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Proton, electron and coordinates

  1. Mar 27, 2007 #1
    1. The problem statement, all variables and given/known data

    A proton sits at coordinates (x,y) = (0,0), and an electron at (d,h), where d>>h. At time t=0, a uniform electric field E of unknown magnitude but pointing in the positive y direction is turned on. Assuming that d is large enough that the proton-electron interaction is negligible, the y coordinates of the two particles will be equal (at equal time)

    a) at about y=d/2000;
    b)at an undetermined value since E is unknown;
    c)at about y=d/43;
    d)nowhere: they move in opposite directions

    2. Relevant equations
    E=F/q
    F=ma


    3. The attempt at a solution
    The correct answer is a) but I don't know why. I have no idea how the answer connects d, a x-coordinate, with the final y-coordinate.

    This is my attempt::

    E=F/q
    Since the E is the same and the q is the same, the 2 Forces must be the same
    (mass of electron)(acceleration of electron)=(mass of proton)(acceleration of the proton)
    (acceleration of electron)=1833(acceleration of proton)

    S=(0.5)(acceleration of electron)t^2+(0.5)(acceleration of proton)t^2
    of course, S=h-0 or S=h, and (acceleration of electron)=1833(acceleration of proton)
    h=(0.5)(1833a(proton))t^2+(0.5)(a(proton))t^2
    a(proton)=h/(917t^2)
    I have no idea how to go from there....please help
    Thanks a lot
     
  2. jcsd
  3. Mar 27, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    It makes no sense. I suspect a typo: The answers show "d" where they should show "h".
     
  4. Mar 27, 2007 #3
    Thanks, Doc Al
    I got the problem, and it is y=h/1834
     
  5. Mar 29, 2007 #4
    this is quite easy actually. Since the time elapsed is the same, the ratio between their displacements should be the inverse of that between their mass which is about 2000(very rough approximation:tongue: ). So we get y=h/2001 so (a)
     
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