∘)\cos\theta=\sin(\theta+90^\circ)cosθ=sin(θ+90∘)

    we get

    v=23.1sin⁡((49πt−30∘)+90∘)v=23.1\sin\big((49\pi t-30^\circ)+90^\circ\big)v=23.1sin((49πt−30∘)+90∘)

    v=23.1sin⁡(49πt+60∘)v=23.1\sin(49\pi t+60^\circ)v=23.1sin(49πt+60∘)

  6. Match with the options

    This corresponds to:

    Option D


Final Answer

23.1sin⁡(49πt+60∘)\boxed{23.1\sin(49\pi t+60^\circ)}23.1sin(49πt+60∘)​

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