OMFG, I love that demo. What happens if you use something more ferrous for the outside tube, like iron?
That’s one thick pipe.
I’m guessing it’s NPS 8, SCH 160 which would give it an outer diameter of 8 5/8" and a wall thickness of just over 9/10". Could be a bit smaller - check here. If you were to call a tubing supplier in your area they could probably quote it based upon that designation.
Where you’d get the magnets I really don’t know but I’d start with a science surplus and give them a call if you can’t find what you need.
The magnet would probably stick to it.
2" x 2" x 2" neodymium magnet, $195.35. 2" diameter sphere, $102.96.
Edit: K&J Magnetics: DY0Y0 and K&J Magnetics: DY0Y0-N52, $153.86 and $206.46, respectively.
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Here’s another video, showing a smaller magnet and longer, smaller diameter tube. (Should be cheaper.)
Using a ferrous material would not work. The magnetic field caused by the induced current is reverse polarity to that of the magnet and this magnetic field has no other forces to overcome in order to push the magnet away.
If you used a ferrous material, the force of attraction would overcome the field caused by the induced current, put simply, it would just stick.
United Nuclear has a huge assortment of neodymium magnets and related items. The ones at the link I think are all N45 and N48 which it seems are the strongest you can get which aren’t too fragile to work with. At the top of the page though they say they also have N50 and N52 but “they are terribly fragile and are known to shatter , chip or break even with careful handling.”
But if you get something in the 1 inch and up category (and even some approaching that size), you really do need to be EXTREMELY careful. Please read the cautions on their site and whatever comes with the magnet you buy. Just a few millimeters can make a big difference between the magnet not noticing that metal desk top and deciding that it is its soulmate. If your finger or a pet’s paw happens to get caught in between it can easily get broken/crushed - especially with the very large magnets. If you haven’t worked with them before, it’s difficult to really grasp just how powerful they are until you get used to it.
I know I sound like a nagging little wuss, but the web site will back me up on this.
This.
That demonstration is of an eddy current brake. It’s how trains and roller coasters stop.
Lenz’s Law - induced current generates a magnetic field that tends to oppose the magnetic flux that caused it (My physics teacher used to call this 'the Sod’s law of Physics).
I made use of this to brake the DC motor on my spin art machine - in the ‘off’ position, the switch shorts the motor terminals - the current generated by the motor acting as a dynamo flows in the opposite direction - that is, trying to turn the motor the other way.
Not quite so impressive: http://www.youtube.com/watch?v=H31K9qcmeMU but available here: http://www.teachersource.com/product/eddy-current-tubes-super-large/magnetism-fields
[QUOTE=deltasigma;16764621[They have an N45 nickle plated 2" cube for $120]
(Supermagnet # 222 (2 Cube) - $225.00 : United Nuclear , Scientific Equipment & Supplies, United Nuclear , Scientific Equipment & Supplies)
[/SIZE][/FONT]
[/QUOTE]
Looks like they have better prices than the place I found.
Thanks! After seeing them used on Mythbusters I’ve wanted to get one (or two). I’ll remember this thread (I hope) when I eventually get around to wanting one badly enough.
Does the pipe wall need to be that thick? I’ve got a long section of 4" copper pipe but no sufficiently large/strong magnet to try this. I have a dozen Nd ball magnets but they’re only about the size of a marble and don’t produce satisfactory results.
Glad to help. You might want to check out Evernotefor clipping things you want to remember. There’s a free and paid version and the free version isn’t crippled, you just don’t have a huge amount of bandwidth for synching, but you probably won’t even notice. And it’ll synch with anything except maybe your toaster (although I wouldn’t bet against that. 
)
I believe generally you get copper tubes not pipes. Pipe sizes and schedules are NOT valid for TUBES.
I see a number of ebay dealers who can draw the above tube for you. Not sure if the policy allows me to provide links but you can search for it. You can also buy a round bar of copper and ask a machinist to drill it for you.
The thicker the wall, the more eddy currents you create and the slower the fall. To a first approximation, the speed of the magnet’s fall is inversely proportional to the thickness of the walls; so if the wall of your pipe is one-quarter the thickness shown, and all other things being equal, it’ll fall four times as fast.
Anything made of that much copper is going to be very expensive. McMaster has bronze, 9"ODx7"ID, in a 6.5" length for $467. Bronze will work as well as pure copper (it’s conductive and nonferrous), and pure copper would likely be even more expensive.
Aluminum will work as well, and should be much cheaper than copper, but McMaster doesn’t have round aluminum tubes with OD larger than 5 inches; you’ll have to look elsewhere.
The higher the conductivity, though, the slower the falling speed. (It’s another inverse proportionality.) Given the relative conductivities of aluminum and copper, a magnet falling through a aluminum tube will fall about 58% faster than a magnet falling through a copper tube of the same dimensions. Bronze generally has even lower conductivity than aluminum, so it’d be even worse.
Well, you could somewhat compensate for the lower conductivity by making the walls thicker, which might even be practical since aluminum is much cheaper than copper.
I don’t think that the falling speed is actually inversely proportional to the wall thickness, though (or more precisely, I think that one would very quickly reach the limits of applicability of that approximation). The problem is that magnetic fields fall off rapidly with distance, so the outer portions of a thick pipe are going to matter less than the inner portions.
I’ll back deltasigma up on this. I have a number of smaller neodymium magnets, and they’re far stronger than you’d ever thing. People are constantly amazed at their strength. I can only imagine the damage that would occur if my finger was caught between a large one and a chunk of metal.