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Frostbite Theater


What happens when a magnet is placed on a superconductor? Play the video to find out!

Announcer: Frostbite Theater presents... Cold Cuts! No baloney!

Joanna and Steve: Just science!

Joanna: Hi! I'm Joanna!

Steve: And I'm Steve!

Joanna: And this is a container of liquid nitrogen!

Steve: And this is a ceramic disk made from barium-yttrium-copper oxide and this is a small yet powerful magnet.

Joanna: Let's see what happens when we place the ceramic disk in the liquid nitrogen!

Steve: Okay!

Now, when the disk is warm, the magnet just sits there. There's really nothing special about it.

Joanna: In 1911, a Dutch scientist named Heike Kamerlingh Onnes was experimenting with liquid helium at temperatures near absolute zero. He was studying to see how the electrical resistance of the element mercury changed as it got cold.

Steve: When electricity flows through a conductor, like copper, some of the electrical energy is changed to thermal energy because of electrical resistance. Resistance is sort of like friction for electricity. The more resistance an object has, the more difficult it is for electricity to flow through it.

Joanna: Onnes discovered that mercury loses all of its resistance when it is cold enough. This is called superconductivity.

Steve: This device is called a cavity. It's made from the element niobium. Niobium also becomes superconductive when it gets cooled to near absolute zero. Here at Jefferson Lab, we study inside of atoms using a machine called an electron accelerator. Our accelerator is made from over 300 of these cavities. We use superconductive cavities so we don't waste energy by generating waste heat. Now, our little ceramic disk also becomes superconductive when it gets cold enough. Happily, we don't need to cool it to near absolute zero. Relatively warm liquid nitrogen is cold enough!

Joanna: Not only do superconductors have zero electrical resistance, they also do not allow magnetic fields to pass through them. The exclusion of magnetic fields within a superconductor is known as the Meissner effect.

Steve: So if we put our magnet back on the disk...

Joanna: It floats!

Steve: The superconductor can't allow the magnet's magnetic field to pass through it. The superconductor can't get rid of the magnet, so it does the next best thing. It sets up electrical currents on its surface. The magnetic fields produced by the surface currents exactly cancel the magnetic field of the magnet inside the superconductor. Of course, the surface currents create a magnetic field outside of the superconductor also. This is what is causing the magnet to float.

Joanna: Since the superconductor has no electrical resistance, the surface currents will flow forever. As long as the superconductor stays cold enough, the magnet will keep on floating!

Thanks for watching! I hope you'll join us again soon for another experiment!

Steve: Let's see how fast this thing can go!

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