# Coloumbs law with inverse cubic

1. Sep 16, 2013

### yusufali

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. Sep 16, 2013

### voko

You have been instructed to use $(1 + x)^{-n} \approx 1 - nx$, when $x << 1$. Why are you ignoring this?