Science is Magic! Watch a Magnet Sail Around a Möbius Strip

Bad Astronomy
The entire universe in blog form
July 12 2013 8:00 AM

Watch a Levitating Magnet Sail Around a Möbius Strip!

File this under “OMG this is so cool why didn’t I think of it?!”: a video of a superconducting magnet sailing up, down, and around a track shaped into a Möbius strip:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

I love this so so so much. It’s all explained in the video (starting around two minutes in) but in case you missed it:

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1) There exists a kind of material that, if you chill it down to very low temperatures, becomes a superconductor, meaning electrons can flow through it freely without resistance. What that means is that a lot of weird stuff can occur. For one thing, if you induce an electric current in a superconductor, that current will flow forever without resistance.

As an analogy, imagine filling your sink with water. If you give the water a brief swirl with your hand the water will flow around the sink. Eventually, though, friction slows it and the water will be still once again. But if the water were frictionless, it would swirl forever. A superconductor is like a frictionless sink, filled with electrons instead of water. Once electrons start to flow, making a current, they never stop.

2) Another important thing is that if you stick a conductor in a moving magnetic field (or move a magnet over a conductor), it will induce a current in the conductor, which will start electrons flowing. One of the basic laws of electricity and magnetism is that the flowing electron current (which we call electricity) will itself generate a magnetic field, and it’ll do so in a way to resist the original changing magnetic field. It’s a bit like if you feel yourself falling over, and you flail your arms (or grip something quickly) to get your balance back.

3) Still with me? Now combine this. In a superconductor, if it’s set up properly, you can generate a flow of electrons that’ll whiz around inside it. If you then put that superconductor on a track, it will levitate above the track because gravity pulls it down, changing the magnetic field it feels, so it sets up its own magnetic field to resist that. Same thing if you hang it upside down; gravity pulls it away from the track, so it changes its own field to hold on to the track tighter.

superconductor puck floating over a track
A supercool super-cooled superconducting, um, superpuck.

Photo by The Royal Institution, from the video

4) Finally, if you look at the magnetic field of the track, it changes rapidly near the edges, toward the inside and outside. That acts like a buffer, keeping the superconductor on the track (because, again, the superconductor’s own magnetic field will resist that change, forcing it to the middle of the track). But the field doesn’t change along the track, so the superconductor doesn’t care about moving in that direction. Poke it, and it’ll glide, floating, along the track.

5) Now we can combine everything. Chill a puck made of yttrium barium copper oxide (or as we in the science biz call it, YBCO, to sound—wait for it, wait for it—cool) with liquid nitrogen so that it becomes a superconductor. Put it on the track. The magnetic field lines of the magnets in the track grip the puck invisibly, and it can glide up and down the track. Yay!

All of the above is well-known and most students of physics have seen this demo a zillion times. But the idea of shaping the track into a Möbius strip? That is pure genius! A Möbius strip is a shape that has only one side. Here’s how to make one: Take a strip of paper that is much longer than it is wide. Give one end a half twist, so it’s flipped over, and attach it to the other end. Voila! A Möbius strip. Take a pencil, put the tip anywhere on the strip, and start drawing a line along it. After a moment, you’ll meet up to the point where you started. If you hadn’t twisted the paper, and just attached one end to the other to form a squat cylinder, that won’t happen. There’s an inside face and an outside face. In a Möbius strip there’s only one face, connected to itself.

So in the video, the floating superconductor slides along the track, flips upside down, keeps moving, and eventually comes back to the place it originally started. If you’re impatient, the cool stuff starts at 4:45 in the video.

This is really just a clever way to show an old demo, but I love it. It’s got everything! Exotic materials, liquid nitrogen, superconducting magnets, and a Möbius strip all combined in a live-action Escher drawing. It’s nerdvana.

Hmmm, well, actually, it doesn’t have everything. What it really needs is for the puck to have a little fan on it, a propeller, so it can keep going around and around forever. Or until it warms up and the superconductivity collapses (maybe attach a hose to it somehow to keep it supplied with liquid nitrogen…?). That would make a fun exhibit at a museum, too.

Science! I love this stuff!

Tip o' the Meissner Effect to Philippe Roux on G+.

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