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Homework Help: Coloumbs law with inverse cubic

  1. Sep 16, 2013 #1
    1. The problem statement, all variables and given/known data
    A permanent electric dipole consisting of charges +q and -q
    separated by the fixed distance s. Charge +Q is distance r from
    the center of the dipole. We'll assume s << r.

    a) Use the binomial approximation (1+x)-n ~= 1-nx if
    x << 1 to show that the net force exerted on the dipole by charge +Q
    is Fnet = 2KqQs/r3

    2. Relevant equations

    F = Kqq/r2

    3. Attempted Solution

    F = +kQq/(r+s/2)2 - kQq/(r-s/2)2
    F = -2kQq(rs)/( (r+s/2)2 * (r-s/2)2 )

    i cannot figure out the rest from there...
  2. jcsd
  3. Sep 16, 2013 #2
    You have been instructed to use ## (1 + x)^{-n} \approx 1 - nx ##, when ## x << 1 ##. Why are you ignoring this?
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