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Asked in AIPMT 2011 Mains · Dipole in a uniform field
Given: m=0.4 J T⁻¹, B=0.16 T.
Idea: U=-mB cos θ, and the stable equilibrium is the energy minimum.
The minimum is at θ=0, with the moment along the field, where cos θ=1.
U=-mB=-0.4×0.16.
U=-0.064 J.
The negative sign is the point: the aligned state sits below the zero of this scale, which is why it is the stable one.
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