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Alternating Current question

2024 · 1 Feb · Shift 1 · Q71
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Alternating Current question

2024 · 1 Feb · Shift 1 · Q71

JEE MainPhysicsAlternating CurrentMCQ+4 / −1
A parallel plate capacitor has a capacitance C=200 pF\mathrm{C}=200~ \mathrm{pF}C=200 pF. It is connected to 230 V230 \mathrm{~V}230 V ac supply with an angular frequency 300 rad/s300~ \mathrm{rad} / \mathrm{s}300 rad/s. The rms value of conduction current in the circuit and displacement current in the capacitor respectively are :
  1. A
    14.3 μA14.3 ~\mu \mathrm{A}14.3 μA and 143 μA143 ~\mu \mathrm{A}143 μA
  2. B
    13.8 μA13.8 ~\mu \mathrm{A}13.8 μA and 13.8 μA13.8 ~\mu \mathrm{A}13.8 μA
  3. C
    13.8 μA13.8 ~\mu \mathrm{A}13.8 μA and 138 μA138 ~\mu \mathrm{A}138 μA
  4. D
    1.38 μA1.38 ~\mu \mathrm{A}1.38 μA and 1.38 μA1.38 ~\mu \mathrm{A}1.38 μA
View written solutionFree

Correct answer: B

  1. Given data
  • Capacitance: C=200 pF=200×10−12 F=2×10−10 FC = 200\,\text{pF} = 200 \times 10^{-12}\,\text{F} = 2 \times 10^{-10}\,\text{F}C=200pF=200×10−12F=2×10−10F
  • RMS voltage: Vrms=230 VV_{\text{rms}} = 230\,\text{V}Vrms​=230V
  • Angular frequency: ω=300 rad s−1\omega = 300\,\text{rad s}^{-1}ω=300rad s−1

We need the rms conduction current in the circuit and displacement current in the capacitor.


  1. Current in a capacitor in AC

For a capacitor connected to AC,

Irms=Vrms⋅ωCI_{\text{rms}} = V_{\text{rms}} \cdot \omega CIrms​=Vrms​⋅ωC

because capacitive reactance is

XC=1ωCX_C = \frac{1}{\omega C}XC​=ωC1​

and hence

Irms=VrmsXC=Vrms ωCI_{\text{rms}} = \frac{V_{\text{rms}}}{X_C} = V_{\text{rms}}\,\omega CIrms​=XC​Vrms​​=Vrms​ωC

So,

Irms=230×300×2×10−10I_{\text{rms}} = 230 \times 300 \times 2 \times 10^{-10}Irms​=230×300×2×10−10

=230×6×10−8= 230 \times 6 \times 10^{-8}=230×6×10−8

=1380×10−8= 1380 \times 10^{-8}=1380×10−8

=1.38×10−5 A= 1.38 \times 10^{-5}\,\text{A}=1.38×10−5A

=13.8×10−6 A= 13.8 \times 10^{-6}\,\text{A}=13.8×10−6A

=13.8 μA= 13.8\,\mu\text{A}=13.8μA


  1. Conduction current and displacement current

In a capacitor circuit:

  • The current in the connecting wires is the conduction current.
  • Between the plates, the equivalent current is the displacement current.

By Maxwell’s theory, in a capacitor these two are equal at every instant, and therefore their rms values are also equal.

Hence,

Iconduction,rms=Idisplacement,rms=13.8 μAI_{\text{conduction,rms}} = I_{\text{displacement,rms}} = 13.8\,\mu\text{A}Iconduction,rms​=Idisplacement,rms​=13.8μA


  1. Checking options
  • A: 14.3 μA14.3\,\mu\text{A}14.3μA and 143 μA143\,\mu\text{A}143μA ❌
  • B: 13.8 μA13.8\,\mu\text{A}13.8μA and 13.8 μA13.8\,\mu\text{A}13.8μA ✅
  • C: 13.8 μA13.8\,\mu\text{A}13.8μA and 138 μA138\,\mu\text{A}138μA ❌
  • D: 1.38 μA1.38\,\mu\text{A}1.38μA and 1.38 μA1.38\,\mu\text{A}1.38μA ❌

So the correct option is B.

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