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