Coulomb's Law

In 1785 Charles-Augustin de Coulomb hung a charged sphere on a fine torsion wire and measured how hard a second charge twisted it. The result is the electric twin of Newtonian gravity: the force between two point charges grows with the product of the charges and falls off with the square of the distance. Electricity is by far the stronger force: the electric repulsion between two protons is about 10³⁶ times their gravitational attraction — which is why a rubbed balloon can lift paper against the pull of the entire Earth.

Worked example: two 1 µC charges held 10 cm apart feel F = kₑ × (10⁻⁶)² / (0.1)² ≈ 0.9 N — roughly the weight of an apple, from specks of charge. Enter charge magnitudes here; the sign of the product only tells you whether the pair attracts (opposite signs) or repels (like signs).