A homopolar motor has a magnetic field along the axis of rotation and an electric current that at some point is not parallel to the magnetic field. The name homopolar refers to the absence of polarity change.
Moving electric charges (an electric current) in a magnetic field experience a force that is perpendicular to both their direction of movement and the magnetic field, called the Lorentz force
. In the homopolar motor shown on the right, the electric current produced by the battery moves radially through the disk magnet, which has a magnetic field along its longitudinal axis. The resulting Lorentz force in the tangential direction produces a torque in the magnet, which is free to rotate with the attached screw.
It is not necessary for the magnet to be electrically conductive, or to move. One can attach the magnet to the battery and allow the wire to rotate freely while closing the electric circuit even at the axis of rotation. Again, where at some point along the electric loop the current in the wire is not parallel to the magnetic field, there occurs a Lorentz force that is perpendicular to both. This Lorentz force is tangential and produces a torque in the wire, so that the wire rotates.
In contrast to other electrical motors, both the orientation and magnitude of the magnetic field and the electric current do not change.
Like most electro-mechanical machines a homopolar motor is reversible so that when electrical energy of a suitable kind is put into its terminals, mechanical energy can be obtained from its motion and vice versa, (see homopolar generator for details on construction and theory of operation).
The homopolar motor was the first ever device to produce rotation from electromagnetism. It was first built and demonstrated by Michael Faraday
in 1831 at the Royal Institution
Sources of confusion
People are sometimes confused by the fact that there are no changes in the magnetic field or electric current, and no recognizable North-South pole interaction between the magnet and the electric circuit. People often think that field lines
cannot be used to understand homopolar machines, or that the field lines rotate -- see Faraday Paradox
. Others refer to special relativity
to explain the homopolar motor. The homopolar motor also may seem to require a conducting magnet.
The homopolar motor can be well explained by the Faraday model of lines of force, with a tangential force (hence, a torque) resulting where the electric current makes an angle with the magnetic lines of force. The homopolar motor provides a simple demonstration of the Lorentz force