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Question #42786

Difficulty: 8/10 | 🟢 Free

The Question

A parallel plate capacitor consists of two rectangular plates of length LL and width ww, separated by a distance d=4.00 mmd = 4.00 \text{ mm}. A dielectric slab of length LL, width ww, thickness t=2.00 mmt = 2.00 \text{ mm}, and dielectric constant k=2k = 2 is partially inserted a distance xx between the plates, parallel to the plates. A constant potential difference VV is maintained across the capacitor. The electric force FF pulling the slab into the capacitor is calculated from the system's parameters. If the maximum absolute errors in measuring the slab thickness and plate separation are Δt=0.03 mm\Delta t = 0.03 \text{ mm} and Δd=0.024 mm\Delta d = 0.024 \text{ mm} respectively, what is the maximum percentage error in the calculated value of FF? (Assume VV, LL, ww, and kk are exact constants, and edge effects are negligible).

Options

  • A.

    3.4%3.4\%

  • B.

    2.7%2.7\%

  • C.

    2.1%2.1\%

  • D.

    0.6%0.6\%

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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

Dielectric Slab Displacement|Symmetry and Virtual Work|Relative Error Propagation in Powers

Why This Question Is Tricky

Common Mistakes

The problem tests the student's ability to correctly apply the Principle of Virtual Work to find the force on a dielectric and then rigorously propagate measurement errors through a non-linear algebraic expression.

A common trap is failing to treat the effective plate separation deff=d−t(1−1/k)d_{eff} = d - t(1 - 1/k) as a composite variable dependent on both dd and tt.

Students often incorrectly assume F∝t/d2F \propto t/d^2 (leading to 2.7%2.7\%) or simply add the base relative errors Δt/t+Δd/d\Delta t/t + \Delta d/d (leading to 2.1%2.1\%), ignoring the cross-dependence in the denominator d(d−αt)d(d - \alpha t).

Another trap is making a sign error during logarithmic differentiation, leading to the subtraction of error terms instead of adding their absolute values for the maximum possible error (leading to 0.6%0.6\%).

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterElectrostatic Potential and Capacitance - Class 12
ConceptDielectric Slab Displacement
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree