Two small spheres spaced 35.0 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 2.2 x 10-21N?

The coulomb force between two equal charges is

       F=keq²/r²

Where ke is the electrostatic constant and it's given as:

              = 8.988 x 109Nm²C^-2

   r = separation distance between the two spheres.

and

    F =repulsion force between the spheres.

Therefore we can calculate q through plugging in the values provided

     2.2 x 10-²¹ = 8.988 x 109  x q²/0.35

Solving for q gives:

    q = 0.17 x 10^-15C

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

        e =1.6 x 10^-19C)

Thus:

    Ne = q/e

          = 0.17 x 10^-15 / 1.6  x 10^-19

Hence the answer is:

         = 1062 electrons