My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a very simple explanation to what is turning out to a very complex theory (or law).

  • $\begingroup$ Welcome to StackExchange! In general, questions should be somewhat specific, so reference requests for basic introductions are generally not paid much attention, but you may find the Hyperphysics explanation at hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html to be a helpful starting point. $\endgroup$ – DumpsterDoofus Jan 11 '14 at 23:02
  • $\begingroup$ There is current inside the wire - electrons moving from the minus pole to the plus pole of the battery. There are magnetic field lines emanating out from the magnet. On the wikipedia page of the motor, you can see a very nice picture of this. The battery puts the electrons in motion, so the magnetic field from the magnet can push them around, making the wire rotate. $\endgroup$ – Mathusalem Jan 11 '14 at 23:27
  • $\begingroup$ + I'd never seen that before. What a nifty demonstration! If you have a wire cutting through magnetic field lines a current is induced. Then the reverse has to be true also, and that's what you're seeing. $\endgroup$ – Mike Dunlavey Jan 11 '14 at 23:40
  • $\begingroup$ Is the magnetic field emanating from the neodymium magnet or is the magnetic field from the battery charging the copper wire? $\endgroup$ – user37213 Jan 12 '14 at 1:45
  • $\begingroup$ The magnetic field comes from the magnet. The battery provides the moving electrons. Electrons moving in a magnetic field sense a force unless they are completely parallel to the field. $\endgroup$ – Mathusalem Jan 12 '14 at 10:22

When you have current flowing in a conductor at right angles to magnetic field lines (or at some angle other than parallel, the conductor experiences a force at right angles to both of them. (It's described with the right-hand rule.)

Your Wikipedia link gives a very good explanation.

EDIT: let me see if I can draw what I think is going on in the cylindrical portion of the nail: enter image description here

Here B is the magnetic field coming up through the nail and exiting horizontally out the side (and then bending back down toward the opposite pole of the magnet). I is the electric current, traveling along the length of the nail. Near the surface of the nail, I and B are at 90 degrees to each other. F is the resulting force, which goes in one direction all around the outside of the nail, causing it to spin. (I may have the polarity wrong.)


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