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Question #42974

Difficulty: 8/10 | 🟢 Free

The Question

A small square conducting loop of mass mm (in kg), side length aa (in m), and electrical resistance RR (in Ω\Omega) is attached to the lower end of a massless rigid rod of length LL (in m). The upper end of the rod is pivoted so the loop can oscillate as a pendulum in the vertical xyxy-plane (xx is horizontal, yy is vertical). A time-independent magnetic field B⃗=(B0+αx)k^\vec{B} = (B_0 + \alpha x) \hat{k} (in T) exists in the region, where B0B_0 and α\alpha are positive constants. An external horizontal driving force F(t)=F0cos⁡(ωt)F(t) = F_0 \cos(\omega t) (in N) is applied to the loop. Assuming small amplitude oscillations, negligible self-inductance, and no air drag, what is the average electrical power (in W) dissipated in the loop when the system is driven at its velocity resonance frequency?

Options

  • A.

    F02R2α2a4\frac{F_0^2 R}{2 \alpha^2 a^4}

  • B.

    F02Rα2a4\frac{F_0^2 R}{\alpha^2 a^4}

  • C.

    F02R2B02a2\frac{F_0^2 R}{2 B_0^2 a^2}

  • D.

    F02RB02a2\frac{F_0^2 R}{B_0^2 a^2}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Intuition_Breaker

Train your brain to resist "obvious" answers that are designed to deceive your intuition.

Concepts Behind This Question

Lenz’s Law Energy Conservation|Driven Oscillations and Resonance|Faraday’s Law Vector Form

Why This Question Is Tricky

Common Mistakes

This question traps students who blindly apply the motional EMF formula ε=Blv\varepsilon = Blv using the background field B0B_0, incorrectly yielding a damping coefficient b=B02a2/Rb = B_0^2 a^2 / R.

It tests the critical understanding that in a uniform field, a translating loop experiences no net flux change; the damping arises purely from the magnetic field gradient α\alpha.

Furthermore, it probes the mechanics of driven damped oscillators: students often misapply energy conservation by treating the system as a conservative Simple Harmonic Oscillator, failing to recognize that at velocity resonance, the external driving power is entirely dissipated by the non-conservative eddy current damping.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterElectromagnetic Induction - Class 12
ConceptLenz’s Law Energy Conservation
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree