QUESTION $3$

Two small spheres are separated by $4\mathrm{.}0cm(0\mathrm{.}04m).$ The spheres have the same magnitude charge of $1\mathrm{.}9{10}^{-6}C$ and feel a $20.N$ electric force pulling them together. First decide if the two spheres should have the same sign charge or opposite signs. Then calculate the electric potential energy in the system of having these point$-$charge$-$like spheres this distance from each other.

Please write your answer without units. If your answer is positive, just write the number; if it is negative, include $a-$sign.