Question

A small plastic sphere with a charge of -6.0 nC is near another small plastic sphere with a charge of -14 nC . If the spheres repel one another with a force of magnitude 8.3×10−4 N , what is the distance between the spheres?

Answer 1

Here is how to answer the question.

Use the formula:
F = kq1q2/r^2 

F = force = 8.3x10^-4N 
q1, q2 = charges 
r = separatioon 
k = constant = 9x10^9 in MKS units 

so we have 

r = sqrt[k q1 q2/F]
r = sqrt[9x10^9*(-14)x10^-9 * (-6)x10^-9 / 8.3x10^-4 ]
r = 0.0302 m

So the distance between spheres is 0.0302 m