Question Details

A capacitor of capacitance ‘C’, is connected across an ac source of voltage V, given by V=V0 sinωt The displacement current between the plates of the capacitor, would then be given by :

Options

A

B

C

D

Show Answer

Correct Answer :

Option C

I_d = V_0 \omega C \cos \omega t

Solution :

The correct answer is:
I_d = V_0 \omega C \cos \omega t

Step-by-step derivation:

1. Recall the definition of displacement current (I_d):
Displacement current is defined in terms of the rate of change of electric flux (\Phi_E) between the plates of the capacitor:
I_d = \epsilon_0 \frac{d\Phi_E}{dt}

2. Express electric flux in terms of voltage:
For a parallel-plate capacitor with plate area A and separation distance d, the electric field E between the plates is given by:
E = \frac{V}{d}
The electric flux through the region between the plates is:
\Phi_E = E \cdot A = \frac{V \cdot A}{d}

3. Relate flux to capacitance:
The capacitance C of a parallel-plate capacitor is given by:
C = \frac{\epsilon_0 A}{d}
Rearranging this gives:
\frac{A}{d} = \frac{C}{\epsilon_0}
Substitute this back into the electric flux expression:
\Phi_E = V \left(\frac{C}{\epsilon_0}\right)

4. Substitute flux into the displacement current formula:
I_d = \epsilon_0 \frac{d}{dt} \left( \frac{V C}{\epsilon_0} \right) = C \frac{dV}{dt}

5. Calculate the derivative of the AC voltage source:
The voltage across the AC source is given by:
V = V_0 \sin \omega t
Differentiating V with respect to time t:
\frac{dV}{dt} = \frac{d}{dt} (V_0 \sin \omega t) = V_0 \omega \cos \omega t

6. Find the final displacement current:
Substitute the derivative \frac{dV}{dt} into the equation for I_d:
I_d = C (V_0 \omega \cos \omega t) = V_0 \omega C \cos \omega t

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