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Question #89433

Difficulty: 5/10 | 🟢 Free

The Question

A semi-infinite distributed capacitive network is constructed along the xx-axis from x=Lx = L to x→∞x \to \infty. The network is filled with a non-uniform dielectric material. For any small segment of length dxdx at position xx, the series capacitance is given by Cs=ϵ0ks(x)AdxC_s = \frac{\epsilon_0 k_s(x) A}{dx} and the shunt capacitance to ground is Cp=ϵ0kp(x)wddxC_p = \frac{\epsilon_0 k_p(x) w}{d} dx, where A,w,A, w, and dd are geometric constants. The dielectric constants vary spatially as ks(x)=k0(xL)2k_s(x) = k_0 \left(\frac{x}{L}\right)^2 and kp(x)=k0λk_p(x) = k_0 \lambda, where k0k_0 and λ\lambda are positive dimensionless constants. Given the geometric relation wd=AL2\frac{w}{d} = \frac{A}{L^2}, and defining a characteristic capacitance C0=ϵ0k0ALC_0 = \frac{\epsilon_0 k_0 A}{L}, what is the exact equivalent capacitance of the entire network as seen from the input terminal at x=Lx = L?

Options

  • A.

    C02(1+1+4λ)\frac{C_0}{2} \left( 1 + \sqrt{1 + 4\lambda} \right)

  • B.

    C0λC_0 \sqrt{\lambda}

  • C.

    C0(1+λ)C_0 \left( 1 + \sqrt{\lambda} \right)

  • D.

    C02(−1+1+4λ)\frac{C_0}{2} \left( -1 + \sqrt{1 + 4\lambda} \right)

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Variable_Constraint

Develop pure symbolic logic skills by mastering algebraic constraints without relying on numerical data.

Concepts Behind This Question

Infinite Series Capacitance|Potential vs Field Relationship|Non-Uniform Dielectric Filling

Why This Question Is Tricky

Examiner Levers

Dimensional Analysis

Tests if the student verifies units/dimensions before selecting.

Difficulty Boost : +2

Common Mistakes

This question tests the student's ability to transition from discrete, uniform infinite ladder circuits to a continuous, non-uniform field-theory model.

The primary trap lies in misapplying the standard characteristic capacitance formula for uniform distributed lines (Ceq=cscpC_{eq} = \sqrt{c_s c_p}).

Students who rely on rote memorization will locally evaluate this formula at x=Lx=L and incorrectly choose C0λC_0 \sqrt{\lambda}.

The rigorous approach requires establishing a differential recurrence relation (charge conservation) that accounts for the spatial variation of the dielectric constant, leading to an Euler-Cauchy differential equation.

This highlights the danger of ignoring spatial gradients in distributed parameters and forces a fundamental derivation from electrostatic principles.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterElectrostatic Potential and Capacitance - Class 12
ConceptInfinite Series Capacitance
Difficulty5
Cognitive Load6
Target Solve Time140 sec
AccessFree