Explanation:
If the two charges are point charges - i.e., they don't have a size - the force between these charges depends on the
• Magnitude if each charge, q1 and q2
• Sign of each charge (+ or -)
• Distance between the charges, r
This is essentially Coulomb’s Law:
FE = (kq1q2)/r2
For collections of charges, you need to find the electric field E, and then use this fields to find a force on a small test charge q in the field. The test charge is always small to help you map the electric field, but not disturb it.
