Two small spheres separated 3cm apart carry equal charges. How many excess electrons must be placed on each sphere if the force of repulsion between them is to be 10^17N?

let the charge on spheres is q

so F= 1/4∏ε qq/.03²

  given F= 10^-19

       1/4∏ε qq/.03² =10^-19 

 9 *10⁹ * q² /.03² =10^-19  

               q=10^-16

1 electron has  1.6 * 10^-19 C  

 ne =q

 n=q/e =10^-16/1.6 * 10^-19 C 

           =625