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Homopolar motor
From Wikipedia, the free encyclopedia
A homopolar motor can take many forms, but all have a magnetic field that does not change in strength or direction, hence "homo-polar", and all have their electrical circuit in two parts that are able to move relative to each other while maintaining a sliding or rotating electrical contact at two places (or an even number of places). In contrast to other electrical motors the quantity of magnetic flux passing inside the closed loop of the electrical circuit does not change.
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When a source of electrical power forces a current through the electrical circuit a mechanical force is developed between the two parts of the electrical circuit in accordance with the Lorentz force equation. The source of the magnetic field may be attached to the moving part or the stationary part or neither. It makes no difference as there is no force on the magnet per se.
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 1821 at the Royal Institution in London.
People are sometimes confused by the fact that there is no force on the magnet per se in a homopolar motor, unlike other kinds of electrical motor. Instead there are equal and opposite forces or torques on the two parts of the electrical circuit that carry the current, one of which must be able to slide or rotate (the rotor) while remaining in electrical contact with the other (the stator). This confusion has led to erroneous claims of free energy from homopolar machines called "N-machines".
However, if the magnet is electrically conductive it may perform double duty as both the source of the magnetic field and as part of the electrical circuit. As a result it will experience a force due to its current-carrying function only.
Another source of confusion is the idea that, just as Einstein's theory of relativity tells us there is no such thing as absolute linear motion, so it should be with rotation, and therefore only relative rotation between the magnet and some other part of the machine should matter. The theory of relativity is sometimes thought to support this idea, and sometimes thought to be wrong because it does not. However absolute rotation of a reference frame can be determined by the need for fictitious forces to explain motion in that frame.
When a magnet with a symmetrical field is rotated about its axis of symmetry (as it may or may not do in a homopolar machine), people often ask whether the field lines rotate with the magnet. Field lines can be a useful visualization-aid in predicting the behaviour of some electro-mechanical machines, but are misleading in the case of homopolar machines. There are no field lines mentioned in special relativity or Maxwell's equations or the Lorentz force equation.
A magnetic field merely has a magnitude and direction at every point in space, and is only defined relative to an inertial frame of reference (i.e. a non-accelerating, non-rotating frame of reference). No one has ever succeeded in making a device that can tell whether or not a symmetrical non-conducting magnet, hidden inside a black box, is rotating about its axis of symmetry.
However the Lorentz force law predicts that a rotating conductive magnet should be detectable, at least in principle, by the electric field produced when its free charges separate radially due to the (absolute) rotation of the conductor within its own magnetic field. This is the basis of one construction of a homopolar generator. The failure to appreciate this difference between conducting and non-conducting magnets is yet another source of confusion.