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Re: [Phys-l] Magnetism and rust



A guess. Rust begins to form when iron (usually in a pitted part of the metal) oxidizes, or loses electrons in an interaction with the water and the oxygen in the water that come in contact with the surface. This adds excess electrons to the metal. The iron ions that are in the water on the surface lose another electron in the water (They become Fe3+) and then are attracted to the extra free electrons in the metal, where rust forms via a complete circuit. In general, this happens randomly as you get concentrations of extra electrons in the metal near the pitted part and still covered by water. Now if you have a magnetic field present, that would concentrate the extra electrons as they move in circular paths. That would concentrate the rust in the area of the magnetic field.

Of course, it might just be a concentration of water as others have suggested.

Bill




On Dec 4, 2010, at 11:37 AM, Peter Schoch wrote:


As I was cleaning up this morning -- putting things away for winter -- I discovered something unusual that I can't explain using any Physics I know...

I have a steel router table. On it I have a magnetic feather-board -- a strong, ceramic magnet in a holder that then has 'fins' on it to keep the work being routed tight to the fence. The table, despite it being cold and damp was rust free -- EXCEPT under where the magnet was! As I lifted off the feather-board, I found a perfect rust outline of the magnet inside the feather-board!

Now, why would that be? If all the other surfaces are rust-free, what would cause the magnet to induce rust? (I'm always looking for practical examples of physics to show/describe to my students.)

Any suggestions are appreciated,
Peter


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