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Homework Help: Electron Uniform Field Question

  1. May 21, 2010 #1
    Hi, I have a problem working out the answer to one part of a type of uniform field question. My answers to the first three parts of the question are correct, but I'll write out those parts as well because I think it's necessary, to understand the second part:

    An X-ray tube is operated at 25 kV and draws a current of 3 mA

    (a) Calculate:
    (i) the kinetic energy of each electron as it hits the target ( Answer: 4x10-15 J )
    (ii) the velocity of impact of the electron as it hits the target ( Answer : 9.38x107 m s-1
    (iii) the number of electrons hitting the target each second ( Answer : 1.875x1016 electrons )

    The following part is the part I'm having difficultly on:

    (b) What happens to the kinetic energy of the electrons?

    My guess was it changed to electrical potential energy, but I don't think that is correct because that's what it would have if it was at the negative plate of a uniform field, but this electron must be at the positive end because it gained kinetic energy. Sorry to bother you all with this problem, I should know the answer but I'm just not sure what the question wants from me, I would consult the answer sheet but it only provides the answers for questions with numerical solutions.

    Thanks in advance for any correspondence.
  2. jcsd
  3. May 21, 2010 #2
    Right, the electron travels from the negative end to the positive end. This corresponds to a drop in potential energy for the electron. This results in an increase in its kinetic energy, which accounts for its acceleration across the potential. You might ask how the electron loses potential energy if it travels from the negative to the positive end, when the positive end is at 25kV higher than the negative end. Remember a proton in an electric field moves in the direction of the electric field whereas an electron would move against the electric field. Therefore the potential difference is of the opposite sign for an electron.
  4. May 21, 2010 #3
    As far as I can tell, that doesn't answer part (b), does it?
  5. May 21, 2010 #4

    - caniexplore
  6. May 21, 2010 #5


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    Homework Helper
    Gold Member

    I guess it depends if the problem statement is asking "what happens to the kinetic energy of the electrons," before or after the electron hits the target. The way I read it, it's kind of ambiguous. You might wish to play it safe and elaborate about what happens to the energy (kinetic or otherwise) before and after it hits the target.

    I think with all the other replies, you have a handle on what happens to the kinetic energy of the electrons before it hits the target.

    But this kinetic energy is converted to other forms of energy after it hits the target. It is an x-ray tube after all. That should give you a hint. You might wish to do a little research on how x-ray tubes work to form your final answer.
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