If the magnet in the video is neodymium I wouldn’t be surprised if it was impossible for someone to separate it from metal without a tool of some sort. I.e. if you stick something on your fridge with it it’ll be there forever.
I’ve accomplished the same trick (to a smaller extent) by using a new roll of aluminum foil (which comes wrapped around a cardboard tube) and a 1/2 inch cube neodymium magnet.
From a physics point of view: Where does the energy go? The magnet, after falling for a foot, should have a speed of 8 feet per second. In reality it’s only going about 1/10th that. Is the energy converted to an imperceptible amount of heat? Does it warm the magnet or the aluminum foil?
As a general rule, whenever the question is “where does the energy go?”, the answer is almost always “heat”. In this case, the electrical currents are in the pipe (or the foil), so that’s where the heat is generated, via the resistance of the material.
Yeah, that’s why I said “to a first approximation”. Adding to the complication is the fact that the dipole approximation is usually only valid at distances very far from the magnet, but usually these demos use a magnet that’s of comparable size to the pipe.
You can actually estimate the terminal velocity of the magnet pretty straightforwardly by assuming that the magnet is an idealized dipole and that the walls of the pipe are “thin”. (It’s an exercise in the new E&M textbook by Zangwill.) Thickening the pipe wouldn’t make the problem that much harder, but treating the magnetic field as anything other than a dipole would be tricky.
Heat. The magnet is inducing electrical currents in the metal tube (or in your case, the roll of foil). The tube/foil has resistance, and so you end up with ohmic heating. You can calculate the expected temperature rise
E = m[sub]mag[/sub] * g * h
where E = total energy transferred from magnet to metal, m[sub]mag[/sub] = mass of magnet, g = acceleration of gravity, h = height of fall
deltaT = E/(C*m[sub]metal[/sub])
where deltaT = temperature increase of metal, C = specific heat of metal, m[sub]metal[/sub] = total mass of metal tube or foil roll
A 1/2" cube magnet doesn’t weigh very much, and a foot isn’t very far; that’s not much energy compared to the heat capacity of the pound or two of metal in the tube/roll. Even in the case of the OP’s video, the copper tube is probably being warmed up more by contact with the experimenter’s hands than it is by the magnet falling through it.
One place where eddy current dissipation can result in substantial temperature rise is in the resistance mechanisms of some exercise machines, like bicycle trainers or elliptical trainers. The user is spinning a metal rotor, and magnets next to the rotor create eddy currents that result in drag. In this case you’ve got a sustained power input by the user into a relatively modest amount of metal. Were it not for air movement over the spinning rotor, it would eventually get smoking hot.
On of the follow-on videos on YouTube is a demonstration of that very thing.
I need to warn people about that second link…it’s dangerous to go there. There are so very many things that you never knew you needed from AS&S.
Great video.
I remember a demo in a physics class long ago but had forgotten about it.
Last night, I amused myself for a bit using a spherical magnet and an aluminum foil roll as suggested by bizerta. This noticeably slows the fall but not the dramatic magic trick in the vid.
Lynn, when I was a kid, my brother and I would convince my mom to drive us to AS&S in Chicago and we’d go nuts with a couple bucks saved allowance. The store was chaotic and really, really fun. They moved to a new location some time ago and it lost a bit of the fun but it is still an easy place to kill an hour. They have many ‘exhibits’ made from stuff they sell and there’s all sorts of crazy stuff hanging from the walls and ceilings.
I now live about a half mile from the place and still get there from time to time to browse. I’ll make a point to visit soon to check the magnets and maybe see if they have suitable nonferrous tube.
I haven’t read the whole thread (or watched the video) but I saw some of the prices up thread I wanted to mention that I’ve done this trick with smaller neodymium magnets and 1/2 (3/4?) inch copper pipe I have laying around my house and it works just fine. This doesn’t have to be a $200 project.
I’d have to look when I get home, but I’d guess thisis the magnet I used and I have a chunk of copper that was a few inches long that I did it with.
Since reading this thread, I’ve been playing around with pipe and magnets.
I found a three foot piece of (nominally) 1" copper pipe down in my basement which is a bit over one inch inside diameter.
As luck would have it, I had a 1" rare earth coin magnet stuck on my fridge. It takes about five seconds to fall through and I think it’s a really cool demonstration.
Another neat thing which I hadn’t expected is that the magnet doesn’t tumble as it falls but rather immediately rights itself parallel to the pipe walls.
I also tried it with a smaller but taller 5/8" magnet and it isn’t quite as dramatic but still noticeable. This magnet does sometimes tumble and looks weird falling in slow motion. I’d say the two magnets are similar strength so, not surprisingly, the proximity to the pipe walls factors in here, too.
I went to American Science & Surplus this afternoon to check out more magnets. They were out of the one really wanted but the guy said they’d probably back back in a couple weeks. The coin magnet I have isn’t as thick as these and I’m hoping they’ll be a little stronger. I did pick up a couple more of the 5/8" ones.
I barely made it out of there without buying a small venus flytrap. Something tells me that when I go back again to check on the magnets, I’ll be bringing one home.