Q.Compute the torque experienced by a magnetic needle in a uniform magnetic field.

Step 1. A magnetic needle of pole strength $q_m$, length $2l$, in a uniform field $\vec B$ tilted at angle $\theta$, feels a force $q_m\vec B$ at the north pole and $-q_m\vec B$ at the south pole -- equal, opposite, so the net force is zero.

Step 2. Being equal, opposite, and not collinear, the two forces form a couple. Taking moments about the centre, each pole's force has a perpendicular moment arm of $l\sin\theta$ from the other pole's line of action. …

• Forgetting the net force on the needle is zero in a uniform field, …