Magnetic torque on
If two like poles of two separate magnets are brought near each other and one of the magnets is allowed to turn it will promptly rotate to align itself with the first. In this example, the magnetic field of the stationary magnet creates a magnetic torque on the magnet that is free to rotate. This magnetic torque τ tends to align a magnet's poles with the magnetic field lines. A compass, therefore, will turn to align itself with earth's magnetic field.
Magnetic torque is used to drive electric motors. In one simple motor design, a magnet is fixed to a freely rotating shaft and subjected to a magnetic field from an array of electromagnets.
By continuously switching the electric current through each of the
electromagnets, thereby flipping the polarity of their magnetic fields,
like poles are kept next to the rotor; the resultant torque is
transferred to the shaft. See Rotating magnetic fields
