Four charges $q_1=1\mu C,q_2=2\mu C,q_3=-3\mu C$ and $q_4=4\mu$ are kept on the vertices of a square of side $1m$. Find the electric potential energy of this system of charges.

 In this problem, 

$r_{41}=r_{43}=r_{32}=r_{21}=1m$

and $r_{42}=r_{31}=\sqrt{{\left(1\right)}^2+{\left(1\right)}^2}=\sqrt{2}m$

Substituting the proper values with sign in Eq. (ii), we get

$U=\left(9.0\times {10}^9\right)\left({10}^{-6}\right)\left({10}^{-6}\right)\left[\frac{\left(4\right)\left(-3\right)}{1}+\frac{\left(4\right)\left(2\right)}{\sqrt{2}}+\frac{\left(4\right)\left(1\right)}{1}+\frac{\left(-3\right)\left(2\right)}{1}+\frac{\left(-3\right)\left(1\right)}{\sqrt{2}}+\frac{\left(2\right)\left(1\right)}{1}\right]$

$=\left(9.0\times {10}^{-3}\right)\left[-12+\frac{5}{\sqrt{2}}\right]$

$=-7.62\times {10}^{-2}J$

Note: Here, negative sign of  $U$ implies that negative work has been done by electrostatic forces in assembling these charges at respective distances from infinity.