<P> Any accelerating electric charge, and therefore any changing electric current, gives rise to an electromagnetic wave that propagates at very high speed outside the surface of the conductor . This speed is usually a significant fraction of the speed of light, as can be deduced from Maxwell's Equations, and is therefore many times faster than the drift velocity of the electrons . For example, in AC power lines, the waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distant load, even though the electrons in the wires only move back and forth over a tiny distance . </P> <P> The ratio of the speed of the electromagnetic wave to the speed of light in free space is called the velocity factor, and depends on the electromagnetic properties of the conductor and the insulating materials surrounding it, and on their shape and size . </P> <P> The magnitudes (not the natures) of these three velocities can be illustrated by an analogy with the three similar velocities associated with gases . (See also hydraulic analogy .) </P> <Ul> <Li> The low drift velocity of charge carriers is analogous to air motion; in other words, winds . </Li> <Li> The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas (sound waves move through air much faster than large - scale motions such as convection) </Li> <Li> The random motion of charges is analogous to heat--the thermal velocity of randomly vibrating gas particles . </Li> </Ul>

What name is given to the rate of flow of electric charge