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Question #42556

Difficulty: 8/10 | 🟢 Free

The Question

A refrigerator is powered entirely by a decelerating flywheel-generator system. The flywheel has a moment of inertia I=2 kg m2I = 2 \text{ kg m}^2 and an initial angular velocity ω0=10 rad s−1\omega_0 = 10 \text{ rad s}^{-1}. The generator experiences a magnetic damping torque due to eddy currents given by τ=−kω\tau = -k\omega, where k=0.5 N m s rad−1k = 0.5 \text{ N m s rad}^{-1} and ω\omega is the instantaneous angular velocity. The mechanical power dissipated by this damping is converted entirely into electrical power to drive the refrigerator. The coefficient of performance (COP) of the refrigerator varies with the instantaneous angular velocity and is given by COP=cω\text{COP} = c\omega, where c=0.03 s rad−1c = 0.03 \text{ s rad}^{-1}. What is the total heat extracted from the cold reservoir as the flywheel comes to a complete stop?

Options

  • A.

    20 J20 \text{ J}

  • B.

    30 J30 \text{ J}

  • C.

    15 J15 \text{ J}

  • D.

    10 J10 \text{ J}

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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

Refrigerator Coefficient of Performance|Eddy Currents Dissipation|Conservative Force Field

Why This Question Is Tricky

Common Mistakes

This question tests the integration of a thermodynamic process over a non-steady-state power input driven by electromagnetic damping.

Students commonly fall into the trap of calculating the total initial kinetic energy (E=100 JE = 100 \text{ J}) and multiplying it by either the initial COP (30 J30 \text{ J}) or a linear average COP (15 J15 \text{ J}).

By making the COP dependent on the decaying angular velocity, the problem forces a differential setup where cooling power dQ/dtdQ/dt must be integrated over time or angular velocity, revealing that the total heat extracted depends on the ω2\omega^2 weighted integral of the COP, not a simple static average.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterThermodynamics - Class 11 Part 2
ConceptRefrigerator Coefficient of Performance
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree