μblog: engineering from the trenches

My office-mate recently told me about a very simple motor that can be made from a magnet, AA battery and a piece of wire, so I figured it was time to look it up. One such design was found on  dangerouslyfun.com, a web site with a few clever how-to guides (with videos). Although well laid out, the guide is a little light on the theory of operation, so here goes.

The Lorentz-force equation states that the net force (F) resulting from a magnetic field (B) on a particle with charge (q) moving with velocity (v) is :  F = q(v x B), where x is a cross product. To put simply, the force is maximal when v and B are at right angles to each-other, and will be orthogonal to both v and B. The coin shaped magnet probably has it’s north and south poles oriented on the flat, round faces, so the lines of magnetic flux (or the field lines) go from south to north (animated gif). Since electronic current is nothing more than the flow of electrons, their velocity is most orthogonal (or closest to 90 degrees) to the B field in the flat bottom wire segments of the top figure. The force exerted on the wire segments is then either forwards or backwards in the plane of the magnet depending on the flow of the current. To make things clear, the magnet is connected to the negative terminal of the battery and is used as a current return path from the positive terminal via the wire. The result is that the wire loop spins, not unlike a more typical electric motor. The authors of the original howto suggest that rare-earth magnets are used as typical ceramic magnets are too weak.

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