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Question #85399

Difficulty: 5/10 | 🟢 Free

The Question

An infinitely extended non-conducting solid slab of thickness 2d2d (in m\text{m}) has a uniform volume charge density ρ>0\rho > 0 (in C/m3\text{C/m}^3). The slab is fixed at an angle θ\theta to the horizontal. A solid conducting sphere of mass mm (in kg\text{kg}), radius RR (in m\text{m}), and positive charge QQ (in C\text{C}) is projected directly up the upper surface of the slab with an initial center-of-mass velocity v0v_0 (in m/s\text{m/s}). The coefficient of static friction between the sphere and the slab is μ\mu. Assume the charge QQ is distributed such that it does not affect the uniform field of the slab, and μ\mu is sufficiently large for the uncharged sphere to roll. The acceleration due to gravity is gg (in m/s2\text{m/s}^2) and the permittivity of free space is ϵ0\epsilon_0 (in C2/(N⋅m2)\text{C}^2/(\text{N}\cdot\text{m}^2)). Which of the following gives the maximum charge QmaxQ_{\text{max}} for which the sphere will roll without slipping during its ascent, and the maximum distance LL it travels up the incline before coming to instantaneous rest?

Options

  • A.

    Qmax=ϵ0mgρd(cos⁡θ−2sin⁡θ7μ),L=7v0210gsin⁡θQ_{\text{max}} = \frac{\epsilon_0 mg}{\rho d} \left( \cos\theta - \frac{2 \sin\theta}{7 \mu} \right), \quad L = \frac{7 v_0^2}{10 g \sin\theta}

  • B.

    Qmax=ϵ0mgρd(cos⁡θ−2sin⁡θ5μ),L=7v0210(gsin⁡θ−Qρdmϵ0)Q_{\text{max}} = \frac{\epsilon_0 mg}{\rho d} \left( \cos\theta - \frac{2 \sin\theta}{5 \mu} \right), \quad L = \frac{7 v_0^2}{10 \left(g \sin\theta - \frac{Q\rho d}{m\epsilon_0}\right)}

  • C.

    Qmax=2ϵ0mgρd(cos⁡θ−sin⁡θμ),L=v022gsin⁡θQ_{\text{max}} = \frac{2\epsilon_0 mg}{\rho d} \left( \cos\theta - \frac{\sin\theta}{\mu} \right), \quad L = \frac{v_0^2}{2 g \sin\theta}

  • D.

    Qmax=mgϵ0ρd(cos⁡θ−2sin⁡θ7μ),L=7v0210gsin⁡θQ_{\text{max}} = \frac{mg}{\epsilon_0 \rho d} \left( \cos\theta - \frac{2 \sin\theta}{7 \mu} \right), \quad L = \frac{7 v_0^2}{10 g \sin\theta}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Multi-Concept_Bridge

Master problems that require linking physics and math concepts to solve complex competitive questions.

Concepts Behind This Question

Charged Conducting Sphere|Static Friction Threshold|Mechanical Energy Conservation|Integration

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 couple electrostatics with rigid body dynamics and energy conservation.

The primary trap lies in the Static Friction Threshold: students often forget that an electrostatic force normal to the surface alters the normal force (N=mgcos⁡θ−QEN = mg \cos\theta - QE), which in turn dictates the maximum available static friction (μN\mu N).

A secondary trap is assuming the uniform electric field does work on the sphere as it moves up the incline; however, since the field from an infinite slab is strictly normal to the surface, the electrostatic force does zero work, leaving the stopping distance LL purely dependent on gravity and initial kinetic energy.

Furthermore, dimensional analysis can eliminate options where the permittivity ϵ0\epsilon_0 is incorrectly placed, rewarding students who verify units.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterElectric Charges and Fields - Class 12
ConceptCharged Conducting Sphere
Difficulty5
Cognitive Load6
Target Solve Time140 sec
AccessFree