Coloumb's law

Meaning: Two point charges exert forces on each other along the line joining them. The forces are equal in magnitude and opposite in direction. Like charges repel and opposite charges attract. The magnitude grows with the product of the charges and falls off as 1/r², an inverse-square law like Newton's law of gravitation.

Equation: F = k|q₁q₂|/r². F is the electrostatic force in newtons (N). k is Coulomb's constant, about 9.0 × 10⁹ N·m²/C². q₁ and q₂ are the charges in coulombs (C). r is the distance between the charges in meters (m).

On the MCAT: Most often this is a scaling question. If one charge doubles and the distance triples, the force becomes 2/9 of its original value. Passages may ask for the net force on one charge from several others, which you find by vector addition. Others compare the electric force with the gravitational force between particles, or ask how a medium such as water, which has a high dielectric constant, weakens the attraction between ions.

Common confusion: Students think the larger charge feels the larger force. Both charges feel forces of equal magnitude, as Newton's third law requires. Students also mix up the powers of r. Force and electric field (E = kq/r²) go as 1/r². Electric potential energy (U = kq₁q₂/r) goes as 1/r.