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Homework Help: Electric field direction with an electron?

  1. Apr 28, 2013 #1
    1. The problem statement, all variables and given/known data
    If we have an electric field with a positive and negative plate:

    The field lines will be going from the positive to the negative, so down.

    But, if we put an electron between these two plates, it will be attracted to the positive plates, so it will have a force vector pointing up.

    The electron has more potential energy at B and less at A

    If the electron moves from point A to point B; was this caused by the field? Will the electric field move the electron from point A to point B, even though the electron wants to attract to the positive terminal?

    So my basic question is: Will an electric fields force vector be DOWN for a proton and UP for an electron? Or will it always be DOWN (from positive to negative) no matter what? Thereby increasing the electrons potential energy?



    2. Relevant equations

    3. The attempt at a solution
    Not really sure at the attempt, because it's not really a question from my book, but more of a question from myself.
  2. jcsd
  3. Apr 28, 2013 #2


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    Field lines are defined to point from positive to negative. If we draw out the lines, they'll point downward in your picture, as you said.

    Now, let's remember how to relate electric forces to electric fields. If we have an electric field, E, and a charge, q, then the electric force, F, can be found with F = q*E. So if we put down a positive q (like a proton), the force will be in the direction of the field (since q won't change the sign of E). However, if we put in a negative charge (like an electron), the negative q value will cause the force to have an opposite sign from the field. We know that it will point in the opposite direction as the field.

    We can see that this works. If the field lines point down, and we put in a proton, it will move down, which we just predicted. If we put in an electron, it will move up, as we just predicted and as you stated. This is actually why we use the convention of pointing field lines from positive to negative.

    That's the relationship between fields and forces to remember. Does that make sense?
  4. Apr 28, 2013 #3
    Yes, thanks. I seemed to have gotten confused, but its clear now.
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