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Homework Help: A simple electric field question

  1. Aug 4, 2010 #1
    1. The problem statement, all variables and given/known data
    http://img19.imageshack.us/img19/7662/screenshot20100804at214.png [Broken]

    2. Relevant equations
    the coulomb law for electrostatic force.

    3. The attempt at a solution
    This is supposedly a very straightforward question but I can't get any of the four answers so I just wonder if anyone is kind enough to check my working for me.

    Basically I just found the net force acting on the electron and got something like [tex]\frac{e^{2}}{32\pi \epsilon_{0} x^{2}}[/tex]. Then I just use the work done = force x distance formula but I did not arrive at any of the answer.

    Am I missing something?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Aug 4, 2010 #2
    The net force varies as the electron approaches the protons. since it's distance from the protons changes as does the angle they're in.
  4. Aug 4, 2010 #3
    OH!! Right I somehow forgot about that. Thanks.
  5. Aug 4, 2010 #4
    Now I'm not sure I would calculate it like that, since it'd probably involve integrating along the whole length of the electron's path (to calculate the area in the F-s diagram to get the work done). A much easier way to do this is consider the changes is potential energy, which we know is transferred into kinetic energy.
    The potential energy from a charge Q at distance r is -(1/4e0*pi)*Q/r. When you have more than one charge, this energy is added together. So first r = 5x, then r=3x. the difference*e(charge) gives the work done on the electron.
  6. Aug 4, 2010 #5
    yes I did that, thanks for the help anyway :D
  7. Aug 30, 2010 #6
    The difference of potential energies at final and initial position gives the answer
    Last edited by a moderator: May 4, 2017
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