Final answer:
A bar magnet perpendicular to a uniform magnetic field experiences torque but no net force, causing it to rotate until aligned with the field.
Step-by-step explanation:
When a bar magnet is oriented perpendicular to a uniform magnetic field, there is no net force on the magnet since magnetic fields exert forces on moving charges, not static magnets. However, the magnet will experience a torque because the magnetic field exerts forces on the individual magnetic moments of the material that makes up the bar magnet. These individual forces create a rotation effect, or torque, on the magnet because forces on one end of the magnet push in a different direction than those on the opposite end. The torque will cause the bar magnet to align with the direction of the magnetic field. This is analogous to how torque is produced on the shaft of a loop of current in the presence of a magnetic field, with forces perpendicular to the field on different segments producing equal torque in the same rotational direction.
The magnitude of this torque is determined by the magnetic dipole moment of the bar magnet and the strength of the magnetic field. The magnet will rotate until the magnetic dipole moment is aligned with the magnetic field, thus minimizing the potential energy of the system