F = q(E + v × B) A single compact law that tells every free charge how the electromagnetic field will act on it. The electric term accelerates the particle along the field lines whether it is at rest or in motion. The magnetic term, born of the cross product, appears only when the particle moves and then stands forever perpendicular to the velocity, bending paths into circles and helices without ever changing the particle’s kinetic energy. Lorentz assembled the complete expression in 1895 while constructing his electron theory of matter. The same relation later became one of the constraints that special relativity had to preserve in every inertial frame.