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Homopolar motor: Wikis


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DIY simple homopolar motor made with drywall screw, alkaline battery cell, wire, and neodymium disk magnet. The screw and magnet contact the bottom of the battery cell and are held up by magnetic attraction. The screw and magnet spin.

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.

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.



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 itself. It was first built and demonstrated by Michael Faraday in 1821 at the Royal Institution in London. [1]

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.

See also


  1. ^ Faraday, Michael. Electro-magnetic rotation apparatus. Quarterly Journal of Science, 1821, XII, pg. 186-187

External links



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