Two Spheres

Two spheres that are $35\text{ cm}$ apart have equal charge. How many excess electrons must be on each sphere if the magnitude of the force of repulsion between them is $2.2\times10^{-21}\text{ N}$ ?

Round your answer down to the nearest whole number.

Details and Assumptions :

• The charge of a single electron is $e=1.6\times10^{-19}\text{ C}$ .
• For this problem, assume the electrostatic constant is $k_e=8.988\times10^9\text{ N m}^2\text{ C}^{-2}$ .