As I mentioned in my old piece about rare-earth magnets, there's a little cocktail-party physics demo I like to do.
(The deal is, I drink some cocktails, and then I do the demos.)
This demo shows magnetic eddy-current braking down the inside of a conductive tube. I take a length of aluminium tube, roll a plastic ball down it to demonstrate that it contains no gimmicks, and then drop a little rare-earth magnet down the tube.
You can hear the magnet going ting-ting-ting down the tube, but it takes a surprisingly long time to come out the other end. When I do this trick with a magnet that fits the tube quite closely, it takes about 30 seconds before it comes out. The plastic pellet takes only about 0.6 seconds.
You can also demonstrate magnetic braking with a chunk of copper and a decent-sized rare-earth magnet. If you slide the magnet up and down the copper, there's an oily feeling of resistance that gets stronger the faster you move the magnet. It fades away to nothing as the movement speed drops, though, which is why magnetic braking is such a great way to get precision balance scales to settle.
A more dramatic demonstration is to use a horizontal spinning disk of non-ferromagnetic, highly-conductive metal, preferably copper. It'll grab and throw a strong magnet that you bring close to it.
(More boringly, you can just use the magnet to slow down a less ferocious disk.)
In theory, you could even use this principle to achieve magnetic levitation. All you'd need would be two copper cylinders in the oh-so-safe "mangle" configuration, spinning like crazy around their long axes. Then a strong enough magnet could be suspended by the Lenz's Law eddy-current effect between and above the cylinders.
You'd have to be out of your freakin' mind to make such a thing, of course.
I give you: The one, the only, Bill Beaty. (More videos here.)
(Via. I should have noticed this when it was new, more than a year ago, but I didn't. I presume that in the intervening time at least one crank has decided that this, at last, must be the secret of antigravity/perpetual motion/free beer.)
Oh, and before someone asks me what a ball-bearing motor is: It's this.
And herewith, a more recent BillB video, partly just to get him a few MetaCafe hits (the older vids, like the one above, are on YouTube), and partly for "it's like cryogenic napalm":
Make Some "Liquid Nitrogen" - Awesome video clips here
20 August 2008 at 9:15 am
Does it matter which way the cylinders are rotating?
because they seem to be spinning so that they will draw in anything toward the table. It might (only a little) safer if they span so that they expelled anything away from the table. In that way the table could act as a bit of a guard.
I dont undestand the physics so it may not work that way.
20 August 2008 at 1:58 pm
On his page for this contraption, Bill mentions that the magnet should float either way, but should float much higher if the cylinders are spinning so as to "blow" it upward rather than "suck" it downward.
20 August 2008 at 3:13 pm
Any reason eddy current braking couldn't work on a bicycle? Or would it be too slow/not enough aluminium?
20 August 2008 at 4:12 pm
Even if you used a few kilos of magnets and a nice thick solid-aluminium wheel rim, I think it'd be quite difficult to tell when the "brake" was operating :-).
You could maybe come up with some geared contraption that, say, spun a copper disk past magnets much faster than the wheels were rotating. But if you're going to go to that much trouble, you might as well do it properly and make an electric-motor-assisted bike with regenerative braking.
Also bear in mind that conservation of energy applies, here. The energy you dump by eddy-current braking goes into heating up the conductive disk. If you're just waving a magnet over a slab of copper or damping the oscillation of a balance beam then the temperature change in the metal will be too small to notice, but if you want to bring a bike and rider to a halt, any form of energy-dump brake will turn your joules of momentum into joules of heat, and get rather hot.
21 August 2008 at 6:47 am
Who wouldn't want a bicycle with brakes that glowed red with latent energy when you arrive at the office?
21 August 2008 at 9:18 am
Rubber rim brakes get quite warm, and I've read that on long downhill stretches it's not unknown for them to get so hot they can blow tires, so it's much of a muchness. Pity, liked the idea of non-wearing brakes.