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Derive equation for electric potential of electric dipole

  1. Jan 25, 2012 #1
    I'm not understanding how the equation of the electric field due to an electric dipole is derived. This is how my book derives it:

    Say you have electric dipole composed of charges +q and -q a distance d apart, with the negative charge at the origin of the z-axis. Then, at any point z, the E field is:

    [itex]\frac{kq}{(z-\frac{1}{2}d)^{2}}[/itex] - [itex]\frac{kq}{(z+\frac{1}{2}d)^{2}}[/itex]

    Below is the exact picture of the situation from my book:


    But shouldn't the equation be:
    [itex]\frac{kq}{(z-d)^{2}}[/itex] - [itex]\frac{kq}{z^{2}}[/itex]


    In my book's equation, it looks like they're just treating the two charges as if they're at the same point (the midpoint, corresponding to (1/2)d)....I know that for large z, this wouldn't matter much, but still, what if you want small z...
    Last edited: Jan 25, 2012
  2. jcsd
  3. Jan 25, 2012 #2
    You have misunderstood the position of the origin. z=0 is between the charges (the dot).
  4. Jan 25, 2012 #3
    Oh! wow im stupid thanks lol
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