A bar magnet of magnetic moment M is placed at right angles to a magnetic field B. If each pole experiences force F, what is the magnet length?
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Question

A bar magnet of magnetic moment M is placed at right angles to a magnetic field B. If each pole experiences force F, what is the magnet length?

G1 Physics · Accepted Answer

Step 1: A short magnet of pole strength m and magnetic length ℓ has dipole moment M = mℓ. Step 2: For its axis perpendicular to uniform B, the force on each pole is F = mB, giving torque τ = Fℓ. Step 3: The dipole-torque formula gives τ = MB, hence Fℓ = MB ⇒ ℓ = MB/F. Final: option B. Final answer: (A) MB/F.