A point charge at rest or in uniform motion is surrounded by static or quasi-static electric and magnetic fields. As long as the point charge remains in its state of rest or uniform motion, these fields move with the point charge as though they were rigidly attached to it. However, if the point charge is subjected to a sudden acceleration caused by some external force, then pieces of the electric and magnetic fields break away from the point charge and propagate outward as a self-supporting electromagnetic wave pulse.

Such a wave pulse is a disturbance of the static or quasi-static electric and magnetic fields. The disturbance arises from the need of the electric and magnetic fields to readjust from their initial configuration (before the acceleration) to their new configuration (after the acceleration). Figure 13.1 illustrates how a propagating disturbance accomplishes the readjustment of the electric field of an accelerated point charge. Initially, the point charge was at rest at the point P; it then received a sharp acceleration during the interval PP ?; and it finally continued with uniform velocity during the interval P ? Q. The initial electric field lines converged on the point P; the final electric field lines converge on the point Q. We know from relativity that the readjustment of the electric field cannot be accomplished instantaneously. Instead, it is accomplished by a disturbance, or kink, in the field lines that travels outward at the speed of light. In Fig. 13.1,...