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Question #43443

Difficulty: 8/10 | 🟢 Free

The Question

A closed electrical circuit consists of a capacitor of capacitance CC and two wire loops, Loop 1 and Loop 2, connected in series. The total self-inductance of the two loops is L0L_0. Loop 1 is a small planar loop of area A1A_1 fixed at the origin in the xyxy-plane. Loop 2 is a wire bent into the shape of an open hemispherical dome of radius aa. The circular base of the dome has area A2=πa2A_2 = \pi a^2. The center of this base is located at a distance rr from the origin along the zz-axis, where r≫ar \gg a and r≫A1r \gg \sqrt{A_1}. Loop 2 is forced to rotate about an axis parallel to the xx-axis passing through the center of its base, with a constant angular velocity ω\omega. At t=0t=0, the circular base of Loop 2 is parallel to the xyxy-plane. Assume the resistance of the circuit is negligible and that at t=0t=0, the magnetic fluxes produced by the two loops reinforce each other. Let q(t)q(t) be the instantaneous charge on the capacitor, and define a constant M0=μ0A1A22πr3M_0 = \frac{\mu_0 A_1 A_2}{2\pi r^3}. By applying Gauss's Law for Magnetism to determine the flux linkage, identify the correct differential equation governing q(t)q(t).

Options

  • A.

    (L0+2M0cos⁡ωt)q¨−2M0ωsin⁡ωtq˙+qC=0(L_0 + 2 M_0 \cos \omega t) \ddot{q} - 2 M_0 \omega \sin \omega t \dot{q} + \frac{q}{C} = 0

  • B.

    (L0+2M0cos⁡ωt)q¨+qC=0(L_0 + 2 M_0 \cos \omega t) \ddot{q} + \frac{q}{C} = 0

  • C.

    (L0+M0cos⁡ωt)q¨−M0ωsin⁡ωtq˙+qC=0(L_0 + M_0 \cos \omega t) \ddot{q} - M_0 \omega \sin \omega t \dot{q} + \frac{q}{C} = 0

  • D.

    (L0+2M0cos⁡ωt)q¨+2M0ωsin⁡ωtq˙+qC=0(L_0 + 2 M_0 \cos \omega t) \ddot{q} + 2 M_0 \omega \sin \omega t \dot{q} + \frac{q}{C} = 0

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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

Gauss’s Law for Magnetism|Dot Product Geometry|L-C Circuit Analogy

Why This Question Is Tricky

Common Mistakes

This question tests the synthesis of Gauss's Law for Magnetism, dipole field geometry, and Faraday's Law in a time-varying circuit.

The primary trap lies in the time derivative of the flux linkage.

Students often incorrectly assume that the induced EMF in an LC circuit is simply −Lq¨-L \ddot{q}, treating inductance as a constant.

Here, the rotation of the loop creates a time-varying mutual inductance M(t)M(t).

By failing to apply the product rule to ddt(M(t)I)\frac{d}{dt}(M(t) I), students miss the −2M0ωsin⁡(ωt)q˙-2M_0 \omega \sin(\omega t) \dot{q} term entirely, which acts as a parametric damping factor.

Additionally, the problem requires applying ∇⋅B=0\nabla \cdot \mathbf{B} = 0 to simplify the flux integral over a complex hemispherical surface to a simple dot product over its circular base, and remembering the factor of 2 for mutual inductance in series-aiding coils.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterMagnetism and Matter - Class 12
ConceptGauss’s Law for Magnetism
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree