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A short bar magnet of magnetic moment 0.4 J T⁻¹ is placed in a uniform magnetic field of 0.16 T. The magnet is in stable equilibrium when the potential energy is:

1. -0.064 J
2. zero
3. -0.082 J
4. 0.064 J

Correct answer: -0.064 J

Solution

The potential energy of a magnetic dipole in a uniform magnetic field is given by U = -MBcosθ. For stable equilibrium, θ = 0°, so cosθ = 1. Substituting M = 0.4 J T⁻¹ and B = 0.16 T, we get U = -(0.4)(0.16)(1) = -0.064 J.

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