# Intriguing physics demo

I came across this nice demo that labels itself ‘the world’s simplest electric train’. It is very clever. I would never have thought of it. It is fun to think about the physics behind it.

1. moarscienceplz says

I get that this is a kind of motor-generator, and I can even guess how it would work once the “train” is moving through the coil, but what I don’t get is how it appears to be self-starting. No one has to “kick-start” the motion. How can what starts out as static charges on the two magnet assemblies induce the changing magnetic field that you need to get motion?

2. moarscienceplz says

Oh, I get it. I assumed it was lacquered magnet wire, but it is actually bare copper so the current from the battery directly flows through the wire, using the nickel-plated magnets as contacts.

3. dmcclean says

That’s cool. I like it it takes advantage of current flowing through the magnets. You can rearrange it to have the battery connected directly to the ends of the coil and use just the magnets as the “train” and the same basic thing would be happening, but it wouldn’t be nearly as cool to watch.

4. Rob Grigjanis says

dmcclean @3:

You can rearrange it to have the battery connected directly to the ends of the coil and use just the magnets as the “train” and the same basic thing would be happening

I don’t think so, because in that case the induced magnetic field inside the coil would be constant for most of the coil’s length, and there would be no force on the magnet inside the coil. You need the induced field to vary (in the direction of the coil’s axis) at the locations of the magnets, and this is accomplished by having the current go from one magnet, through the coil, and out at the other magnet.

5. dmcclean says

Good point Rob. Might work for a coil not much longer than the magnet, but wouldn’t work for a really long coil like that. It’s an even better idea than I thought.

My mistake.

6. Holms says

Rob, that is not correct. The field does not need to vary in magnitude, it only needs to have a direction. Normally, this is not provided by a straight wire carrying a current, as the magnetic field produced will be perpendicular to the current and will therefore wrap around that cable. The great benefit provided by the coil configuration is that there is now row upon row of current bearing wire, and since the current is flowing in the same direction across all of them (all clockwise, or all anticlockwise) the fields along the inner surface of the coil will all align to point all up or all down. Thus, the inner region of the coil gains a magnetic field where the poles are always pointing to either end of the tube, even if we imagine the tube to have infinite length.

dmcclean had the right idea: hooking the coil to a current would do much the same thing. It’s called a solenoid. It may even cause the little ‘train’ move faster, by divesting it of the mass of the battery.

7. Rob Grigjanis says

Holms @6: The force on a magnetic dipole μ in a magnetic field B is the negative gradient of the potential energy, which is simply the negative of their dot product. So, with negative signs cancelling;

F = (μ·B)

so the field B has to be varying at the location of the magnet, and in the direction of μ, for there to be a force.

In a solenoid (see here), the field is only varying at the ends, where the current is leaving/entering. In the case of this train, that is at the locations of the magnets, hence the force on them.

8. dmcclean says

Holms,
No, the error was mine. You and my original comment would be right if the magnets were monopoles.

Those are a lot harder to come by, though, so it would ruin the whole “household items” vibe. 😉

9. ericjuve says

When I saw it I just had to make one and I will admit I was surprised that it worked at all much less as well as it did. I used an AA battery and 4 ea 1/2″ dia x1/4″ thk N42 magnets from KJ. I wound the coil using bare 22ga copper wire on a 1/2″ mandrel on a lathe. When I cut the coil loose it unwound enough to give clearance for the magnets. The magnets need to oppose the induced field on the front and attract from the back. Really fun to show to people.

10. Mano Singham says

ericjuve,

That is excellent. Well done!

Now you can answer a question that has been bugging since I saw the demo.

Are the polarities of the two magnets at the two ends of the cell the same or opposite? At the beginning of the video, he attaches one magnet and then when he goes for the second it seems like he has to force it just so slightly to get it to stick, as if that dipole is in the opposite direction to the first magnet. Is that the case?

11. Rob Grigjanis says

Mano, the polarities have to be opposite for both ends to feel the force in the same direction, because the component of the magnetic field along the coil’s axis (call it x) is increasing entering the mini-solenoid, and decreasing when leaving it, and the force goes like μdB/dx, where B is the x component of the field. If dB/dx has different signs at the ends, so does the magnetic moment, to get the same sign force.

12. ericjuve says

Yes that is the case. It took me a couple tries to realize the correct configuration though. It is of course important to get the north/south and the +/- oriented correctly. I made 500′ of wire into coils and ended up with a LOT of track (it helps having an engine lathe to wind them). It seems that a winding pitch of 10 coils per inch is optimum for the battery/magnet configuration that I have. I’m going to set it up for my Grandson this Christmas, he’s only 2 1/2 but he loves magnets and I’m hoping to keep him interested in science and engineering.

13. ericjuve says

Thank you Rob for a more definitive explanation, my basic physics is a little rusty…

14. Rob Grigjanis says

ericjuve, my experimental physics is non-existent. I’d end up garrotting myself with the copper wire 🙂

15. Mano Singham says

ericjuve,

There is something magical about magnets, so mysterious in its operation that even the Insane Clown Posse