This Ain't No Music Thread

[Johnny Pneumatic]

Halbach arrays - those fantastic arrangement of magnets that virtually cancles out the magnetic flux field on one side - prompted an idea in my little head. If you want to know more about Halbach arrays Google or Wiki them, or you can look here and find out how to make your own: http://www.matchrockets.com/ether/halbach.html

Ok, now for the idea I had. If you'll note the ring-like Halbach array in my provided link you'll see how it's magnetic flux field is laid out. Interesting, no?
The idea/question I had is what would happen if one made such a thing and put a disk of iron, steel, ferromagnetic whatever, inside the thing? What about another ring-like Halbach array made the other way - so that the field is on the outside, rather than inside? Of course I could always make one and try it myself, and I intend to make a flat Halbach array, but I don't know where to get magnets of the shape I'd need to make a ring-like array.

[Johnny Pneumatic]

What I think would happen in my ideas is that in the case of a metal disk it would be attracted basically evenly around it's diameter by the magnetic ring and thus hover within it. It wouldn't fall out either end since it's being attracted to the magnetic ring. In the case of having a reversed Halbach array ring inside another I think it should repel from the larger outer ring, and nothing keeps it from falling out either side - and the field might indeed force it out - so it will do so. As to if I'm correct in my conjecture is what I'm curious about.

[Cuddles]

The metal disk would be attracted due to induced magnetism exactly opposite to the field inducing it. This would imply that a putting a reversed ring inside would also be attracted, since the poles would align themselves so opposite poles would be facing each other. The attraction would be much stronger in the second case, but since there is no vertical force in either case there would be no way of preventing the ring/disk falling out. A small vertical force may be produced by induction as the disk falls, but since this force only exists while the disk is moving it would not be able to prevent it falling. The same effect can be observed by dropping a magnet down a copper tube - the magnet falls slower than free-fall, but does not ever quite stop.

Interstingly the horizontal Halback array is used in syncrotron light sources. The alternating magnetic field causes electrons in a beam to oscillate, giving off a wide range of wavelengths of light.

[Johnny Pneumatic]

The metal disk would be attracted due to induced magnetism exactly opposite to the field inducing it.


Right. And since it's being attracted to the ring from all sides about the disk's radius that means it's being pulled no more in one direction than another; thus it would hover, no?

This would imply that a putting a reversed ring inside would also be attracted, since the poles would align themselves so opposite poles would be facing each other.

What? Why? I'm talking about a Halbach array that has been made by glueing the magnets together. Some kind of really strong glue, think epoxy. J-B Weld even. The magnets don't have a choice about flipping around N to S or S to N, they don't effect each other enough to force the glue to break.



The attraction would be much stronger in the second case, but since there is no vertical force in either case there would be no way of preventing the ring/disk falling out.

There should be a verticle force in the case of the metal disk. How many times have you seen a piece of iron repelled from a magnet?

[DrCron]

Right. And since it's being attracted to the ring from all sides about the disk's radius that means it's being pulled no more in one direction than another; thus it would hover, no?

Nope. Its unstable, positive feedback makes it accelerate towards the nearest magnet if upset. You could, if it was placed perfectly, and there were no disturbances have it sit there, but as soon as it moves, it wont have the same force acting on each side, and will quickly slam against the side.

[Bruno Putzeys]

There's a chap called earnshaw who mathematically proved that no static arrangement of magnets is stable.