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Question #43391

Difficulty: 8/10 | 🟢 Free

The Question

A long non-conducting solid cylinder of radius RR has a time-varying volume charge density given by ρ(r,t)=ρ0(rR)t\rho(r, t) = \rho_0 \left(\frac{r}{R}\right) t, where ρ0\rho_0 is a positive constant, rr is the radial distance from the central axis, and tt is time. The cylinder rotates about its central longitudinal axis with a constant angular velocity ω1\omega_1. A closed conducting loop is formed in a plane perpendicular to the cylinder's axis (far from the ends). The loop consists of a stationary radial wire from the axis to r=Rr=R, a stationary circular arc of radius RR, and a conducting rod of length RR pivoted at the axis. The rod rotates with a constant angular velocity ω2\omega_2 in the same sense as ω1\omega_1, continuously expanding the sector area of the loop. At t=0t=0, the rod coincides with the stationary radial wire. Find the magnitude of the total induced electromotive force (EMF) in the loop at time tt. Assume all quantities are in SI units and ignore self-inductance.

Options

  • A.

    15μ0ρ0ω1ω2R4t\frac{1}{5} \mu_0 \rho_0 \omega_1 \omega_2 R^4 t

  • B.

    110μ0ρ0ω1ω2R4t\frac{1}{10} \mu_0 \rho_0 \omega_1 \omega_2 R^4 t

  • C.

    13μ0ρ0ω1ω2R4t\frac{1}{3} \mu_0 \rho_0 \omega_1 \omega_2 R^4 t

  • D.

    16μ0ρ0ω1ω2R4t\frac{1}{6} \mu_0 \rho_0 \omega_1 \omega_2 R^4 t

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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

Biot-Savart Law Vector Symmetry|Motional EMF in Rotating Rods|Vector Decomposition Ambiguity

Why This Question Is Tricky

Common Mistakes

This question tests the synthesis of Ampere's law for continuous rotating charge distributions with Faraday's law in a non-steady state.

The primary trap is the 'Time-varying Fields' misconception: students often mechanically apply the motional EMF formula ∫(v⃗×B⃗)⋅dl⃗\int (\vec{v} \times \vec{B}) \cdot d\vec{l} and forget that when the magnetic field itself is explicitly time-dependent, a transformer EMF ∫E⃗⋅dl⃗\int \vec{E} \cdot d\vec{l} is also induced.

Calculating only the motional EMF leads to Option B.

Furthermore, assuming a uniform magnetic field based on the center value bypasses the necessary spatial integration of the non-uniform field, leading to Option C.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterMoving Charges and Magnetism - Class 12
ConceptBiot-Savart Law Vector Symmetry
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree