Plastic Car

Aside

I talked to Sherry Marshall from the Oklahoma Museum Network today and she shared a little magnet experiment with me. She said she had been carrying around some neodymium magnets in her pocket that one day she was sitting in her car and got to wondering if she could use the magnets to search for metal components. She started running the magnets over everything in the car looking for metal and couldn’t find anything but a couple mounting screws in the stereo. Surprisingly, the interior of her car was almost entirely made up of other materials.

Homopolar Motors

Homopolar motors are very simple motors that consist of a battery, a magnet, and a bit of wire connecting the two. I made one with a coat hanger serving as part of the circuit, and the battery freely hanging off of it. Here are some pictures, and a very short video:

Floating Magnets

To brainstorm ideas for a magnet exploration I turned to one of the Exploratorium’s secret little archives of activity ideas, Paul Doherty’s personal website. He has a whole giant section on magnets with tons of interesting demos and experiments. Click Here to take a look at it.

I have been wanting to play around with one of these activities The Garden Of Magnets, for a long time now, because I love thinking about the way that simple systems can create complex patterns and interactions and this seemed like a perfect example. To make a floating magnet garden all you do is make a set of foam circles and attach a magnet to each one so that they all have the same orientation (I put staples in the foam and stuck each magnet to the staples so it would stay on the raft).

When I floated all my magnets in the water I was fascinated by the way they interacted with each other. Their behavior seemed to me to have a lot of personality, so at one point I added eyes to rafts and watched as they all moved over to give each other space, just like people do! Then I discovered that if I put one magnet in with the opposite orientation from all the others I got very different results. Eventually the rebel magnet would cause the entire system to collapse on itself, with all the magnets flipping over and grabbing onto each other until they were just one big pile and half the water had splashed onto the table!

Another interesting thing I noticed about this experiment is that the tone of this video became very important to me. I really wanted it to feel strange, and I wanted the little magnet rafts to come across as animate. I started this activity because I was interested in what it might show me about magnetic fields and how they interact, but in the end the narrative was what kept me playing with them for so long. A big part of taking time to try new experiences as a learner is to retain your ability to empathize with your visitors and students, and at the moment I’m finding it really easy to understand why narrative making opportunities get emphasized so much when we talk about exhibit and activity design.

Magnets Have Nothing To Do With It

When I was an Explainer I always carried magnets around in my pocket, but oddly enough the main thing I did with them was demonstrate their ineffectiveness. When kids were asked “why do you think that’s happening,” one of the number one answers I heard was “magnets,” even at exhibits that had nothing what-so-ever to do with magnets. I would then pull out my magnets and let the kids test their idea.

it didn’t occur to me at the time, but looking back I realize that listening to common misconceptions like this one can reveal a lot about children’s deep understanding of phenomenon. As a thought experiment I decided to revisit the number one exhibit that attracted misplaced magnet theories and see if I could make some hypotheses about what those children were thinking. Here’s what I noticed:

1) This table is made of metal, a material that can be easily magnetized. Perhaps the children are aware of this phenomenon and associate metals with magnetism.

2) The rolling objects are round and black, just like some of the most common magnets found in science kits and on refrigerators. Perhaps the children associate round black objects, particularly ones with holes in them, with magnets.

3) The rolling objects tilt at an angle that appears to defy gravity. Perhaps the children suspect magnets when they encounter materials that seem to cause gravity defiance, because they know a magnet can be used to suspend objects that would otherwise fall.

I now plan on spending some time lurking near Turn table to see if I can find any magnet-confused subjects to test my theories on. This thought experiment has reminded me that underneath an incorrect answer there is often a correct idea worth digging for.

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!