Question

A tiny bar magnet is kept close to a long current carrying straight wire placed along the z axis. When the center of the magnet is at (a, 0.0) and it is oriented along the z axis, it experiences

• A. no net force but a net torque.
• B. neither net force nor net torque.
• C. both a net force and a net torque.
• D. a net force but no net torque.

Answer 1

The correct option is A no net force but a net torque.

Here, from figure we see the direction of magnetic field.

Since, the magnet is small (i.e, dimension of magnet<<$a$), we can assume that the magnetic field due to current carrying wire at the location of magnet is uniform.

And, we know that in a uniform field , net force on a magnet is zero. And since field B is perpendicular to magnetic axis , and not parallel, hence there will be a net torque acting on the magnet given by $\hat{M}\times \hat{B}$.

Hence, answer is option-(A).