This post is being written at the request of Brooke's boyfriend's sister's daughters, who saw the Quantum Levitation video posted by the Superconductivity Group at Tel Aviv University. The boyfriend's sister's daughters are in fourth grade, and would like to investigate cold, floating superconductors for their fourth-grade science fair project.









Do you have to have parental permission to buy liquid nitrogen, or did I waste my childhood years not buying liquid nitrogen when I could have? If you can either be a nine-year-old owner of liquid nitrogen, or your parents can sign for you, will either you or your parents have fingertips left at the end?

I have a few questions:We'll have to check back with the girls in a few weeks.

For now, though, here's an explanation aimed at the grade in which I learned how to do long division and not how to levitate ceramic material. Not that I'm jealous.

First of all, what's a superconductor?

Hold your horses; let's start with regular conductors. Conductors are materials that let electricity travel through them. Electricity is really just electrons moving in a particular direction, so a conductor is something that the electrons can move through. Metal and water are good examples of conductors.

The conductors, though, make life a little bit difficult for the electrons. They don't just let them travel peacefully--they resist the electricity, and the amount of difficulty electricity has in moving through a conductor is called resistance. You can think of resistance like friction. If you're on a slippery slide, there's not a lot of friction, and you slide down really fast with little effort. If the slide is old and dirty, there is a lot of friction, and you have to scoot along (and it's boring).

So there are conductors. But then there are superconductors. At normal, everyday temperatures, superconductors are normal, everyday materials. But at a certain cold temperature, called the critical temperature ,the superconductor all of a sudden starts to live up to its name. It lets electricity flow through it with no resistance. Imagine the smoothest slide in the world, coated in vegetable oil.

What this means is that if you took some superconducting wire, made a loop out of it, and zapped some electricity into it, the electricity would travel around that loop forever.

This should freak you out. It freaks me out.

When they're cold enough, superconductors act totally different from the way they do when they're warm. What if every time you stepped outside during winter, you suddenly had X-ray vision or could run as fast as a car? (Superhero!)

What does electricity have to do with the whole floating thing?

If you watch the video, you'll see a superconductor that is cold enough to act like a superconductor (and you'll see a hand, which I will call The Claw, since Halloween was a few days ago).

This superconductor is an extremely thin ceramic material spread on top of sapphire--so basically a sliver of dinner plate on top of a piece of jewelry. It floats above (and below) a magnet. The Claw can tilt the superconductor any way it wants, and the superconductor stays stuck like that, in midair. The Claw also spins the superconductor, and it will keep spinning, also in midair.

In the coolest part of the video, The Claw puts the superconductor on top of a circular race track. The race track is made of magnets. The superconductor levitates above the race track, of course, and The Claw gives it a little push. It races around like a hoverboard! A hoverboard that would give you some pretty extreme frostbite.



Iron filings, like the ones you find in the

toys that let you draw a beard on a guy (below),

follow the magnetic field of a magnet (Credit: BBC).





It floats because of another characteristic of superconductors, one that has to do with magnetic fields. Every magnet has a north pole and a south pole. The magnetic field curves from one pole to the other pole, like the picture on the left.

Brooke's boyfriend's sister's daughters:

Please bring Wooly Willy to the science fair. Source.

Superconductors are not friends with magnetic fields. They especially don't like to have magnetic fields inside them.

The superconductor basically says, "Get this parasite out of me," and expels the magnetic field (the same way you get expelled from school if your science fair project doesn't work).

Then, when you put a superconductor near a magnet, it repels the magnet. When you put the north pole of one magnet against the north pole of another magnet, they push apart. The superconductor is basically acting like another magnet and repelling the first magnet. If the repulsion is stronger than gravity, the superconductor floats! This is part of what's going on in the video.

The lines are the magnetic field, and the purple

ball is the superconductor. When the superconductor

isn't cold enough, the magnetic field goes straight

through it (left). But when the superconductor is cold

enough, the field lines bend around it (right).

Source.

What's the other part? How does it stay locked in one position?

In the case of the floating superconductor in the video, the magnetic field actually sneaks in. Even though the superconductor wants the magnetic field to stay outside, the superconductor is too thin to completely keep it out.

The magnetism flows through the superconductor,

but not everywhere--just along those few tubes in the

middle.

Source.

magnetic tubes

But it doesn't get in everywhere, like in the picture above on the left. Magnetism just goes through in a few places, called

If the superconductor changes position, the magnetic tubes have to change positions, too. But they don't want to! Since the superconductor wishes there were no magnetism inside it at all, it wants to keep the magnetic field trapped in a small space that is the same all the time.

So when The Claw tilts the superconductor, the tubes change position, but the superconductor says, "All right--that's enough of that! No more moving around!" and locks itself in place so that the tubes will stay in place. That's how the superconductor stays tilted.

In conclusion, superconductors are seriously weird, science is cool, and liquid nitrogen will hurt if you stick your hand in it, so please don't do that, but please do send us a picture of your blue ribbon (and the URL of the online store that sells yttrium barium copper oxide at the cheapest price).