Yup, it's pretty cool. This was one of the most impressive demo's my physics professor did in our freshman physics class. The point is that as the magnet is falling through the copper tube it creates an electric current which then creates a magnetic field in the opposite direction of the magnet's movement. In other words, this is the practical application of the Right Hand Rule [1].
Two fun facts about it: first, if you cut a small notch along the length of the tube, this will not happen as the current cannot go around the tube.
Second, imagine a superconducting tube with an extremely powerful magnet right in the center of it. Now, try to get the magnet out without cutting the tube.
You can also think of it as magnetic flux lines encountering "drag" as they pass through conductors.
The limiting behavior of this view is in type II superconductors, which entirely "pin" flux lines -- they present an effectively infinite resistance to the movement of flux lines. That's why a magnet levitates over such a superconductor. [1]
Technically, this has nothing to do with the Right Hand Rule. That is just a human convention to assign a positive and negative signs to a particular direction. Electromagnetism doesn't care about our silly human notations.
Yeah I'm actually pretty surprised that this made it to the front page. Pretty novel and I thought more people would know about it (being a typical physics class demonstration). Cool nonetheless.
I would wrap an electromagnet around the magnet in the tube, then turn on current to create a field in the opposite direction. Hm... If I pulse the field quickly, maybe it would even remove itself :)
The problem with the youtube link however is that any video with 'magnet' in the title brings you uncomfortably close to thousands of youtube nutters posting about perpetual motion machines.
Lenz's Law in action, lots of good demos online, or take any strong magnet and move it rapidly while in close proximity to any non-magnetic conductor (brass, copper, aluminum, etc.) You can feel the force exerted by the generated field. Automobile speedometers used to operate on this principle.
be very careful when handling such powerful magnets - if you have something metalic on you, they might jump off the table and smack you really hard. bones will be broken.
6.5 inch outer, 5.5 inner, 12 inches tall (twice what the video shows) for only $700. So I'd say $300 would be a good estimate of what you can get that piece of copper for.
That magnet is also probably pretty expensive, that's a big one.
The interesting part is that NMR magnets often have a second magnet within them that creates an opposing force outside of the bore of the NMR (or at least they used to do it that way).
If you notice on the video the yellow chain which keeps ferrous metals away from the instrument isn't really that far away. Also, when he drops the aluminum disc, it actually starts to accelerate after falling only a foot.
Because of the "counter" magnetic field produced, NMRs have a much small/weaker external magnet field that you'd expect.
This effect gets used in modern trains and roller coasters in the form of linear eddy current brakes. In practice, this looks like an electromagnet that's held just over the rail.
With electromagnet and iron rail the same effect is used for almost an century on streetcars under the name "electrodynamic brake". Idea is that single assembly combines this for small braking strengths and friction braking for complete stop (the electromagnet is on springs and can come into complete contact with rail, which is used as equivalent of parking brake).
So it's ok to post links to third party instead of the original content now ?
here's the link to the video instead of this portal profiting from it: http://www.youtube.com/watch?v=keMpUaoA3Tg
He had the large square magnet and the large round one within arms reach of each other so that he could swap them with one hand without walking away. That kind of freaks me out a little.
You really don't need anything as fancy. A small magnet and a narrow aluminum tube will do just fine. It won't be quite as impressive, but drop a pebble from the same height not through the tube and watch the delay for the magnet dropping. A four foot length of tube about 1/2 to 3/4 inch in diameter would do nicely.
Edit: try cast iron if you must have a big pipe. It's likely to be much cheaper. The thickness shouldn't make much of a difference either, so maybe even a cooking pot with a cut off bottom would work.
Would a thinner wall tube work just as well? It seems the effect depends on electrical conduction which should be sufficient even with much less copper. Or does it depend on how many flux lines are hitting the copper cross section?
Not sure where to get one at that thickness or diameter, but a specialized plumbing supply company can get you most standard size and diameter of copper piping.
That one is huge, though. In theory, this should work with smaller, but powerful magnets, and thinner tubes, correct?
No. Because copper isn't ferromagnetic, it will not be magnetized by the magnet, and consequently the magnet and the copper will not be attracted to each other. (In fact copper is diamagnetic, so it will actually slightly repel.)
I wonder, with a tube that is of sufficient thickness and a magnet of sufficient strength, would it be possible to levitate the magnet inside the tube?
The best way to do it is with a superconducting magnet.
And no, it doesn't quite levitate, but it takes a very long time for it to fall through. I think the video we watched in high school physics class had it take about fifteen seconds to go an inch and a half.
[+] [-] IgorPartola|12 years ago|reply
Two fun facts about it: first, if you cut a small notch along the length of the tube, this will not happen as the current cannot go around the tube.
Second, imagine a superconducting tube with an extremely powerful magnet right in the center of it. Now, try to get the magnet out without cutting the tube.
[1] http://en.wikipedia.org/wiki/Right-hand_rule
Edit: Of course, there's nothing special about the tube being made of copper. Any conducting substance will do.
[+] [-] taliesinb|12 years ago|reply
The limiting behavior of this view is in type II superconductors, which entirely "pin" flux lines -- they present an effectively infinite resistance to the movement of flux lines. That's why a magnet levitates over such a superconductor. [1]
You can even image these "vortices" directly [2]
[1] http://en.wikipedia.org/wiki/Flux_pinning [2] http://www.youtube.com/watch?v=7_ZgiumS41Q
[+] [-] huhtenberg|12 years ago|reply
Any non-magnetic conducting substance that is. Otherwise the magnet will just stick to the tube wall.
[+] [-] sillysaurus2|12 years ago|reply
It'd be completely impractical, of course, but a whole lot of fun.
[+] [-] orblivion|12 years ago|reply
[+] [-] vecter|12 years ago|reply
[+] [-] smitec|12 years ago|reply
[+] [-] reddog9287|12 years ago|reply
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[+] [-] dublinben|12 years ago|reply
This digg page is blogspam.
[+] [-] aunty_helen|12 years ago|reply
[+] [-] fnordfnordfnord|12 years ago|reply
[+] [-] InclinedPlane|12 years ago|reply
[+] [-] baq|12 years ago|reply
[+] [-] malandrew|12 years ago|reply
[+] [-] adsr|12 years ago|reply
[+] [-] aaronsnoswell|12 years ago|reply
[+] [-] dm2|12 years ago|reply
6.5 inch outer, 5.5 inner, 12 inches tall (twice what the video shows) for only $700. So I'd say $300 would be a good estimate of what you can get that piece of copper for.
That magnet is also probably pretty expensive, that's a big one.
[+] [-] frankus|12 years ago|reply
[+] [-] refurb|12 years ago|reply
If you notice on the video the yellow chain which keeps ferrous metals away from the instrument isn't really that far away. Also, when he drops the aluminum disc, it actually starts to accelerate after falling only a foot.
Because of the "counter" magnetic field produced, NMRs have a much small/weaker external magnet field that you'd expect.
[+] [-] asteli|12 years ago|reply
More reading: http://en.wikipedia.org/wiki/Eddy_current_brake
[+] [-] dfox|12 years ago|reply
[+] [-] ta_tatata|12 years ago|reply
[+] [-] plq|12 years ago|reply
Here's the answer: http://youtube.com/watch?v=KFPvdNbftOY
[+] [-] chrissyb|12 years ago|reply
This might also pique you interest http://www.youtube.com/watch?v=_dQJBBklpQQ
[+] [-] noonespecial|12 years ago|reply
[+] [-] BorisMelnik|12 years ago|reply
[+] [-] jtreminio|12 years ago|reply
[+] [-] liebfraumilch|12 years ago|reply
[+] [-] JackFr|12 years ago|reply
[+] [-] ThePhysicist|12 years ago|reply
http://www.youtube.com/watch?v=A1vyB-O5i6E&list=PL8E7ED16454...
The physics behind this are explained here: http://en.wikipedia.org/wiki/Magnetic_levitation
[+] [-] deletes|12 years ago|reply
[+] [-] IgorPartola|12 years ago|reply
Edit: try cast iron if you must have a big pipe. It's likely to be much cheaper. The thickness shouldn't make much of a difference either, so maybe even a cooking pot with a cut off bottom would work.
[+] [-] mark-r|12 years ago|reply
[+] [-] jules|12 years ago|reply
[+] [-] ingebretsen|12 years ago|reply
Not on the same scale, but appears to be designed to demonstrate the same concept.
[+] [-] Loughla|12 years ago|reply
That one is huge, though. In theory, this should work with smaller, but powerful magnets, and thinner tubes, correct?
[+] [-] TrainedMonkey|12 years ago|reply
[+] [-] martinvol|12 years ago|reply
[+] [-] tedsanders|12 years ago|reply
Source: I am a magnet scientist.
[+] [-] blacksmith_tb|12 years ago|reply
[+] [-] abcd_f|12 years ago|reply
http://www.bicycleman.com/recumbent-exercise-bikes/images/Ma...
[+] [-] lostlogin|12 years ago|reply
[+] [-] 556790|12 years ago|reply
[+] [-] omegaham|12 years ago|reply
And no, it doesn't quite levitate, but it takes a very long time for it to fall through. I think the video we watched in high school physics class had it take about fifteen seconds to go an inch and a half.