Coulomb's law

Meaning: Coulomb's law gives the electrostatic force between two point charges. The force acts along the line joining them, repels for like charges, attracts for opposite charges, grows with the product of the charges, and falls off with the square of their separation. Each charge feels an equal and opposite force, even if one charge is larger. A medium such as water weakens the force relative to vacuum.

Equation: F = k|q₁q₂|/r², where F is the force magnitude in newtons (N), k ≈ 8.99 × 10⁹ N·m²/C², q₁ and q₂ are charges in coulombs (C), and r is the separation in meters (m).

On the MCAT: Expect ratio problems: doubling one charge doubles F, while doubling r cuts F to one-fourth. Other questions ask where a third charge feels zero net force, or compare the electric force with gravity, which has the same inverse-square form but is purely attractive. The law also leads to electric field (E = F/q) and electric potential energy (U = kq₁q₂/r).

Common confusion: Students often use 1/r instead of 1/r². Tripling r gives one-ninth the force. Note that force goes as 1/r², but potential energy goes as 1/r.