electrostatic force

Meaning: The electrostatic force is the force one charged object exerts on another because of their charges. It acts along the line joining the two charges. Like charges repel and opposite charges attract. The two objects exert forces of equal magnitude and opposite direction on each other (Newton's third law). With several charges, the net force on one is the vector sum of the individual forces.

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

On the MCAT: Expect ratio questions. Doubling one charge doubles F, and doubling r cuts F to one-quarter. Passages may compare it with gravity (same inverse-square form, but the electrostatic force can repel) or place a test charge near other charges or between plates and ask for the direction of the net force.

Common confusion: A larger charge does not push harder on a smaller one; the two forces are equal in magnitude. Also keep distance dependence straight: force falls as 1/r², but electric potential energy, U = kq₁q₂/r, falls as 1/r.