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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]
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莫比烏斯帶上的懸浮超導磁 (Levitating Superconductor on a Mobius strip)

103 分類 收藏
Ryoya 發佈於 2019 年 11 月 29 日
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