Question

If the distance between two charges is increased to three times the original distance, how will the electrical force between the charges compare with the original force?

It will increase to three times the original force.
It will increase to nine times the original force.
It will decrease to one-third the original force.
It will decrease to one-ninth the original force.

Answer 1

It follows Coulomb's inverse-square law: F ∝ 1/r^2. Increasing r to 3r gives F' = F/(3^2) = F/9, so it decreases to one-ninth the original force.