Make: the simplest electric car toy, a homopolar motor


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Simplest electric train is somewhat cooler.


One thing I’ve noticed in the last 40 years or so is the growing tendency to assign physicist’s names to things (even when not strictly correct). It’s actually a bit depressing because it reinforces the celebrity view of science, which actually tends to be the work of much larger groups of people. The forces on a charged particle due to the surrounding electric, magnetic or electromagnetic fields derives from Cavendish and Faraday’s original experiments through Maxwell’s equations, J J Thomson’s experiments with cathode ray tubes, and Heaviside’s correction and modernisation of Thomson’s formulae. Lorenz really just came up with an equation, and so we should perhaps write “The Lorentz equation”, but certainly not the Lorenz effect or the Lorenz force because they were known about and described long before Lorenz. It is a bit like confusing the train timetable with the train.
(This is not specifically about the Lorenz but the nomenclature. For instance, we refer to the “Geiger-Marsden experiment” but we don’t call the deflection of alpha particles the “Geiger-Marsden effect” or the “Rutherford force”).
In maths we nowadays tend not to refer to Newtonian calculus (thus omitting Leibniz from the record). Euclidean geometry is a convenient shorthand for “geometry of three mutually perpendicular dimensions in a flat spacetime”, but also because we have no records of anybody else’s work - Euclid was the one chosen by chance and survivor bias for fame. But where the development of ideas is well attested, it’s rarely the case (Einstein being an exception) that one can point at a name and say “This is where it all started”.


I’ve made both of the simple homopolar motors illustrated in the Homopolar motor article on Wikipedia, but I must say this design is charmingly elegant.

I especially like the asymmetric magnet-wheels so you don’t need an infinite roll of foil roadway. (-:

Listening to people who think they understand electricity and motors try to explain the dangling-drywall-screw version can be quite entertaining. (And can sometimes reveal just how very little the self-described ‘expert’ actually does understand.)


I see, I see… E cross B… (strokes beard)


interesting exercise; calculate the force between two wires by working out the Lorenz contraction of the electrons due to their relative velocity, thus working out the excess or defect charge, and working out the resulting electrostatic force. If you do this, you get the correct answer - with no magnetic fields involved.


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