Problem 1

Two small spheres spaced 35.0cm apart   have equal charge.  How many excess electrons must be present on each sphere if the  magnitude of the force of repulsion between them is ?

Solution

The Coulomb force between two equal charges (spheres) is

We know : , , electrostatic constant: . Then we can find q :

Then we can find the number of excess electrons (since we know the charge of a single electron :