Magnet levitation?

Question

I have to do a demonstration on MagLev trains, and I was wondering how I could demonstrate this.

I was thinking of getting 2 donut magnets and slipping them thru a wooden dowel so the top magnet levitates over the bottom one.

Any other ideas?

Answer 1

It's a decent way to demo it, but your should be clear that this is not the way MagLev trains work.

They use superconductors (usually in the train) and electromagnetic fields on the track. Superconductors have the remarkable ability to refuse to let magnetic fields too far into their interiors, something called the Meissner Effect.

So the magnetic field from the electromagnet wants desperately to get into the superconductor on the bottom of the train, but the superconductor refuses to let it in very far, thus the magnetic field essentially piles up below the superconducting plate lifting up the plate and anything else attached to it, i.e. the car of the train. Some maglevs reverse this arrangement with the superconductor on the track and the electromagnet on the train, but I think the former is more common.

Suddenly, this train floats above the track, with no contact at all necessary with the rails, and some magnetic fields on the side which center the train on the track, so it doesn't have to touch there either. Now the only resistance is the air resistance as the speedy train slices through the air.

The reason why the donut magnet idea isn't used for trains is because there is no stable potential in such an arrangement, the magnet on top is trying desperately to slide to the side (which is why your donut arrangement will have one of the magnets tilted). So at speed, there will be a lot of friction from whatever you use to keep the top magnet from sliding over, negating some of the advantages of magnetic levitation.

Physicists know that you can't freely float one magnet over another one due to something called Earnshaw's theorem, which is a complicated math theorem, but can be simply demonstrated by the following ... get a reasonably small box no bigger than a foot on each side. Now use a knife to cut the top off of the box in any wild pattern you like. Next, stretch an old pantyhose or latex sheet over the top like a drum head and secure with a big rubber band or tape. Now put a small ball bearing (not too heavy) onto the top. No matter where you place it, no matter what shape you cut the box, it will always roll off, there is no stable potential on the top. (If you cut it straight across, and if your bearing is heavy enough, you might be able to make it stay, but it's cheating, because the weight of the ball makes a slight depression that holds it.)

There is a much better demo than this, although not superconducting, you won't need the dowel, nor will you need liquid nitrogen to cool a high-temperature superconductor.

It's a diamagnetic levitator, and it definitely isn't easy to make, but it is instructive.

http://sci-toys.com/scitoys/scitoys/magnets/suspension.html

A much easier way, if you have $15 or so to spend, is just to buy a ready made diamagnetic levitation kit, like this one ...

http://www.envisionlabs.com/cgi-bin/shopper.cgi?preadd=action&key=000D10500

Answer 2

They have this cool globe of the earth you can buy that's magnetically levitated without the need of any solid contact. It requires electronic feedback stabilization, though, so you have to plug it in. It's extremely difficult to get sustained magnetically levitation without a power source, if only for feedback or refrigeration.