My copper mystery: Is it diamagnetic or not?

[evansste]

I have learned that copper is supposed to be diamagnetic. So the results of an experiment that I recently performed, confuses me.

I took a small piece of aluminum foil and quickly moved it near the surface of a really strong neodymium iron boron magnet. Because aluminum is paramagnetic, the foil reacted to the magnetic field. It was as if it had "run into" interference. I could visibly notice the field tug on the foil.

So then I took a thin, copper-coated board that I have. These are phenolic boards and are about one sixteenth of an inch thick. They have a layer of copper on one side and are used to make printed circuit boards. I ran the board over the magnet, and the same thing happened. The board reacted just as the aluminum foil did. It seemed as if the field "tugged" on the board. If copper is diamagnetic, why would this happen?

In order to make sure that the copper was to blame for the "tug" affect, I etched the board, removing all copper from it. Now there is no tug when I run the board through the magnetic field.

I then decided to try a piece of graphite foil. When I ran the graphite foil through the field, nothing happened. This makes sense because graphite is diamagnetic.

So, what's with the copper? Why does copper act as if its paramagnetic when every place on the internet says that copper is diamagnetic? Any ideas?

I also placed a penny on my finger and ran my finger over the magnet. Once again, there was a reaction. The penny fluttered, as if something was trying to grab it. So what's the deal with copper? Is everyone lying, or is there some other reason why this happens?

I've thought of trying to get a sheet of copper that is known for being really pure; believing that maybe it has something to do with the purity of the copper. However, I don't want to spend money for an ultra-pure sheet of copper only to find that there is a valid reason why copper seems to be misbehaving. It may have nothing to do with purity at all.

Thank you for your time. Your insight into this mystery is greatly appreciated.

[Enthalpy]

You have observed eddy currents in the metal. They result from conductivity and are used to build truck brakes for instance, and can be felt by hand under favourable conditions. You didn't observe them in graphite because of the smaller conductivity.

 

DIamagnetism is fainter and can't normally be felt by hand. More, it does not depend on movement.

[evansste]

I considered, for a moment, that it could have been a result of some form of inducted current (eddy currents). After all, I was moving the sheet, which technically provides a situation of a moving charge (current) in a magnetic field. But then I dismissed this idea because I figured that the direction that I was moving the sheet would have caused the force to push the sheet sideways, and not feel as if it was grabbing it. This is because the field from the magnet is pointing vertically, and I was moving it horizontally, which would cause it to move in the other orthogonal direction (from one side to the other).

But reading your response has caused me to think about this again, and what you're saying makes sense. The field from the magnet isn't only pointing up and down, but ultimately points in a circular direction. This is because the magnet has edges, and the magnetic field encircles the edges of the magnet. So some components of the magnetic field would make it move up and down, as well as side to side.

So if the magnet had no edges, then the sheet would only move from one side to the other as I move the sheet through the field. However, every magnet has edges, and the edges are where I felt the tug take place. This all makes sense. Thanks for your help.

[John Cuthber]

Th e effect of eddy currents is to hold the magnet "back", as if it's dragging on the metal.

Copper is diamagnetic.

You probably won't detect the small diamagnetic effect if the metal an magnet are moving, because the eddy currents will have a bigger effect.

[swansont]

There are youtube videos of people dropping magnets down copper tubes which demonstrate the damping effects of the eddy currents. Or you can use magnets from a dead hard drive and some aluminum to see the same thing

 

http://blogs.scienceforums.net/swansont/archives/6374

[Tres Juicy]

There are youtube videos of people dropping magnets down copper tubes which demonstrate the damping effects of the eddy currents. Or you can use magnets from a dead hard drive and some aluminum to see the same thing

 

http://blogs.scienceforums.net/swansont/archives/6374

 

Magnetic braking is cool