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Re: Bar magnets



Ludwik Kowalski wrote:

I replaced the attractive magnetic poles by electrified pith balls
(sphere A and sphere B) and performed an Interactive Physics
simulation (see the details below). The result is that the two
pith balls had no trouble of finding the equilibrium position;
they did not jump to each other suddenly, as my bar magnets
did.

Does it mean that Interactive Physics is not good for this kind
of simulation or that the experiment was not good enough to
prevent small sidewise and vertical oscillations?

Try reducing the damping force and see if you can get the instability
to appear. You may also have inadvertently picked a region where the
interaction of the linear and inverse square forces are not unstable,
which, it seems to me, is where the absolute value of the slope of
the inverse square curve is less than the k of the spring. In that
range the change in electric force will be smaller for a given change
in distance than the change in spring force. When the distance gets
small enough, then the slope of the inverse square gets larger than
that of the spring force and a small change in distance will lead to
a smaller change in spring force than increase in electrostaic force,
and the instability should appear. If you increase the charge on the
two spheres (easier than changing the distances, in IP), you should
be able to create the situation where the slope of the inverse square
is greater than that of the spring force.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

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