Four identical particles each have charge q and mass m. They are released from rest at the vertices

Question

of a square of side L. How fast is each charge moving when their distance from the center of the
square doubles?

Answer 1

Calculate the potential energy of a particle at original location
( in general at distance integral from infinity to d of k Q1Q2 dr /r^2
= k Q1Q2 /r )
Do it again for bigger distance.
(1/2) m v^2 = decrease in potential energy