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Accelerating electrons in a cathode ray tube

  1. Feb 4, 2015 #1
    I have a question which has intrigued me for quite some time. If the electrons are accelerated using a potential difference in a cathode ray tube, the electrons should get accelerated till the time they reach the anode only. Once they cross over (ie. move to the other side of the anode), they should begin to slow down, now being attracted by the anode. Hence, on the whole, I suppose there should be no net gain in energy for the electrons, unless the anode is placed very close to the front glass screen of the CRT, in which case there's not enough time available to the electron to be slowed down appreciably. This is a conundrum for me. Can anyone explain this?
    Thanks
     
  2. jcsd
  3. Feb 4, 2015 #2

    davenn

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    greetings
    Welcome to PF :)

    what do you mean by that ?

    Do you understand how a CRT tube works and what/where the anode in a CRT is ?


    Dave
     
  4. Feb 5, 2015 #3
    Yes, I do. Please refer to the diagram here: http://design.osu.edu/carlson/history/images/crt.gif
     
  5. Feb 5, 2015 #4
    Cross over to the other side means go through the anode. Coming from the left they are naturally attracted/accelerated by the positive anode. However, once they go through the anode in the diagram, they should begin to be slowed down (the same attraction forces still operate).
     
  6. Feb 5, 2015 #5

    davenn

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    OK ... you do have a misunderstanding of how a CRT works

    neither of those anodes are the main anode for the tube
    The main High Voltage anode isn't shown in that pic as it is the front of the screen
    depending on the size of the screen, that HT voltage will vary .... Small Oscilloscopes may be ~ 1-5 kV
    the old large 26 inch colour TV CRT's had up to 25 kV on them

    This HT voltage is what allows the to continue on to the screen and strike the phosphor dots

    ( There may be CRT's that don't have an HT voltage on the front, instead use some other method, but I'm not personally aware of them)

    The Crookes tube is a bit of a really basic version ... but not really a CRT as we know. It only showed one thing on the front of the display
    http://en.wikipedia.org/wiki/Crookes_tube



    cheers
    Dave
     
  7. Feb 5, 2015 #6
    I suppose my question is not clear to you. To put it in a simpler manner. if I have electrons (low energy), and wish to accelerate them before allowing the high energy electrons to strike something, how do I do it?
     
  8. Feb 5, 2015 #7

    sophiecentaur

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    Your point is absolutely fine, in principle. In an electron gun, the electrons are attracted by the 'First' Anode (Which is part of the electron gun assembly, initially and there is a strong field between them (Many Volts, small distance). The ballistics of the situation could take those electrons far beyond the first Anode, before they slow down and head back to it, if that Anode Potential were all that counted. But there is also a potential difference between Anode and Screen (that's also an anode), so the electrons carry on towards the (highly positive) back of the screen. In practice, the whole of the Electron source (back end) is kept at a 'safe' voltage so that you can supply the grids and deflection coils / plates with reasonable voltages from video amplifiers etc. That big fat wire that goes to the front, side of the tube, carries the EHT from the diode stack and is the hairiest part of the setup.

    The reason for having an electron gun at all is to produce the narrow, steerable electron beam which, once formed, will carry on to the screen phosphors. The energy from this acceleration is what produces the light output from the screen. A Crooke's tube has no focussed beam and no electron gun. You can make a crook's tube work with the sort of vacuum that you can get with a hand cranked school vacuum pump. A round bottomed flask with an anode, fed through a sealed hole in the bottom is not hard to make and to get results with. But be careful with kV power supplies!!!!
     
  9. Feb 5, 2015 #8

    davenn

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    You haven't asked anything different, the answer remains the same :)
    As already discussed, you use a high voltage positive plate/screen etc to attract them

    Dave
     
  10. Feb 5, 2015 #9

    sophiecentaur

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    If the object you are trying to bombard is a conductor, you put it at a high potential. If it is not, then you can put an anode behind it or, even better, use an electron gun to focus your beam at it. Your original point about the electrons 'going past' an anode and still having energy, is a valid one.
    What actually do you (the OP) want to do with these electrons?

    PS there can be a problem with an insulator charging up and then repelling electrons from the beam.
     
  11. Feb 5, 2015 #10

    nsaspook

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    Usually the electrodes are shaped to focus the fields with the anode at the effective ground potential of the circuit.
    http://www4.nau.edu/microanalysis/microprobe-sem/images\Electron_Gun.jpg
     
  12. Feb 5, 2015 #11

    sophiecentaur

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    It will depend upon what sort of CRT you are dealing with. A TV CRT certainly needs the Grids to be at around Ground potential so that relatively low video voltage drivers can be used. In any case, the accelerating potential, to produce high energy will have to be at the 'front end (call it a second anode, because that is its function) because it is not convenient to produce modulating signals, sat down at lots of -kV. The absolute potential has no relevance except for practicality and it is by far the easiest system to have the EHT supply taken to the front of the tube, away from all the other electronics.
    That diagram is of the electron gun section and the supply is only 'high voltage' and not EHT. It doesn't seem to include the focussing electrodes, which are needed for spot forming.
     
  13. Feb 5, 2015 #12

    nsaspook

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    Correct. I was just trying to show that the potentials and fields would be focused to accelerate the electrons past the (at effective ground potential IRT the electron) anode but after the anode the potentials and fields from that circuit would be much less and not slow the electron as it moved into space past it.
     
  14. Feb 25, 2015 #13
    The acceleration of the electrons is caused, not by the potential of the first anode, but by the potential gradient between anode and cathode. Once the electrons pas the anode, the potential gradient is much reduced. There may be a second anode to help.
     
    Last edited by a moderator: Feb 25, 2015
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