Answer
5.08 m
Work Step by Step
Set the magnitudes of the two forces equal to each other; solve for the distance.
$$F_{electric}=F_{gravitational}$$
$$ k\frac{e^2}{r^2}=mg$$
$$r=e\sqrt{\frac{k}{mg}}$$
$$r=(1.602\times10^{-19}C)\sqrt{\frac{8.988\times10^{9}(N \cdot m^2)/C^2}{(9.11\times10^{-31}kg)(9.80m/s^2)}}$$
$$r=5.08m$$
