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Homework Help: Force on an electric dipole

  1. Jan 22, 2009 #1
    1. The problem statement, all variables and given/known data

    A dipole, consisting of a charge -q at position vector r and a charge +q at position vector r+s, is sitting in an external electric field E(ext).

    (a) Write down the total force on the dipole F(dipole) due to the external field

    (b) Show that when s is small,
    F(dipole) approx. = (dot product of p and del operator)*E(ext)
    where p is the electric dipole moment, p = q s

    (c) In what limit does the result become exact?

    2. Relevant equations

    p = q s
    F = torque = cross product (p and E(ext))

    3. The attempt at a solution

    (a) F(dipole) = cross product (p and E(ext)) = q* cross product (s and E(ext))

    stuck on (b) and (c)

    Please help!
  2. jcsd
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