caps and pvc?

From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Wednesday, April 15, 1998 11:22 PM
To:  Tesla List
Subject:  Re: caps and pvc?

Tesla List wrote:
> ----------
> From:  rpittman-at-juno-dot-com [SMTP:rpittman-at-juno-dot-com]
> Sent:  Tuesday, April 14, 1998 9:32 AM
> To:  tesla-at-stic-dot-net
> Subject:  Re: caps and pvc?
> >Polyethylene is a low loss plastic, PVC is high loss plastic.
> Ok being a science teacher I have to ask, What is it that makes something
> high loss and other things low loss?  Crystal structure, molecular make
> up,  does the chlorine cause the loss?  Cause polyethylene is just a
> bunch of c=c and H right, while the poly vinyl chloride is a vinyl
> molecule with some chlorine?  I am sure this is over simplified but just
> curious.  Thanks for the help.
> Adios
> Ronnie


Excellent question...and unfortunately one with no simple answers! A
VERY high-level and over-simplified explanation follows.

Individual atoms or molecules in a dielectric material will tend to
"line up" in the presence of an applied electric field, depending upon
the degree of overall electrical charge balance within the atomic or
molecular unit structure (or, in some cases, the glass or crystalline
structure). Certain molecules, called polar molecules, possess a
permanent electrical dipole even with no externally-applied field. These
molecules will have a more positive charge distribution on one end, and
a more negative charge distribution on the other end. PVC is an example
of a polar molecule because the more electronegative chlorine ions that
repace the hydrogen in about 25% of the bonds create an asymmetrical
charge distribution within the material. Other, non-polar, materials
(like Polyethylene) may show only a relatively small degree of
polarization under an applied electric field. 

Now if an external electric field is suddenly applied, the polar
molecules attempt to line up with the field. However, this does not take
place instantly. Similarly, if the polarity of the applied electric
field is reversed, the dipoles take a finite time to reverse their
orientation. It's sort of like trying to rotate a rod lengthwise through
a viscous medium - it takes energy to rotate the polar molecules, and
some of this energy ends up heating the dielectric material (similar to
frictional heating). Dielectric losses within glasses, ceramics, and
liquids are often considerably more complex than the simple explanation
for polar molecules above, and can involve movement of ions or even
electron-hole pairs within the dielectric material's crystalline or
glassy structure. 

The degree of polarization governs the dielectric constant for the
material. Although the polar molecules in PVC result in its higher
dielectric constant, they also cause PVC to have a higher dielectric
loss than polyethylene.

Hope this helped a bit...

-- Bert --