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.
See also Electric Motors
Homopolar motors necessarily have a single-turn coil, which limits them to very low voltages. This has restricted the practical application of this type of motor.
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.
The homopolar motor was the first ever device to produce rotation from electromagnetism. It was first built and demonstrated by Michael Faraday in 1821 at the Royal Institution in London.
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.
Barlow's Wheel is the name given to an early demonstration of a homopolar motor, designed and built by English mathematician and physicist, Peter Barlow in 1822. An electric current passes through the hub of the wheel to a mercury contact on the rim; this is contained in a small trough through which the rim passes. Due to health and safety considerations brine is sometimes used today in place of mercury. The interaction of the current with the magnetic field of a U-magnet causes the wheel to rotate. The presence of serrations on the wheel is unnecessary and the apparatus will work with a round metal disk, usually made of copper.
"The points of the wheel, R, dip into mercury contained in a groove hollowed in the stand. A more rapid revolution will be obtained if a small electro-magnet be substituted for a steel magnet, as is shown in the cut. The electro-magnet is fixed to the stand, and included in the circuit with the spur-wheel, so that the current flows through them in succession. Hence the direction of the rotation will not be changed by reversing that of the current; since the polarity of the electromagnet will also be reversed."
(Excerpt taken from the 1842 edition of the Manual of Magnetism, page 94)
Topics of Interest
A homopolar generator is a DC electrical generator that comprises an electrically conductive disc rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim. The device is electrically symmetric (bidirectional), the polarity depending on the direction of rotation and the orientation of the field, and generates continuous direct current. It is also known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disc. Although the potential difference is low, typically 0.5 to 3 volts, they can source tremendous electric current (10 to 10000 amperes) because the homopolar generator has very low internal resistance.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)