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Homework Help: Electric Field and dipole moment

  1. Feb 7, 2008 #1
    There is a dipole moment that is symmetric along a y-axis and is along an x-axis. A charge -q is placed a distance a along the -x direction and a charge +q is placed a distance a along the +x direction, making the distance between the 2 charges 2a. I'm supposed to show that the electric field at a distant point on the +x axis is
    E ~ (4*Ke*q*a)/(x^3).

    I set the equation so that E = (-Ke*q)/(x+a) + (Ke*q)/(x-a) and ended up with
    E = (2*Ke*q*a)/(x^2 - a^2), which for a distant point may round to (2*Ke*q*a)/(x^2). The answer they're looking for is different. What am I doing wrong?
  2. jcsd
  3. Feb 7, 2008 #2


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    E=Keq/r^2 not Keq/r.
  4. Feb 7, 2008 #3
    !!!.......Such a simple mistake. Heh...oh well. Thanks, I got it.
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