# Another use for rare earth magnets

http://www2.citytel.net/~solar/milledrillpage.html Scroll down to item #5

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Regards.
Ken.
http://www2.citytel.net/~solar/
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Ken Davey wrote:

Another fun use for rare earth magnets which gets a rise out of people who haven't seen it before is this recipe:
Ingredients:
1 cylindrical rare earth magnet about 1/2" diameter by 1/2" long.
1 piece of 1/2" copper water pipe about a foot long.
Holding the pipe vertical, drop the magnet into it and watch how long it takes it to ooze all the way down through it.
The hard part is finding a "simple" way to explain the effect to non-techies.
Jeff
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Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
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How long does it take to 'ooze'?
Wes
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snipped-for-privacy@lycos.com wrote:

Mine takes about 5 seconds to drop through about 10 inches of pipe.
BTW, the priciple which makes for the slow decent is that when the magnet moves with respect to the pipe it's magnetic field induces a dc electric current flowing in a circle in the metal of the pipe. That current creates its own magnetic field which exerts a opposing force on the magnet.
Since the induced current and the opposing force on the magnet only occur when the magnet is moving, a balance of forces establishes the steady speed of magnet's decent.
It's akin to the "regenerative braking" used on hybrid cars, except the car's electric motor windings are a lot more complex that the single shorted turn created by the pipe.
Much of the current created when running the car's electric motors as generators when braking gets put back into its battery, while with the magnet and pipe, the current is all wasted in imperceptible resistance heating of the pipe. <G>
At least that's the way I understand it...
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
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Another way to put this is that as magnet induces current in the pipe, the current wastes energy by heating the pipe slightly. That energy has to come from somewhere, which if from the mechanical energy of the magnet. That's why it slows down -- the mechanical energy is converted to heat.
i
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Yeah, that's good. But technically, it's backwards.
The loss due to resistance is why it falls at all, not why it slows down. If you do the same experiment without resistance, the magnet hovers without falling:
http://en.wikipedia.org/wiki/Image:Meissner_effect.jpg
When you add resistance, it causes the energy holding the magnet to slowly be converted to heat which allows the magnet to fall. So the heat loss is why it falls, not why it it slows down.
By talking about the heat loss, you have still failed to explain why the magnet is coupled to the pipe in the first place since the copper pipe is non magnetic or why a magnet will hover above a superconductor without falling at all. I don't know of any easy ways to explain the effect to someone that doesn't already understand electromagnetism.
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Curt Welch http://CurtWelch.Com /
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That's very profound.
I have one more question. How could I make the magnet fall even more slowly. Specifically, if I try to get a pipe with diameter very close to that of the magnet, would that slow down its fall even further?

Of course. What I explained is why the force from the pipe that acts on the magnet, is opposed to its movement. It could not possibly be aiding the movement.

Doubtless.
i
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I think so, but I'm not sure.
I made them fall slower simply by putting 4 together (4 was all I had) and dropping them through the pipe. Can't say I really understand why they fall slower when together. The combined magnetic field would be larger but so would the combined weight so I don't know why the two effects don't just cancel each other out and end up with the magnets falling at the same rate. Maybe the field from one magnet tends to make the field from the others stronger so the combined field is more than the sum of the parts???
I think if you were to replace the copper pipe with a wire coil and then connect the ends of the coil together it might make it fall slower. That is, the magnets would be inside the wire coil. Or maybe just use many small copper wire rings???

Yeah, but your answer I think is the easiest way to make someone grasp what is happening even if the truth is more complex. I'll be using that answer the next time I have to explain it to someone... :)
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Curt Welch http://CurtWelch.Com /
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On 2007-04-15 21:51:34 -0700, snipped-for-privacy@kcwc.com (Curt Welch) said:

That is the case. Less air space is better. The pipe and the magnet will be in closer proximity to the magnetic fields. The field weakens through space. Making the walls of the pipe thicker, or using a material that is a better conductor will also make the magnet fall slower.

That is not really true. Consider a permanent magnet generator. The magnet is spinning within what? A coil of copper wire. Magnets affect more than just ferrous materials.

I'll try.
A. First, you drop the magnet in close proximity to a conductor. From playing with generators, we've all learned that when we move a magnetic field in proximity to a conductor, we induce electrical current.
B. Now we have current flowing through the pipe.
C. We've learned that when we run electrical current through a conductor, we induce a magnetic field. The current running through the pipe is inducing a magnetic field.
D. The magnetic field is repelling the magnet and opposing the force of gravity.
Now go google or wiki on Lenz' Law.
Rob
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The magnet, in this case, is affecting electrons within the metal. Anything conductive will respond to magnet movement. The key is movement: the field has to be cutting through a conductor to generate electron flow, and whether it's the field that's moving through the conductor (as in an alternator) or the conductor moving through the field (as in a generator) the result is the same. No movement equals no electron flow generated. The magnet hovering over a superconductor assumes that a current was stimulated within the superconductor in the first place. The hovering magnet cannot do that, since there is no field movement. Nothing is free.
Dan
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On Apr 16, 2:35 pm, Dan_Thomas snipped-for-privacy@yahoo.com wrote:

I always thought the act of bringing the magnet into place over the superconducter was what created the current in the superconducter.
Dave
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It can be done by placing the magnet on the superconductor before chilling it. No (apparent) motion is required.
Actually, even the tiniest vibrations are enough to set up currents that disturb the stability of the setup enough to wiggle it a bit more, causing more current, wiggling it more... ad nauseum, until the thing levitates, and becomes quasi-stable.
LLoyd
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On Sun, 15 Apr 2007 16:57:36 -0500, Ignoramus18842

Not much, untill it fits so closely it starts acting like a pneumatic damper. Retarding force is proportional to flux density thru the pipe and pipe wall conductivity. Flux density varies inversly with path length. Making a tighter fit will have a fairly small effect because the gap from magnet to pipe ID is a small percentage of the pathlength from pole to pole of the magnet.
Increasing pipe wall thickness will increase conductivity and thus induced current, thus making it fall slower.
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Ignoramus18842 wrote:

:-) Use a "superconductor" peice of pipe and it wont fall at all. :-)
...lew...
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Thanks for the reply and explaination. Got a tip on where to get one of these magnets?
Wes
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wrote:

http://www.amazingmagnets.com
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ChuckEaves
78 FLH Shovel-in-Progress
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By the way. These rare earth magnets are very useful for hanging tools, car keys, as well as papers on the refrigerator. Be careful around small children. If a child swallows two magnets, they can pinch their intestines. Toy manufacturers mostly stopped making toys with magnets that can be swallowed, at least that's what I heard.
i
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Thanks,
Have some on the way.
Wes
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On Apr 13, 2:00 am, snipped-for-privacy@lycos.com wrote:

Computer hard drives have them. Find some old drives, take them apart, and look for them in the mechanism that swings the recording/reading head across the disc. I have one large magnet from a big old drive that is arc-shaped and has a steel flux plate across it. Getting that plate off is fun, and putting it back on can pinch your fingers hard enough to raise blisters. I use it to demonstrate the magnetic drag phenomenon to my Aircraft Systems class by running a strip of aluminum through it. They need to understand how a magnetic- drag tachometer works (magnet spinning inside an aluminum cup; the magnet is rotated by the tach cable and the cup drives the needle). Newer drives have smaller magnets that are still really handy.
Dan
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The effect is even more pronounced if the "pipe" is a coil of heavy wire, and shorted. More EMF can be generated by the multiple turns than by just one.
We used to do this trick with the really heavy head-positioner mechanisms from old SMD removable disk drives.
The magnet weighed about twenty pounds. The coil of the positioner was around 100 turns of about - oh - ten gauge square magnet wire. If you shorted the coil, the slider (weighing 'bout another pound) would take about a half-minute to fall six inches. It was truly eerie, looking almost like it was viscous-damped.
LLoyd
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