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]
Lucky kids. When I was in school, the only commercial superconductors were liquid Helium cooled Niobium-Titanium alloys, something not many classrooms could handle. ;-)
Luckily, when I got to programming, we already had zeroes and ones. Imagine our predecessors who had to do it with uppercase o's and lowercase l's...
I forget what type we used but it was cooled by liquid nitrogen. That prof. used lots of magnets and superconductors in his research, some cooled by Helium-3. Now that stuff is expensive.
The high-temperature ones were probably Yttrium-Barium-Copper-Oxides (YBCO) I've seen this demo done with them. The magnet's motion is almost undetectable, they appear suspended in the copper pipe until they start to warm up.
The inverse square law of force strength falloff would make it very hard to scale this linearly, that combined with the fact that weight increases with the cube of size means that the magnets involved would have to be incredibly strong.
Not impossibly strong mind you, if magnets can hold a hundred ton train car off the ground I am sure they could slow the fall of an elevator.
I'd not want to be inside that elevator with anything magnetic in my pocket, though.
Yup, magnetic field falls off as 1/(distance3). That's why being 5 feet from an NMR magnet is bad news bears, while being 8 feet away is fine (numbers made up, do not test). I remember seeing signs all over the place at our labs at the uni, saying to not enter if you wear a pacemaker or if you are wearing a wrist watch.
But it's possible to the same experiment with a battery and an electro-magnet, isn't it? (Not sure how the power-density of lithium-ion batteries would relate to floating 80 kgs down a tube... but seeing as people have demonstrated hover-boards, it should be doable) ? Assuming the purpose is a) float down tube, and b) SCIENCE! -- and not c) with rare earth magnets...
Even hard disks are not affected by external magnetic field in the way most people expect. Strong magnetic fields interfere with the operation of drive (seeking voice coil) but accidental erasure of data on platters by external magnetic field is relatively improbable.
Generally most of the damage caused by strong external magnetic fields to any electronics are caused by second order effects, i.e. something gets confused and causes data-destroying failure.
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.
No one is comparing or likening charge to some other thing. We simply give opposing charge "directions" a name. Everyone uses the name. It's a convention.
The comparison that is being made is the direction your thumb points when your fingers wrap in a particular direction. It's not merely giving it a name - it's specifically likening the vector to the relative orientation of your fingers. That's why the comparison is memorable. If it were merely a name with nothing that it is compared to, it would not be memorable nor helpful.
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.
The latch on our clothes dryer broke; the door would no longer snug up and maintain the seal or necessary pressure for the safety switch. I now have a 2x2x1" rare earth magnet holding the door shut against the dryer frame. Its strong enough to keep the door snug, but I can still open the dryer door with a good tug.
I'm not sure why I had a 2x2x1" neodymium magnet laying around, but I'm glad I did.
It is interesting that with the neodymium magnets being rare earth materials and apparent limits being placed on their mining/import/export etc., the price of them has sky-rocketed. There was a time when all the bass cabinets for bass players were getting smaller and smaller and lighter and lighter thanks to these magnets because it meant you could have a smaller speaker driver (hence less weight) but now they're having to resort to the heavier speakers again.
A shame really! I never did buy any light cabinets but always wish I had! At least Class D amplifiers are doing the rounds and so massive wattage amplifiers are now tiny and lightweight.
Copper compared to many metals isn't particular expensive per KG, it is just relatively expensive in the class of metals that we use a crapload of (for example aluminimum is around $1.8 per KG) it is still much cheaper than tin at ~$22.
The main reason that copper is so valuable is it has so many uses and is very easily recycled(I saw an estimate that said 80% of all the copper ever mined is still in use) so holds it's value (You often get 90-95% of its commodity value for recycling).
Copper is always in demand, has a bajillion uses, and can be scrapped and reworked very easily. This is why you hear about meth heads stripping copper wiring out of old homes. It's not that the copper is incredibly precious. It's that the copper is so easily liquefied into cash at any number of one-stop, no-questions-asked destinations.
Copper is expensive as a building or product material, not in and of itself, but in relation to many of the alternatives in any given application. Small differences in component costs, scaled over large supply chains, add up to huge differences in margin at the macro level.
Here in the UK there is constant copper theft going on. The church up the road here had their roof pinched 3 times in 6 months. But it is good that if you have new heating (with a combi boiler) and your condenser boiler taken out, that massive water tank you had in the loft to hold water now suddenly turns into money!
Copper and aluminum are two commonly used metals that are manufactured by relatively costly process that at the same time allows essentially complete recycling.
On the other hand there are not that many uses for pure copper (most of them are electricity related) as there are many other metals or copper alloys that are significantly cheaper.
That may be, but most of that markup is probably in the part it self, if you compare to a steel tube of the same dimension you'll see an indication of that. I was going by the current price the metal trades for, that is, a ballpark figure as something to "begin to imagine" what it could cost based on the material alone.
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?
You can't make a permanent magnet stronger, I believe. I worded that weird. I meant a smaller magnet, that is powerful, and a smaller tube. My brain doesn't work so well sometimes.
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.)
A lot of reading, plumbing, and electrical work. But when things are running smoothly, we grow complex oxide crystals using pulsed laser deposition and then measure their magnetism with SQUIDs (superconducting quantum interference devices).
I don't think the spinning should have any effect on the speed. Only the vertical movement would create eddy currents. It could be he does it to keep the magnet stable as it falls (like spin on a football).
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.
As magnet falls, it is slowed down, this drag is created because magnetic forces are resisting gravity. So after some time, magnet loses that force, I just wonder how long does it take. It can't fall slowly like that forever.
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.