This extra part of the force is called the

*magnetic*force, and is described by*magnetic**fields*. For slow movement, the*magnetic*force is minimal and Coulomb's law can still be considered approximately correct, but when the charges are moving more quickly in relation to each other, the full electrodynamic rules (incorporating the*magnetic*force) must be considered. In simple terms, the Coulomb potential derives from the QED Lagrangian as follows. The Lagrangian of quantum electrodynamics is normally written in natural units, but in SI units, it is: where the covariant derivative (in SI units) is: where g is the gauge coupling parameter.