Answer to Question #58091 in Electricity and Magnetism for Misha Jain

Question

For a magnetic needle placed in a uniform magnetic field, which of following are correct :
a) F ≠ 0, τ ≠ 0
b) F ≠ 0, τ = 0
c) F = 0, τ ≠ 0
d) F = 0, τ = 0

Answer 1

If the magnetic field is uniform, then the magnetic needle will essentially rotate to align itself with the magnetic field. There will be a force trying to make that rotation happen. But the net force would be zero (the forces acting on the poles are equal and opposite). The needle won't generally fly along the field with a lot of force, unless the field isn't uniform.
Answer: c) F = 0, τ ≠ 0.