Superconductor Magnetic Levitation!

A superconductor is a material that has no electrical resistance when it is cooled to temperatures approaching absolute zero. These materials display something called the Meissner effect—in short, they make magnets levitate! It’s very cool, both figuratively and literally.

The superconductor in this project is YBCO, yttrium barium copper oxide (chemical formula YBa2Cu3O7). It’s a famous high temperature superconductor, “high temperature” in this case meaning 92 K or about -294°F. Toasty warm! As frigid as that is, though, it’s well above the boiling point of liquid nitrogen (77 K, -321°F), so by pouring liquid nitrogen around the YBCO, we can easily get it to go superconductive.

Here’s our pyrex petri dish, insulated with some foam so that the table doesn’t suck all the cold out of it. The disc of YBCO is in the center (for scale, it’s about one inch across), with the neodymium magnet sitting on top (not floating as of yet).

 

Now we get to decant some liquid nitrogen! I love that stuff.

Step 2; hey, that's me!

 

Then we pour the liquid nitrogen into the dish.

 

When the YBCO reaches its critical temperature (it doesn’t take long), the magnet floats!

 

It was difficult to get a sharp photograph, and the black-on-black nature of the two materials didn’t help, so we slipped a piece of paper between the YBCO and the magnet. You can see by the shadow that the magnet is indeed floating.

 

And here’s a video:

 

How does it do that? Well, the short answer is that superconductors push on and rearrange magnetic fields, so that the magnet is buoyed up. Okay, but…how does it do that? This is what’s going on (at least, as understood by someone who’s neither a physicist nor a materials scientist):

When the YBCO is above its superconducting transition temperature (remember, that’s 92 K), it’s just a lump of non-conductive, non-magnetic stuff. The magnetic field lines of a nearby magnet go right through the material; the magnetic flux is permeating the YBCO. There’s nothing exciting happening—the magnet just sits there, surrounded by its normal field.

When the YBCO is cooled to below its transition temperature, it becomes superconductive. The presence of the permanent magnet induces electric currents within the YBCO, which have their own magnetic fields that oppose the field of the permanent magnet (hooray for Faraday’s law!). The material is now diamagnetic: acting in opposition to the nearby magnetic field. The magnetic field within the YBCO has been canceled.

However, the total magnetic flux is conserved, so if there’s no flux inside the YBCO, it must be outside. In other words, the superconductor pushed all the field lines outside itself. This phenomenon of excluding internal magnetic fields is the Meissner effect.

The poles of the induced magnetic field are arranged so as to repel the poles of the permanent magnet. (If the permanent magnet’s north pole is downwards, then the induced field has an upward-facing north pole.) Remember that this is all happening in a superconductor, that is, an environment with no electrical resistance, so the induced currents can change extremely easily as the magnet moves, adjusting as needed to keep the levitation going.

And that’s one of the wonders of electromagnetism!

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Marble Art

Hi everyone, I’m Isabel from ScienceWorks Museum in Ashland, Oregon. I’m dedicating this post to my mom, from whom I stole this idea. Isn’t that what role models are all about?

1. Collect your materials.

stuff you'll needYou’ll need some paints, a piece of paper, tape, small containers to put the paint in, newspaper to protect the conference room table (ahem), paper towels to clean up, and a large container that the paper fits in (I used the white tub in the picture; with kids, it can be a good idea to use something like a cereal box that can be closed up to prevent flying marbles). Don’t forget the marbles!

2. Tape the piece of paper to the bottom of the large container.

good thing it fits!3. Select your paint colors and dispense them into the small containers.

pretty...4. Plop the marbles into the paint.

hexagons and spheres, oh my!5. Roll the marbles around so that they are totally covered in paint.

mmm, painty marbles6. Toss the marbles into the container with the paper.

almost ready to roll!You can tell that this tub has a somewhat convex bottom. But don’t we all?

7. Tilt the container so the marbles roll all around the surface.

awesomeThis is the fun part. If you have a box with a lid, you can even shake the whole thing. (With that method, the art comes out as a surprise, which can be a nice touch.)

8. Take out the marbles, untape the paper, and lay it flat to dry.

by Jackson PollockI used tempera paint and printer paper. My mom used fabric paint (or something else more permanent than tempera) and instead of paper, she used an old LP record. They looked really cool, especially with metallic paints.

As for the science: Do larger marbles make different trails than smaller ones? What happens if you move the container quickly or slowly, at a greater or lesser angle? Why are the paths parabolic?

pretty neat, no?You can also go the arty route and talk about Jackson Pollock and other abstract artists. All in all, it’s pretty fun. Thanks Mom!