Two small diameter, 10gm dielectric balls can slide freely on a vertical channel each carry a negative charge of 1microcoulomb. find the separation between the balls if the lower ball is restrained from moving.

The distance of separation is

Explanation:

The mass of the each ball is  

 The negative charge on each ball is

Now we are told that the lower ball is  restrained from moving this implies that the net force acting on it is  zero

Hence the gravitational force acting on the lower ball is equivalent to the electrostatic force i.e

=>      

here k the the coulomb's  constant with a value  

So  

force is mass*acceleration.

we have a very small mass: 11.4*0.001 kg

if we want 0.035n of force, we need an acceleration:

0.035 = 11.4*0.001*a

a = 0.035/(11.4*0.001) m/s^2

i think it would be 5