Like charged objects can attract

franklinhu_at_yahoo.com
Date: 02/27/05 

Date: 26 Feb 2005 21:45:22 -0800

It is commonly assumed that like-charged objects will repel and
oppositely charged objects will attract. However, there have been
some experiments to show that like-charged objects can appear
to attract in some cases. Some of these cases are described in:

http://focus.aps.org/story/v6/st25

These describe attraction of like charged microscopic balls. However, I
have found an experiment that demonstrates this strange
phenomenon using a simple styrofoam cube and an ordinary PVC
pipe. To run this experiment, I cut a 3/4 inch cube of styrofoam
(the type made from compressed beads) and hung it on a thread.
I then charged up a 1/2 inch PVC pipe using a silk cloth and
brought it near the cube and observed its reactions. The behavior
is far from predictable. It can be either attracted or repelled
to the PVC pipe. However, the most bizarre behavior was that I
could get it into a state where at fairly large distances, the
cube would repell which clearly shows that it was negatively
charged like the PVC pipe. However, if I brought the pipe
close enough, it would actually switch to being attracted to
the pipe. If I allowed the cube to separate from the pipe, it
would then again repel, showing that the cube was still negatively
charged. I have posted a video (3MB) of this experiment at:

http://www.geocities.com/franklinhu/likeattract.mpg

It isn't easy to get it into this state. It mostly will either
be all attractive or all repulsive, but I suspect there is a
certain threshold at which the attractive and repulsive
forces balance, and you have to find that balance point to
see this behavior. It is normally more attractive, so you need
to force negative charge onto the styrofoam block by touching
a highly charged PVC pipe to the foam block.

As to why this happens, I can only theorize. I did try to
eliminate the possiblity that the foam block was simply in
an attractive state and the cotton thread was causing the
initial repelling force. I substituted the thread with
monofilament fishing line which is much less likely to
be repelled by the charge on the PVC pipe. I was still
able to observe the strange behavior.

I suspect that the foam block is mostly neutrally charged
and neutrally charged objects are always attracted to
a charged object like the PVC pipe. So this explains
the inital attraction of the foam block to the pipe.
When I repeat this experiment using a smaller piece of
foam (like a piece of a packing peanut), I can see it
attract but as soon as it touches the PVC pipe, it
immediately gets charged and jumps away from the pipe.
The larger styrofoam block acts differently in that
takes a lot more charge before it starts to display
a repelling nature. The charge also appears to be
localized on one side of the block since it shows
a strong tendency to face one side to the pipe. I
suspect that since the charge is localized, this still
leaves most of the foam block neturally charged, and it still
is attracted to the pipe. However, the negatively
charged area on the foam block is still repelling the
negatively charged PVC pipe. Whether the block is
attracted or repelled depends on the interaction
of these 2 opposite forces. It appears that at close
distances, the force of attraction is superior, but
at larger distances, the force is repelling.

According to standard scientific doctrine, the
attractive force between a neutral and a charged object
should be far weaker (1/r^3) than the repelling force between to like
charged objects (1/r^2). But this has to be
balanced with the fact that the neturally charged
atoms of the block probably far outnumber the
charged ones. The physics and the math required
to prove that there is a balance point is far
beyond my abilities to calculate, however, I think
it is plausable that such a balance point exists.
The key to this balance is the ratio of charged matter
to uncharged matter. It is commonly assumed that
if an object carries a net charge, that all of the
atoms in the object carries the charge, however,
this is not the case if the electrons are not
very mobile (as they are in styrofoam) and the
charge can be localized.

No matter what the true explanation for this
phenomenon is, this experiment dispells the common
notion that similarly charged objects can only
repel. This experiment shows that even if an object
carries a net negative charge, it can still be
attracted to another object carrying a net negative
charge, thus showing that like charged objects
can indeed attract.

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