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I know that in electron guns we see in TV's and lots of other places, we have electron emitter (cold/hot W needle in the simplest case) and electrons are accelerated using lattice with high-voltage potential.

But the question is why doesn't this lattice slow down electrons once they passes through it?

PS. Please correct if lattice is not the correct word here :-D

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2 Answers

up vote 4 down vote accepted

I would say "electrodes" rather than "lattice." In a real CRT you have a very complicated set of electrodes, but let's pretend it's a parallel-plate capacitor with a hole in the positive-voltage plate. A parallel-plate capacitor has a strong field between the plates, but a very weak field on the outside. So once an electron flies out the hole, it feels very little field.

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Zero exterior field is only strictly true in the limit of a infinite plate capacitor, but it is quite a good approximation. –  dmckee Jul 26 '11 at 16:47
@dmckee: Thanks for your comment. I've corrected "feels no field" to "fiels very little field." –  Ben Crowell Jul 26 '11 at 16:55
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After traveling far away from the positive grid, electron has lost half of the energy that it gained when traveling between the electrodes.

Here's a numerical example that maybe explains why it must be so:

Electron that is shot from a far away position towards the negative electrode of an electron gun, loses 3 electron-volts of energy when traveling towards the negative grid, gains 6 electron-volts of energy when traveling between the electrodes, and loses 3 electron-volts of energy when traveling away from the positive grid.

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I deleted my answer, because I do not want to compete with such nonsense. –  Georg Jul 26 '11 at 16:04
Where do you get this figure of 1/2? It doesn't sound right to me. Are you claiming that it's exact? Approximate? –  Ben Crowell Jul 26 '11 at 16:05
"The positive grid" means he knows nothing, There are several positive grids in a CRT. –  Georg Jul 26 '11 at 16:19
Your claim here amounts to "the gun doesn't work because the electron loses the same amount of energy as it gains.", which is nonsense. Perhaps you should look for a reference to back this up. –  dmckee Jul 26 '11 at 16:49
@dmckee: He says "Electron that is shot from a far away position...," which means that his claim isn't that the gun doesn't work. It's certainly true that when an electron is brought from infinity into some electrostatic device and them removed again to infinity, it ends up with the same energy it started with. It's also irrelevant. And I think the 1/2 is simply due to some conceptual confusion, such as a possible belief that when an electron is outside the plates it only feels the field from the nearer plate. –  Ben Crowell Jul 26 '11 at 16:54
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