Sunday, August 21, 2011

Room temperature diamagnetism with pyrolitic graphite

Updated 2011.09.01

I just acquired some pyrolitic graphite and powerful neodymium magnets from United Nuclear. Pyrolytic graphite has the highest diamagnetism of any room temperature material [see correction below]:
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. According to Lenz's law, these electrons will oppose the magnetic field changes provided by the applied field, preventing them from building up. The result is that lines of magnetic flux curve away from the material.
Here are some small pieces of pyrolitic graphite on top of four 0.5" square neodymium magnets:

Click to enlarge
This is not magnetic repulsion. If that were the case the pieces of graphite would simply fly off the magnets. The magnetic field is strongest at the edges of the magnets so the largest piece is trapped in the center; it's pushed inward to the point of least magnetism. If perturbed it will snap back to this same point and orientation. The smaller pieces are standing on edge because they are repelling the magnetic field on both sides and can't fall over. If one is pushed over it will immediately snap back into a vertical position.

In this view you can clearly see the largest piece levitating and the smaller pieces standing on edge. The medium size piece is tilted slightly. This may be because its thickness is slightly uneven, which would make the repulsion slightly stronger on the more massive side. This calls for more experimentation.

Click to enlarge
Needless to say this behavior is completely unintuitive and surprising. When pushed the pieces react in a totally unexpected way. It's as if they've fallen into a sort of bizarre magnetic well. Here's a video showing this behavior in real time.

Superconductors have an even more powerful diamagnetic effect but require liquid nitrogen to cool them sufficiently.

I'll have more on this topic in the future as I've also acquired some bismuth, which has the highest room temperature diamagnetism of any metal.

Correction (2011.08.31)

Here's the difference between bismuth and pyrolitic graphite (without the math):

"The most strongly diamagnetic material is bismuth, although pyrolytic carbon may have a [lower] susceptibility in one plane."

So there you have it. I played around with some bismuth the other day and it was quite interesting. Stay "tuned"...

1 comment:

Otto Belden said...

Very cool! Looking forward to more posts about this stuff to see what you do with it.
- otto