Re: Charge Distribution around pointed areas

[William Beaty <billb@ESKIMO.COM>]

On Sun, 14 Nov 1999, Brian Rintoul wrote:

> If a charged object is spherical, the charge is uniformly distributed over
> the surface, the reason for this distribution is quite clear.  What isn't
> immediately apparent, this Biochemist is, why charge becomes concentrated
> around sharp pointed areas on a conductor's surface.

Here's a non-computer-based realtime simulation technique.  Build a large
number of "pucks" for your air table.  Place a ceramic disk magnet on each
one, with all magnets oriented the same way.  Turn on the air table, and
the magnet-pucks will repel.  With enough pucks, you'll see a
hexagonal crystal array appear.  Note that the pucks cluster along the
edges of the air table with much higher density than the pucks held in the
"array", and cluster slightly more densely in the corners.


Notes:

  Is your air table non-ferrous?  I used an old one made from wood, and a
  couple of nails caused problems.

  Make small pucks, but not too small.  Each one should cover at least two
  air-holes.  That way a plugged hole won't make the puck get stuck.

  To mimimize the number of pucks required, put a barrier across the table
  so that only a small portion is used.  You might wish to further reduce
  the number of pucks by lining three of the sides of the air table
  with small magnets (orient them to repel the pucks.)

I found that the best puck has a "skirt" around the bottom edge.  I cut
out disks from 1/16" plexiglas on a bandsaw, then nibbled out the
centers of half of them with a ream and wire cutters, then glued the disks
onto the "rims" to form something that looks like the following in cross
section.  Lots of labor, but they could lift heavier magnets without
touching down and getting stuck.  Flat-bottom pucks tend to plug the air
holes.  Pucks with a "plenum chamber" and a "hovercraft skirt" work much
better.

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Note that this crystal-array simulation can be used to demonstrate, um,
"heat".  Once the array has settled down, grab one of the pucks and move
it back and forth by hand.  It will affect its neighbors and make them
move too.  To heat the array with "radiation", wiggle a large stack of
ceramic speaker magnets at it from a distance.


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William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
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