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Re: Curvature of Space



At 10:34 AM 2/20/98 -0800, Leigh wrote:
If the water thread is uncharged it will not be deflected
by an electric field. Again, ask "Which way would it go?"
You are forgetting that an electric field exerts no force
on an uncharged object.

Leigh,
If memory serves dielectrics have no net charge and yet are
affected
by electric fields?

They are certainly affected. They will be polarized, and
depending on the shape and/or isotropy of a particular
piece, net torques may be exerted on them, but they will
experience no net force if they remain electrically neutral.

Twice now while teaching introductory E&M to frosh engineering students
I have started with a sticky tape experiment. Students come to college
knowing this mantra: like charges repel, opposite charges attract. So,
I show then how to make a charged piece of scotch tape. They
immediately discover that charged tape is attracted to almost
everything in the room, and are unable to reconcile the observation
with the mantra. Now I have their attention.

The charged tape is attracted to objects that are electrically neutral.
There is a force of attraction between a charged object and an
uncharged dielectric. We see this experimentally.

Now, do I dare "explain" to this group? This is risky, but I'll try.

The electric field of the tape induces a polarization in the
dielectric. The electric field from the tape is far from uniform.
Because of the polarization of the dielectric, the dipoles do feel a
net force in the inhomogeneous field. (to be specific, suppose we have
a negatively charged tape. The polarization means that a small (very
small) charge separation is induced in the dielectric, and the negative
charges are, on the whole, farther from the tape than the positive
charges. The repulsive force on the negative charges is less than the
attractive force on the positive charges, and we have a net attractive
force.

JEG

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John E. Gastineau gastineau@mindspring.com KC8IEW
900 B Ridgeway Ave. http://gastineau.home.mindspring.com
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