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## Homework Statement

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?

## Homework Equations

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)

Please help!