According to Coulomb's law, the electrostatic force between two point charges $q_1$ and $q_2$ separated by distance $r$ is proportional to

1. Coulomb measured the force between two charged spheres with a torsion balance (1785). 2. He found the force grows with the PRODUCT of the two charges: double either charge, the force doubles. 3. He also found the force falls with the SQUARE of the separation: double the distance, force drops to one fourth. 4. Combining both: $F = k\frac{q_1 q_2}{r^2}$. Option B misses the square, option C adds charges instead of multiplying, option D grows with distance. _Source: syllabus Physics Ch 1 "Electric Charges and Fields", §1.6 Coulomb's Law_