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A magnet with a magnetic moment M is rotated from an initial angle of 90° to a final angle of 60°. The work required to rotate the magnet through 90° is expressed as MB (cos 0° - cos 90°). Which of the following statements is accurate?

1. The magnetic moment of the magnet is represented as M.
2. The starting angle for the magnet's rotation is 90°.
3. The ending angle for the magnet's rotation is 60°.
4. The work done to rotate the magnet by 90° is given by MB (cos 0° - cos 90°).

Correct answer: The work done to rotate the magnet by 90° is given by MB (cos 0° - cos 90°).

Solution

The work done in turning a magnetic dipole in a uniform magnetic field is given by W = MB (cos θ₁ - cos θ₂). The given expression W₁ = MB (cos 0° - cos 90°) is correct for θ₁ = 90° and θ₂ = 0°.

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