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In this YouTube video, a dry cell battery, a wound copper wire and a few magnets (see image below) are being used to create what can be described as "train". It looks fascinating but how does this experiment work?

Update 8th August 2016: This problem has been completely worked out in a paper that was published in the January 2016 issue of American Journal of Physics.

Link: Am. J. Phys. 84, 21 (2016); http://dx.doi.org/10.1119/1.4933295

Still shot from magnet train video

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  • $\begingroup$ This is some sort of Homopolar motor. Search Youtube for this term, there are a lot of funny experiments with wires and magnets. $\endgroup$ – Dims Dec 2 '14 at 21:36
  • $\begingroup$ wow. i created an account to ask this ;) $\endgroup$ – naveen Feb 8 '15 at 3:31
  • $\begingroup$ If I had asked it, I would have required someone to do it themselves to verify that it worked. The USPTO has given patents on warp drive and time machines based on theory. $\endgroup$ – EngrStudent - Reinstate Monica Feb 8 '15 at 23:41
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That's a nice video - good find :-)

If you run a current through a coil; it generates an magnetic field inside the coil like this:

Coil

(Image from the Hyperphysics site.)

If the field lines are exactly parallel a bar magnet will feel no net force. However at the ends of the coil, where the field lines diverge, a bar magnet will be either pulled into the coil or pushed out of the coil depending on which way round you insert it.

The trick in the video is that the magnets are made of a conducting material and they connect the battery terminals to the copper wire, so the battery, magnets and copper wire make a circuit that generates a magnet field just in the vicinity of the battery. The geometry means the two magnets are automatically at the ends of the generated magnetic field, where the field is divergent, so a force is exerted on the magnets.

The magnets have been carefully aligned so the force on both magnets points in the same direction, and the result is that the magnets and battery move. But as they move, the magnetic field moves with them and you get a constant motion.

If you flipped round the two magnets at the ends of the battery the battery and magnets would move in the reverse direction. If you flipped only one magnet the two magnets would then be pulling/pushing in opposite directions and the battery wouldn't move.

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  • $\begingroup$ Thank you for the explanation. Is the Lorentz's Force law F=q(vXB) the main equation behind this? $\endgroup$ – noir1993 Dec 2 '14 at 18:09
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    $\begingroup$ @noir1993: No, Lorentz's law gives the force on a charge moving in a magnetic field. This motion is due to the force on a magnetic dipole in a non-uniform magnetic field. $\endgroup$ – John Rennie Dec 2 '14 at 18:32
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    $\begingroup$ "it generates an electric field"? don't you mean "a magnetic field"? $\endgroup$ – thejh Dec 2 '14 at 19:04
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    $\begingroup$ @iamnotmaynard: no, you're getting mixed up between a dipole and a monopole. A dipole feels no net force in a constant field. $\endgroup$ – John Rennie Dec 3 '14 at 6:08
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    $\begingroup$ January 2016's American Journal of Physics has a paper analysing the motion: scitation.aip.org/content/aapt/journal/ajp/84/1/10.1119/… $\endgroup$ – noir1993 Jul 29 '16 at 7:31

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