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Homework Help: Magnitude of Electric field using superposition principle

  1. Feb 16, 2010 #1
    1. The problem statement, all variables and given/known data

    Two charges, +q and -q, are located in the x-y plane at points (0,+d/2) and (0,-d/2), respectively. Calculate the magnitude of the electric field at point P with the superposition principle.
    Data: q=37.0 nanoCoulombs,
    d=4.60mm and P at

    2. Relevant equations
    E = kQ/r^2
    a^2 + b^2 = c^2
    sin = o/h

    3. The attempt at a solution
    I tried to solve this like a normal E field question.
    For the +q charge, I got that the E field would be:
    ( k * -37E-6 (sin2) / .092^2) + ( k * 37E-6 (cos2)
    and For the -q charge, I got that the E field would be:
    ( ( k * -37E-6 (sin2) / .092^2) - ( k * 37E-6 (cos2)
    This came out to 2740 N/C (positive b/c it asks for magnitude) but that was wrong. Can someone tell me where I'm going wrong? I attached the diagram.

    Attached Files:

  2. jcsd
  3. Feb 17, 2010 #2
    I understood what you first did,

    you first said that you will break up E= Esintheta + Ecostheta, one thing I noticed that in the equation E=kq/r^2 , you used for r = 0.092 you used the x distance for the point p, I suggest you have another look at your figure :

    http://img231.imageshack.us/img231/5466/59813260.gif [Broken]

    compare the blue line with the red one , do you still think that the distance between q and p is x (0.092) ?
    Last edited by a moderator: May 4, 2017
  4. Feb 17, 2010 #3
    When I did (.092^2) + (.0023^2), I get that the hypotenuse is .0092...
  5. Feb 17, 2010 #4
    Ah, sorry my mistake, I now get .00847. I will try that. Was the rest of the setup ok? Thanks for your help!
  6. Feb 17, 2010 #5
    Hmmm.... that was still wrong. Am I doing something else incorrectly?
  7. Feb 17, 2010 #6
    i think the required distance between q and p should be sqrt((0.092^2) + (0.0023^2)) i did the calculation and i got 0.092028 (when you said it is 0.00847 you didnt take the square root) ..

    another thing, I think you probably havent considered the elecrtic field produced from positive charge and negative charge, have a look at this figure:

    http://img402.imageshack.us/img402/7681/95152182.jpg [Broken]

    so your problem should look something like the following:

    http://img4.imageshack.us/img4/4996/34194234.gif [Broken]

    notice where r the electric field vectors , find the components of each vectors ,then see whether you will find any compenents that will cancel with each other and pay attention to the directions ..

    by the way , i have one question for you , from where you get sin2 and cos2?
    Last edited by a moderator: May 4, 2017
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