Date: Wed, 21 Mar 2001 22:29:42 -0800 (PST)

Author: William Beaty

Subject: Re: Photoelectric effect with zinc plate


On Wed, 21 Mar 2001, Gerald Zani wrote:

> But the textbooks and my Faculty don't quite agree. They both say that I
> should not need to initially charge the zinc plate negative to see the
> photoelectric discharge.

Aren't there TWO effects here? First, the UV knocks electrons off the
metal surface and they are grabbed by air molecules. Second, if there is
an overall e-field at the surface of the metal, the airborn electrons will
be driven in one direction or another. An uncharged plate wouldn't lose
electrons: they'd stay in the "boundary layer" of air, and would not tend
to drift away. (maybe you can scrape them off?!!)

Hey, what if you blast the surface of the metal with an air jet, then
expose it to UV? The air would do work upon the negatively charged air
molecules and drag them far away, possibly producing very large
potentials. The UV acts like the roller/comb assembly of a VDG generator:
it moves charge off an electrode. The air jet then acts like the VDG
belt: it transports the mobile charge away from that electrode. Hmmmm.
Maybe this is the basis of a VDG generator where the only moving part is
the air pump: surround the UV tube with a zinc cylinder, ground the zinc,
then blast air along the gap between the cylinder and the tube. Negative
air would pour out, and the excess charge could be collected by a metal
screen placed in the air stream.

I've heard of VDG machines based upon air, but I think that they usually
use a kilovolts power supply to knock the charges off the metal electrode.

> My experiments prove that this is not the case. Why don't textbooks and
> faculty agree? Can anyone shed some UV light on the explanation? Do other
> people find that the faculty do not expect to first place an excess of
> negative charge on the zinc plate before discharging it?

Well, if you POSITIVELY charge the plate, and the UV makes it discharge,
then the UV is generating ion-pairs in the surrounding air, which would
make the air become slightly conductive.

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