Re: volume polarization vs. surface charge

["Donald E. Simanek" <dsimanek@EAGLE.LHUP.EDU>]

On Wed, 3 Mar 1999, John S. Denker wrote:

> [A nice discussion, with pictures, of polarization in a dielectric.]
>
> =========
>
> (Of course, in addition to the "del dot P" charges discussed above, you can
> inject real, unbalanced charge onto and into an insulating dielectric, as
> we discussed a few weeks ago.)
>
> Cheers --- jsd

Yes, and these real charges are the ones which are relatively mobile, they
can be added or removed from the insulator, especially if on the surface
of it. While the induced charge, due to a field polarizing the dielectric,
acts as a surface charge, but is bound, and not free to migrate to the
conducting plates of a capacitor and become current, at least not with the
size fields of this demo. You'd have to break up molecules to do that.  I
think that the distinction between induced charge resulting from
polarization of the dielectric, and the excess charges (mobile, loosely
bound) was not clearly brought out in our discussion of the dissectable
capacitor demo.

To recap, in a manner to allow easy disagreement with individual points:

In the dissectable capacitor demo the parts consist of a nest of two metal
cans with an insulating cup between them. The inner can often has a heavy
wire hook coming upward to provide an electrode for the spark gap, and a
convenient hook so an insulating rod can be used to carry the can during
assembly and disassembly. The outer cup generally sits on an insulating
sheet. During assembly and disassembly the parts are only touched with
insulating tools to ensure no significant charge is added or removed from
the parts, and to ensure the operator gets no shocks.

1. The parts, disassembled, are each neutral, having no excess or
deficiency of charge. This can be tested before assembly with an
electroscope. (Though it usually isn't.)

2. The parts are assembled in a manner which doesn't compromise their
neutrality. (Use the insulating tools.)

4. A high voltage source is connected to the outer cans. It "charges" (or
energizes) the capacitor, which involves a current which transfers mobile
electrons from one metal can to the other.

5. None of these electrons come from the dielectric. They come from one
metal can and are transported to the other one.

6. The excess electrons on one can and the deficiency on the other can are
responsible for a strong electric field between the cans.

7. This field may polarize the molecules in the insulating cup, *if* it
behaves as a dielectric. This is incidental to the demo, but does increase
the capacitance of the device. If it didn't happen, the demo still works.
Dielectric effects aren't required for this demo.

8. The strong field also causes electrons to be pulled from the can to the
surface of the glass insulator wherever the two have close enough
proximity. Once there, they "stay put", for they are resting on a
non-conducting material.

9. The voltage source is disconnected. The capacitor remains 'charged'.

10. The cans are pulled apart. The excess charges remain on the surface of
the dielectric, since the field is still strong until the cans are too far
away for the electrons to jump the space between can and the glass
insulator. The parts are completely disassembled and placed on the
insulating sheet.

11. The individual cans can be shown to be uncharged (by testing with an
electrometer), or nearly so. The insulating cup could be tested to show
that it carries charge. This isn't usually done.

12. The cans can be nested to ensure they have the same potential. They
can be grounded to remove any residual excess charge. We can now agree
that these metal cans are neutral (uncharged)?

13. The parts are re-assembled, being careful to touch the cans only with
insulators, and use an insulating rod to carry the inner can into the cup,
so you don't get a shock, and so you are sure you aren't adding or
removing any charge during the reassembly.

14. The metal discharge tool (or a loop of wire) is used to show the
hot spark due to current from the cans through the discharge tool
and spark gap. This current consists of free, mobile electrons.

15. Now where did those electrons come from?

16. From the cans? No, they can be tested after the spark, and shown to be
uncharged. They were uncharged before reassembly also. So they didn't gain
or lose electrons.

17. From the surface of the insulating cup? Yes. It can be tested after
the spark and may have some residual charge, but certainly much less than
it had before re-assembly. That's where the electrons which caused the
spark came from, by way of one of the cans.

18. They didn't come from the polarization charge (if any) due to any
dielectric properties of the insulating cup, for that isn't free to
migrate anywhere.

And, I repeat, dielectric polarization isn't essential to the demo,
needn't be even mentioned in the discussion, and probably shouldn't be
mentioned, for it detracts from the real explanation. I did bring it in
here just to emphasize that it plays no role other than to increase the
capacitance of the device.

So, I'm puzzled. Where's the problem in understanding this? What do some
of you think I'm missing here?

        -- Donald

.....................................................................
Donald E. Simanek
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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