MadSci Network: Physics

Re: In an electron gun, where do the electron come from, where do they go?

Date: Wed Dec 21 00:11:35 2005
Posted By: Zack Gainsforth, Undergraduate, Physics, U.C. Berkeley
Area of science: Physics
ID: 1135101365.Ph

An electron gun is actually a part of an electronic circuit.  The short
answer is that the electrons are recycled, so they aren't lost and don't
pile up anywhere.

Now the long answer:

From your question, it sounds as though you have already read the earlier
explanations regarding the workings of an electron gun, but third parties
reading this post will want to refer to:


Now, your question is about where the electrons come from in the first
place.  When you first turn the electron gun on, they come from the atoms
of the wire filament emitting the electrons.  The filament would normally
run out of electrons if it were not part of an electronic circuit which
supplied new electrons at exactly the rate that it emits them.  The way
this is accomplished is that the electron gun is placed at a very negative
voltage -- for example, the filaments in TV's are around -25,000 volts.  In
a scientific apparatus the voltage can be much, much greater.  In fact, the
voltage is required in order to cause many electrons to "boil off" in the
first place because without it they would promptly return to the filament
due to the electrical attraction between the electrons and the atoms.  An
example where this occurs is an ordinary old-fashioned lightbulb.  Since,
there is not a very high voltage in a lightbulb, very few electrons
ever leave the tungsten filament even though it is white hot.  The few that
do only make it out of the metal a microscopic distance before coming back.

In an electron gun, after the electrons leave the filament, they travel
through a vacuum.  The only purpose of the vacuum is to make sure the
electrons have nothing to hit until they arrive at their destination. In a
TV, the destination is the screen.  The screen consists of glass with a 
phosphorescent coating on the back side (phosphorescent = glows when hit by
electrons or other types of energetic particles).  In addition, there is a
thin coating of a conductive material on the backside which catches the
electrons and allows them to run back through the circuit towards the
filament, but along a different path.

The cloud of electrons referred to in other descriptions is simply a way of
describing the electrons.  By way of contrast, a raincloud is comprised of
individual droplets of rain.  However, you don't see the droplets, you just
see ... a cloud.  Nevertheless, the droplets are there.  

Likewise, a conductor has many, many electrons moving around in it in
random directions.  We don't really know the exact location of any given
electron at a specific time.  Rather, we describe the electrons as a cloud
and visualize the cloud as being made of a bunch of electrons --just like a
raincloud is made of a bunch of water droplets. 

The nature of the atoms doesn't change much during the process of operating
an electron gun since there are so many atoms, and so few electrons being
emitted.  For example, a filament may have thousands or millions of atoms
for each electron emitted.  Additionally, electrons are supplied 
very rapidly by the electronics and so in fact, the filament is never
missing electrons at all.  

In fact, I mentioned that the filament is held at a very negative voltage,
which is the same thing as saying that it is pumped up with extra
electrons.  Therefore, the changes in the atoms are so minimal as to be

Therefore, the complete answer is: the electrons come from the atoms in the
filament, and go to another conductor at a different voltage.  From there,
they are pushed back towards the filament using electronics so that the
atoms in the filament are resupplied with electrons.  This continues as
long as the electronics can continue to push electrons in a circle -- and
how that happens is a question for another day!

Hope this helps!


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