electrostatics

Meaning: Electrostatics is the study of electric charges at rest and the forces, fields, and potential energies they produce. Like charges repel and opposite charges attract. A charge sets up an electric field around it, and any other charge in that field feels a force. Inside a conductor at electrostatic equilibrium the field is zero, and excess charge sits on the surface.

Equation: F = kq₁q₂/r², where F is the force between two point charges in newtons (N), k ≈ 9.0 × 10⁹ N·m²/C² is Coulomb's constant, q₁ and q₂ are the charges in coulombs (C), and r is their separation in meters (m).

On the MCAT: Questions ask how force changes when charge or distance changes (doubling r cuts the force to one-fourth), ask for the net force direction on a charge near two or more others by vector addition, or ask you to read field-line drawings. Related formulas: E = F/q, U = kq₁q₂/r, V = kQ/r.

Common confusion: Force and field fall off as 1/r², but potential energy and potential fall off as 1/r. Force and field are vectors; potential and potential energy are scalars, so add them with their signs, not as directions.