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Re: source of electrons



Or perhaps because of the vacuum between the cathode electron emitter
and the anode screen he doesn't realize that the gun is part of a normal
continous circuit that involves a high voltage power supply connected in
series with the picture tube.
Interestingly, in this case, the pipe is
empty...at least the part of the pipe between the cathode and the anode,
since it is a vacuum. If the accelerating voltage were low enough, you
could see the delay associated with it, but of course, you wouldn't see
anything on the screen since the beam would not make it though the mask.

cheers,

joe

On
Tue, 8 Jan 2002, William Beaty wrote:

On Tue, 8 Jan 2002, Larry Smith wrote:

How would you answer the question a student wrote to me today: "Where does
an electron gun in a TV get an unlimited supply of electrons?"

Might he be using the incorrect "empty pipes" analogy for electric
circuits? This is the one where the wires contain no mobile charges, and
the charges are supplied by a power supply. If we know that the electron
gun (and all the wires) contain thousands of coulombs of electrons, while
the gun emits particles at a very low rate, then the source of electrons
is obvious.

Or is he asking why the gun doesn't eventually become positively charged,
and why the CRT screen doesn't eventually become negatively charged? This
is easy: the inside of the CRT has a conductive coating which connects the
electron beam impact point to the rest of the circuit.


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William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
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Joseph J. Bellina, Jr. 219-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556