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!

It’s all okay

Aside

Hey there, I just wanted to mention, in response to a couple questions I received, that if you didn’t get a chance to post about any of the previous challenges and you still want to, it’s not too late! If you experimented with any of these things, anytime, in any way, then we want to hear about it.

Mythbusters: SEG Edition

Oh, “mythbusters” is a trademarked name?  Okay, I’ll go with B.S.-buster.

For years, the museum industry has been regaled with urban legends that decry the nefarious side of one of our favorite toys, Bucky Balls.  I’ve had parents, kids, my mom, grandparents, and the odd pedestrian in the parking lot all tell me the same apocryphal tale.  Most stories go like this:

“Once upon the time, [your cousin’s cousin /my friend’s sister/ boss’ pediatrician]’s kid ate TWO Bucky Balls and ZOMG they connected in her intestines and BURNED A HOLE through the lining!”

Being an innate skeptic, I am unsure that magnets can actually burn a hole through intestinal walls.  However, after a cursory Google search yielded no results, I decided to try it out myself.

Behold, my super-scientific experiment:

Sliced turkey, emulating the intestinal wall.


Surely this will get to the bottom of the mystery.  I will check on progress in a few days.  I’m positive the results will vindicate my hunch.  (That, and maybe a perusal of medical journals….) And I, the B.S.-buster, will reign supreme.

More updates as events warrant…

700 Square Feet of Sculptures

Ahhh, good old aluminum foil. No tinker’s workshop is complete without a roll of at least one of the elements on the periodic table.

Here at the DHDC, we’ve been using the materials from the Guild to spark visitor interactions. This week, our explainers have been busy sharing aluminum foil pieces with museum guests and asking the open-ended question “What can you make?”

This was so popular with the explainers that it quickly became a regular occurrence in our Science Cafe, where visitors used table space to craft their little hearts out. Some visitors could be persuaded to leave their creations with us so we could put them on display, so a wall of fame was born.

And it grew and grew.

I showed some children how to “cast” items in aluminum by pressing the foil over a surface.

Some children found interesting things to cast.

And those creations turned out to be pretty robust.

Then, the aluminum foil facilitators began to wander around the museum. They discovered that visitors could be coaxed to make aluminum foil T-Rex claws, which prompted many children to play dramatically in our traveling dinosaur exhibit. These wildly slashing dinosaur children turned out to be very difficult to photograph.

Pretty soon, the claws were a meme within the Discovery Center. Now, no matter where we gave the children aluminum foil, they made dinosaur claws for their fingers. Even if we weren’t in the dinosaur exhibit, and even when we didn’t provide instructions on how to make claws, the children made what they saw other children had made.

 

And they remained just as difficult to photograph as before.

Well, the next day we gave no instruction and returned to saying “What can you make?” We got no dinosaur claws, and everything was a surprise again. Guess what this dude is making:

While his sister made a wallet.

The summer camp kids got in on the action. This little gal made a line of accessories to help her dress up like a witch, which included long fingernails and a magical bracelet.

The boy behind her proved you’re never too young to chuck the deuces in the back of a photo. He retrieved his aluminum foil dagger and crown, and an epic battle unfurled.

This raged on for quite some time, and I was lucky to escape without injury while capturing the action.

We learned a lot about facilitating an open-ended creative activity, and we became more aware of how the surroundings often dictate the products our visitors make. I think we’ll continue to work with aluminum foil to hone these skills in the coming weeks.