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  • [MUSIC PLAYING]

  • I wanted to show you one of my favourite props from last

  • year's Christmas Lectures, which is a Mobius strip steel

  • track, covered in maybe 2,000 or 3,000 of these super strong

  • neodymium magnets.

  • [MUSIC PLAYING]

  • So what I want to show you is how this track is going to

  • interact with one of these.

  • This is a high temperature superconductor, made of

  • yttrium barium copper oxide.

  • It's a sort of ceramic material.

  • And a superconductor is something that, when you cool

  • it down, in this case with liquid nitrogen, it loses all

  • its electrical resistance.

  • So this leads to some interesting behaviours, which

  • I'm going to show you.

  • And to do that I'm going to need to hang this thing from

  • the ceiling, which I'll do now.

  • [MUSIC PLAYING]

  • Oh!

  • [MUSIC PLAYING]

  • So we have one suspended, super strong, neodymium

  • magnetic Mobius strip track.

  • What we need now is some liquid nitrogen.

  • So we're down in one of the labs at the RI.

  • And this is where the liquid nitrogen is stored.

  • Right.

  • Got what we need.

  • The superconductor is now cooling down, which will take

  • a minute or so.

  • It needs to be cooled down in situ, because, one, what I'm

  • trying to get to is to have a superconductor kind of locked

  • into this position.

  • So I need to cool it down in situ above the track.

  • I'm cooling it down past it's critical temperature, the

  • temperature at which it will become a superconductor, and

  • its electrical resistance will completely disappear, which is

  • about minus 180 degrees.

  • And the burger tray is there to maintain the spacing

  • between the superconductor and the track.

  • I mentioned that we're trying to lock it

  • into a specific position.

  • So that is the position that we want it to take up.

  • I'm just going to slide this off.

  • This is the best way I've found of doing this.

  • If you ever hold it so quickly enough you can see that it's

  • actually levitating on the track.

  • Actually, you might be able to see a gap underneath it there,

  • sliding back and forth, levitating.

  • Oh!

  • OK.

  • So unfortunately, it warms up quite quickly.

  • Obviously, the effect only works as long as it is at its

  • superconducting temperature.

  • So what's actually going on there, a good way to start to

  • understand that is to think about how magnets interact

  • with ordinary conductors, like this copper tube.

  • So if I just drop this nut through, it falls all the way

  • through very quickly, as you'd expect.

  • If I drop a strong magnet through instead, it takes

  • quite a long time.

  • So the reason that's happening is as the magnet moves through

  • the copper, it induces electric

  • currents in the copper.

  • Those electric currents, themselves, have their own

  • magnetic field.

  • And that magnetic field will always arrange itself so as to

  • resist the motion that's creating it.

  • This is the fundamental principle of electromagnetism,

  • the moving magnets.

  • And the conductors are moving conductors near magnets,

  • creating electric currents.

  • Now, because this is not a superconductor, because it has

  • electrical resistance, the currents that are created as

  • the magnet falls through will always tend to die away.

  • So if the magnet were, for instance, to be stopped by the

  • electric currents, those currents would immediately die

  • away, and the magnet would start falling again.

  • So what actually happens is it just falls

  • slowly through the tube.

  • If, however, this were a superconductor those currents

  • would not die away.

  • As soon as you induce them by introducing the magnets into

  • the tube, those currents would remain and would keep going

  • and would keep going even after the magnet has stopped.

  • So if this were a superconductor, the magnets

  • would basically be locked in the pipe.

  • They would be effectively levitating in the pipe

  • indefinitely.

  • So that's part of the explanation.

  • We are sort of halfway there.

  • And we'll get onto the track now.

  • And to do that, we saw, when I was demonstrating the

  • superconductor on the track down here, that the effect

  • only lasted a few seconds.

  • It warmed up too quickly.

  • It warmed above its critical temperature and stopped

  • levitating.

  • The solution we've decided on for the Christmas Lectures

  • dossier was to make it a little train

  • stroke, boat thing.

  • So we've got a piece of the superconductor embedded in

  • polystyrene with its own little

  • reservoir for liquid nitrogen.

  • So let's cool this down.

  • Again, it's not a burger tray this time.

  • It's a slightly different dish.

  • Gently pick it up.

  • And I think--

  • So now I can send her out.

  • Come all the way back.

  • Come back on top.

  • This is why we made it like a Mobius strip, so that it can

  • go and come back on the opposite side it went around.

  • [MUSIC PLAYING]

  • So if I stop it there.

  • So it's kind of locked into position.

  • So it's not just repelling the magnets and hovering.

  • I can sort of pick it up and then stick it underneath as

  • well, and it will hang there.

  • So as I mentioned with the copper, when you induce

  • electric currents in conductors with a moving

  • magnetic field or changing magnetic flux, the electric

  • currents that you create will always have their own magnetic

  • field that resists that change, that

  • resists that motion.

  • So when it's hovering on top of here, gravity is trying to

  • pull it towards the track.

  • That small motion toward sets up the electric currents in

  • the superconductor, which have a magnetic field that resist

  • that motion.

  • But likewise, when it comes around and is now on the other

  • side, now gravity's trying to pull it away.

  • So the act of trying to fall away from the track sets up a

  • different sets of currents with a different magnetic

  • field, which this time are attracted to the track.

  • So whichever way you try and move it, whether you're trying

  • to move it off to the side, up and down, towards the track,

  • away from the track, the superconductor will resist

  • that motion, effectively locking itself into position

  • relative to the track.

  • So the only direction it can move is the direction which

  • the magnetic field doesn't change, i.e.

  • along the track.

  • So basically, the superconductor can be any kind

  • of magnet it wants to be, or any kind of magnet it needs to

  • be at that moment to keep itself in the right position

  • relative to the track.

  • So this material was a real breakthrough and possibly the

  • first step on the way to what might be the holy grail of

  • superconductor research, which would be a superconductor that

  • would operate at room temperature or close to room

  • temperature.

  • At which point, we might be able to use them to replace

  • all of the conventional conductors, like copper and

  • things, that we use in all our electrical applications today.

  • And all those materials, when they conduct electricity they

  • do it inefficiently to some extent and waste energy.

  • If we could superconduct in those applications, then that

  • would really change the world.

  • [MUSIC PLAYING]

[MUSIC PLAYING]

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B1 中級

メビウスストリップに超電導体を浮上させる (Levitating Superconductor on a Mobius strip)

  • 43 2
    Ryoya に公開 2021 年 01 月 14 日
動画の中の単語