Practice portal › Magnetism and Matter › Bar Magnet and Dipole in a Field
Asked in NEET 2013 · Dipole in a uniform field
Idea: equilibrium is where the torque vanishes; stability is decided by the energy.
Torque on a loop of moment ⃗m: τ=mB sin θ, which is zero at θ=0 and at θ=180°.
So there are exactly two equilibrium orientations.
Energy: U=-mB cos θ, which is -mB at θ=0 and +mB at θ=180°.
θ=0 is the energy minimum, so it is stable; θ=180° is the maximum, so it is unstable.
Two orientations, one stable and one unstable.
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