# Homework Help: Derive Electrostatic Force from Coulomb's Law.

1. Aug 8, 2012

### jhosamelly

1. The problem statement, all variables and given/known data

I should derive

$\vec{F_{q}} = \frac{-q'^{2}}{4∏ \epsilon_{0}} \frac{r'/a}{(r'^{2} - a^{2})} \hat{r}$

from

$\vec{F_{q''q'}} = k\frac{q' q''}{\left|\vec{x'} - \vec{x''}\right|^{2}}$

2. Relevant equations

3. The attempt at a solution

I know that $q'' = -q' \frac{a}{r'}$

and $r'' = \frac{a^{2}}{r'}$

so, I substituted these. but I can't seem to arrive at the correct formula.