Coulomb's inverse-square law and superposition

Coulomb's inverse-square law and superposition

Coulomb's law is an inverse-square relationship: F is proportional to 1/r². If separation doubles, force becomes one quarter; if separation triples, force becomes one ninth. This strong distance effect explains why drawing a sensible geometry matters. When more than two charges are present, calculate or reason about the force from each source separately, including direction, and then add the force vectors. This is superposition. A positive or negative answer can represent direction only after you define an axis; the formula's magnitude form by itself cannot decide attraction or repulsion.

Work it through

Two +2.0 μC charges 0.20 m apart repel with F ≈ 0.90 N. If their separation becomes 0.40 m, the new force is 0.90/4 = 0.225 N. The charge values did not change, so the factor comes entirely from r². In a three-charge line, first sketch every force on the selected charge before adding signed components.

Mastery target

Apply the inverse-square relationship and superposition to compare or calculate electrostatic forces with a consistent axis and direction statement.