Practice portal › Magnetism and Matter › Bar Magnet and Dipole in a Field
Asked in GSEB Board July 2017 · Dipole in a uniform field
Idea: treat the needle as a small dipole -- poles +qₘ and -qₘ a short distance apart -- and ask separately about the twist and the pull.
Torque: the two pole forces act along the local field but at different points, so unless the needle already lies along the field they twist it, τ⃗=⃗m×⃗B. The needle is stated to be non-parallel, so τ eq0.
Net force: in a uniform field the two pole forces are equal and opposite and cancel exactly. Here the field is non-uniform, so the pole in the stronger field is pulled harder and a net force survives.
So a needle held non-parallel in a non-uniform field experiences both a force and a torque.
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