I don't understand French nearly well enough to understand the explanation in this video, but I was amazed at what the physical world can do when I saw this video.
(Video courtesy of Wimp.com.) What's going on here? Well, you've got:
- a series of bar magnets on the bottom,
- a ceramic cylinder,
- an insulating, non-magnetic piece of plastic between the magnets and cylinder,
- and liquid nitrogen being poured into the cylinder before the insulator's removed.
Amazing! For those of you who like puzzles, you've got until tomorrow's post goes up to figure it out. (I'll give you a hint: Walther Meissner.) What's going on here, and how does this work when you're upside-down?
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HOT
Ethan,
Our undergraduate students at Ithaca College Physic have posted similar movies on youtube. The link goes to the race track that one of our students did this summer. The pucks and tracks are all designed by undergraduate physics students.
http://www.youtube.com/watch?v=6lmtbLu5nxw
There doesn't seem to be much explanation. They seem to be saying that they have two tracks, the middle being the opposite polarity from the outer two.
Though they don't say so, the title indicates that the object in the middle is a superconductor? The liquid nitrogen's function is beyond my limited knowledge, but I would guess it does something to the superconductor such that it creates a magnetic "cushion" of sorts, so that the magnetic field "hugs" the superconductor preventing it from touching or moving away. Then, of course, since the fields are along tracks, it's simple to move it along those tracks.
I think I know the guy that made the setup. He's big on teaching this subject to undergraduates, and this is part of how he gets their interest.
I think i smell some YBCO coming from that video. You can see a black disk on the bottom of their little LN2 cylinder. Arrange the bar magnets into a rail and the LN2 cooled YBCO will stay aligned with the rail of magnets. Hooray for science and SC. Now if only we had some RT SCers.
the ceramic cup is made of high T superconductor. (looking closely it appears that the bottom of the cup is darker, and probably the actual superconductor, perhaps YBCO)
you pour the LN2 in to cool the superconductor below it's transition temperature. at that point it becomes superconducting. one property of superconductor is the Meissner effect. the Meissner effect is the expulsion of external magnetic fields from the bulk of the superconductor (in this case, the series of bar magnets creates the external field.) it does this by inducing currents within the penetration depth of the superconductor. these induced currents create their own magnetic field which is opposite in polarity from the external bar magnetic field. if you try to push two oppositely aligned magnets together, you can feel a repulsive force. this is how the cup levitates. the induced magnetic field in the superconductor repulses the external magnetic field of the bar magnets, causing the cup to levitate.
how does it work when it's upside down? the superconductor is not perfect. within the superconductor some of the external field is not expelled. this field gets pinned--it is stuck within the superconductor. when you turn the cup upside down, rather than floating via the repulsive force, the cup actually hangs on the pinned field lines.
Contrary to the hint, I don't think it's the Meissner effect (which I normally associate with field expulsion from a type-I superconductor).
I'd wager that the bottom of the cup is a type II superconductor, with magnetic field lines penetrating the superconductor according to the field conditions during the transition (this is why it's key that the superconductor be cooled down "in place"). Once superconducting, the infinite conductivity of the superconductor "resists" any change in the penetrating field, so that it works rightside-up or upside-down. This is the same as the classic eddy-current demo with the magnet-falling-down-the-copper-tube-slowly, only now the resistivity is zero, so rather than falling slowly, the superconductor doesn't fall at all.
That's awesome - I wish I could afford a superconducting thimble like that one. What I would like to know is how the other ceramic is bonded to the superconductor - was it formed and fired around the superconductor? Come to think of it - given the picture quality - is that even a ceramic cylinder or is the superconductor press fit into a plastic device?
Very cool.
Damn, #7 beat me to it.
Didn't the Mythbusters used this trick once?
Having this video auto-play in a browser is pretty annoying. Every time I come to "Starts With A Bang" I have to find the thing and stop it.
GBJ,
Thanks for bringing that to my attention! Is it fixed now?