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Homework Help: Electron & Proton within a capacitor

  1. Jan 27, 2009 #1
    1. The problem statement, all variables and given/known data
    "The plate separation of a charged capacitor is 0.0800 m. A proton and an electron are released from rest at the midpoint between the plates. Ignore the attraction between the two particles, and determine how far the proton has traveled by the time the electron strikes the positive plate."

    2. Relevant equations
    Not 100% sure.

    3. The attempt at a solution
    I've tried using energy conservation and Newton's Second Law and I've tried to have a look at every other equation I've been given and their derivations. I'm just stuck. I think there might be a conceptual part I'm missing?

    If anyone could just give some hints, I'd be very grateful.
  2. jcsd
  3. Jan 27, 2009 #2


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    Apparently the forces on each would be equal wouldn't they, even though in opposite directions?

    But they have different mass. So ...
  4. Jan 27, 2009 #3
    I get that mass of electron*acceleration of electron=mass of proton*acceleration of proton.

    The thing is, I think I'm understanding what I should calculate wrong - somewhere in my mind I want to find the time it takes for the electron to hit the positive plate and from that find the acceleration of the electron (using x=at2/2) and using the relation between the electron's and the proton's accelerations and from there using previously mentioned equation to find the distance for the proton.

    However I seem to have one too many variables constantly. I just don't know what I should do, it's driving me absolutely nuts.

    And probably I'm making it more difficult than it is as well x|
  5. Jan 27, 2009 #4


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    You may just be making it more difficult than necessary.

    The point is, figure the acceleration for each. You know the distance.

    You remembered your kinematic equations for distance acceleration and time being:

    x = 1/2*a*t2

    Figure the time for the electron.
    Then use that time with the proton acceleration to figure distance for the proton.

    Subtract from the total distance and you have your Δ.
  6. Jan 27, 2009 #5
    I get an answer that's something like .02 mm .. but I've recently learned that in electricity I'm perfectly useless in determining whether the answer is reasonable or not. But I found expressions for their accelerations (being Fc/m) and just .. used that expression. It works for me, at least.

    The only thing I'm not sure about is what you mean by the delta - to me that seems to be figuring out how far it is left for the proton to hit the negative plate. However, I don't think I'm thinking clearly anymore, so I probably should drop it.

    Thanks so much for your help! I really needed someone to hit me over the head and make me think about it the right way.
  7. Jan 27, 2009 #6


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    Never mind that about Δ. I mistakenly had it in mind they wanted to know how much further it had gone.
    Sorry for any confusion my confusion may have caused.
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