A Diamagnetic Demonstration

Our friends stopped by with a simple apparatus to demonstrate the diamagnetic properties of bismuth metal. Diamagnetism is a extremely weak magnetic effect — generally orders of magnitude weaker than everyday permanent magnets, which exhibit ferromagnetism. However it is also an extremely interesting effect because diamagnetic materials are repelled by magnetic fields. This is different than the case with ferromagnets, where one pole of a magnet repels another — rather, the entire material is (weakly) repelled by any magnetic pole.

Now, how might one observe such a weak effect? One way is to build a magnetic levitation rig, but the field configurations there are a little less obvious. With a simple but sensitive balance, we can see the repulsion directly. The balance above has a long wooden beam, a central pivot on two blocks of plastic, and a couple of coins on the far end for counterbalance.

At the business end of the scale, there is a cylinder of bismuth metal about 1 cm in diameter, held in place by a rubber band. We also have a larger rectangular block, which is our test magnet, made of grade N50 NdFeB and painted black. And finally, the Lego Astronaut Twins are here helping out as a scale and position reference.

Moving the block magnet beneath the bismuth, we can see what happens in an animated GIF:

After the balance settles, the resting position of the end with the bismuth is considerably higher. With some calibration in terms of weights and/or positions, one could even measure the exerted force with some precision.

A slight improvement to this apparatus would be to reverse the roles of the bismuth and the block magnet. That is, to affix the magnet to the arm of the balance, and to slide the bismuth beneath it instead. You could then use a nearby block of aluminum to damp the motion of the beam through magnetic (eddy current) damping. Many commercial balance-beam type scales already use magnetic damping so that they settle down to their final values faster.

A pleasant surprise in the freezer

Our automatic ice maker is on the fritz, so we’ve temporarily reverted to making ice in conventional trays. But, imagine our delight at opening up the freezer and finding this!

Several of our ice cubes apparently formed with long spikes on top. This is really *not* what you expect when you start out with liquid water in an ice cube tray.

Snowcrystals.com has a fairly detailed explanation of how these things form, and it’s documented elsewhere as well. (Roughly speaking, supercooled water is pushed up through a hole, somewhat like magma forming a volcano.) It’s relatively easy to form these in your freezer if you start with distilled water, but occasionally– as in our case –they do occur with regular tap water.

Make a physics education video and win the Phylm Prize!

“Phylm,” pronounced as “film,” is a portmanteau built out of the words “physics” and “film.” It’s also the name given to a new award, The Phylm Prize, aimed at spurring interest in physics and the educational use of new media. Translation: it’s a YouTube contest for physics geeks!

We’ve been invited to sit on the panel of judges for the contest, and so we’ll be looking forward to seeing the submissions. Videos up to two and a half minutes long featuring physics will be judged on clarity, accuracy, and creativity. This year’s winner will receive a check for \$100 (US) to be dispersed in June 2007.

You can watch the video announcement at YouTube or (embedded) here:

We are guessing that many of you, our fine readers, already have an interest in physics and/or new media, so get started already! Let’s see your submissions! And don’t let the word “educational” intimidate you– educational propaganda is a highly appropriate diversion for evil mad scientists! (Besides, you could probably use the cash for your world domination scheme.)

Here’s some info from the rules on what kinds of things the clips can contain:

• A critique/analysis of the physics presented in a fictional work. For example,
could the bus in Speed have made “the jump,” or how strong would Spider-Man
have to be to throw a car that far?
• An analysis of physics as revealed by the examination of a real-world video clip.
For example, what forces does a gymnast experience during his routine?
• An explanation/presentation of some physics concept or theory. For example, what is the conservation of energy?

Submissions are due by 12:00 am (GMT) May 1, 2007. We’ll be waiting.