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Alternating Current question

2010 · Shift 1 · Q69
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Alternating Current question

2010 · Shift 1 · Q69

JEE AdvancedPhysicsAlternating CurrentMCQ+3 / −1
An AC voltage source of variable angular frequency ω\omegaω and fixed amplitude V0 is connected in series with a capacitance C and an electric bulb of resistance R (inductance zero). When ω\omegaω is increased
  1. A
    the bulb glows dimmer.
  2. B
    the bulb glows brighter.
  3. C
    total impedance of the circuit is unchanged.
  4. D
    total impedance of the circuit increases.
View written solutionFree

Correct answer: B

  1. Circuit elements and impedance

    The circuit is a series combination of:

    • resistance RRR (the bulb),
    • capacitance CCC.

    For a series RCRCRC circuit, the capacitive reactance is XC=1ωC.X_C = \frac{1}{\omega C}.XC​=ωC1​.

    Hence the total impedance is Z=R2+XC2=R2+(1ωC)2.Z = \sqrt{R^2 + X_C^2} = \sqrt{R^2 + \left(\frac{1}{\omega C}\right)^2}.Z=R2+XC2​​=R2+(ωC1​)2​.

  2. Effect of increasing ω\omegaω

    As angular frequency ω\omegaω increases, XC=1ωCX_C = \frac{1}{\omega C}XC​=ωC1​ decreases.

    Therefore, Z=R2+XC2Z = \sqrt{R^2 + X_C^2}Z=R2+XC2​​ also decreases.

  3. Current in the circuit

    Since the source amplitude V0V_0V0​ is fixed, the current amplitude is I0=V0Z.I_0 = \frac{V_0}{Z}.I0​=ZV0​​.

    Because ZZZ decreases when ω\omegaω increases, I0I_0I0​ increases.

  4. Brightness of the bulb

    The bulb is purely resistive, so its average power is P=Irms2R.P = I_{\text{rms}}^2 R.P=Irms2​R.

    Since current increases, the power dissipated in the bulb increases. Therefore the bulb glows brighter.

  5. Checking options

    • A: the bulb glows dimmer. ❌ False
    • B: the bulb glows brighter. ✅ True
    • C: total impedance of the circuit is unchanged. ❌ False
    • D: total impedance of the circuit increases. ❌ False

Therefore, the correct option is B.

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