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Alternating Current question

2007 · Shift 0 · Q71
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Alternating Current question

2007 · Shift 0 · Q71

JEE MainPhysicsAlternating CurrentMCQ+4 / −1
In an a.c.a.c.a.c. circuit the voltage applied is E=E0 sin⁡ ωt.E = {E_0}\,\sin \,\omega t.E=E0​sinωt. The resulting current in the circuit is I=I0sin⁡(ωt−π2).I = {I_0}\sin \left( {\omega t - {\pi \over 2}} \right).I=I0​sin(ωt−2π​). The power consumption in the circuit is given by
  1. A
    P=2E0I0P = \sqrt 2 {E_0}{I_0}P=2​E0​I0​
  2. B
    P=E0I02P = {{{E_0}{I_0}} \over {\sqrt 2 }}P=2​E0​I0​​
  3. C
    P=zeroP=zeroP=zero
  4. D
    P=E0I02P = {{{E_0}{I_0}} \over 2}P=2E0​I0​​
View written solutionFree

Correct answer: C

  1. Given expressions

    The applied voltage is E=E0sin⁡ωtE = E_0\sin \omega tE=E0​sinωt

    The current is I=I0sin⁡(ωt−π2)I = I_0\sin\left(\omega t - \frac{\pi}{2}\right)I=I0​sin(ωt−2π​)

  2. Identify the phase difference

    Comparing voltage and current:

    • Voltage phase =ωt= \omega t=ωt
    • Current phase =ωt−π2= \omega t - \frac{\pi}{2}=ωt−2π​

    So the current lags the voltage by ϕ=π2\phi = \frac{\pi}{2}ϕ=2π​

  3. Use average power formula in AC circuit

    The average power consumed in an AC circuit is Pavg=ErmsIrmscos⁡ϕP_{\text{avg}} = E_{\text{rms}} I_{\text{rms}} \cos\phiPavg​=Erms​Irms​cosϕ

    where Erms=E02,Irms=I02E_{\text{rms}} = \frac{E_0}{\sqrt{2}}, \qquad I_{\text{rms}} = \frac{I_0}{\sqrt{2}}Erms​=2​E0​​,Irms​=2​I0​​

    Therefore, Pavg=E02⋅I02⋅cos⁡π2P_{\text{avg}} = \frac{E_0}{\sqrt{2}} \cdot \frac{I_0}{\sqrt{2}} \cdot \cos\frac{\pi}{2}Pavg​=2​E0​​⋅2​I0​​⋅cos2π​

    Since cos⁡π2=0\cos\frac{\pi}{2} = 0cos2π​=0

    we get Pavg=0P_{\text{avg}} = 0Pavg​=0

  4. Alternative verification using instantaneous power

    Instantaneous power is p=EI=E0sin⁡ωt⋅I0sin⁡(ωt−π2)p = EI = E_0\sin\omega t \cdot I_0\sin\left(\omega t - \frac{\pi}{2}\right)p=EI=E0​sinωt⋅I0​sin(ωt−2π​)

    Using sin⁡(ωt−π2)=−cos⁡ωt\sin\left(\omega t - \frac{\pi}{2}\right) = -\cos\omega tsin(ωt−2π​)=−cosωt

    p=−E0I0sin⁡ωtcos⁡ωtp = -E_0I_0\sin\omega t\cos\omega tp=−E0​I0​sinωtcosωt

    Over one complete cycle, the average of sin⁡ωtcos⁡ωt\sin\omega t\cos\omega tsinωtcosωt is zero, so average power is again P=0P=0P=0

  5. Check options

    • A: P=2E0I0P=\sqrt{2}E_0I_0P=2​E0​I0​ ❌
    • B: P=E0I02P=\dfrac{E_0I_0}{\sqrt{2}}P=2​E0​I0​​ ❌
    • C: P=zeroP=\text{zero}P=zero ✅
    • D: P=E0I02P=\dfrac{E_0I_0}{2}P=2E0​I0​​ ❌

Hence, the correct option is C.

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