|MadSci Network: Physics|
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: http://www.madsci.org/posts/archives/jun2000/960035115.Eg.r.html and http://www.madsci.org/posts/archives/feb99/917409810.Eg.r.html 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 unmeasurable. 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! Zack
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