Amazing physics

October 8, 2006 | General | By: Mark VandeWettering

Even if you don’t understand the physics of superconductors (and I don’t) this still looks pretty cool.

YouTube – Amazing physics

If you want to learn more about the underlying physics on your commute, you can try listening to the final chapter of the best of the Feynman lectures as an audio book. I did. I’m not sure it helped. I kept feeling something pass over the top of my head, but perhaps some small bit of it stuck.

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Comment from Steve VanDevender
Time 10/11/2006 at 5:36 pm

The physics of why superconductors superconduct is a bit over my head too (something about electrons forming “Cooper pairs”) but the physics of superconduction is really simple — a superconductor has exactly zero electrical resistance. Not a tiny resistance, but truly none at all. If you start current flowing in a loop in a superconductor, it keeps flowing forever (or at least as long as the superconductor stays superconducting, which usually means it has to stay below a certain temperature, and can’t be in too strong of a magnetic field).

The video (which has no audio when I look at it) appears to show a block of a “high-temperature” superconductor, which these days means one that has a critical temperature above that of liquid nitrogen, which is what it looks like they pour over it in that little dish to get it cold enough to superconduct.

Why does the magnet float? Basic electromagnetic theory says that changing magnetic fields induce electric current (motion of charge) and moving charge creates a magnetic field. So moving the magnet toward the superconductor induces current to flow, and it’s confined to flow in loops within the superconductor. And there’s no electrical resistance to dissipate the current and magnetic field in the superconductor once it’s formed. So there’s an equilibrium point where the field induced in the superconductor balances the weight of the magnet, so the magnet floats above the superconductor at a particular height (and the shape of the magnetic field even holds the magnet in a particular position once it’s placed there).

At the end of the video, when the magnetic coupling between the magnet and the superconductor is used to lift the superconductor out of the dish, you then see what happens when the superconductor goes above the critical temperature for superconduction. As it warms up, gradually superconduction breaks down throughout the superconductor, and at that point the current loops in the superconductor dissipate and the magnetic field they create vanishes, and the magnet stops levitating.