alleryA magnetic needle is in stable equilibrium when its magnetic moment m is
AAnti-parallel to B (θ = π)
BPerpendicular to B (θ = π/2)
CAt any random orientation
DParallel to the external field B (θ = 0)
Answer & Solution
Correct answer: D. Parallel to the external field B (θ = 0)
U = −m·B is minimum (−mB) when θ = 0, so m parallel to B is the stable equilibrium.
Anti-parallel is unstable.
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