Speed of electricity

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The Speed of electricity refers to the relatively slow movement of free electrons through a conductor in the presence of an electric field, also known as drift velocity. It is often confused with the propagation speed of an electromagnetic wave. It is the electromagnetic wave that can carry information (data), not the movement of electrons.

Free electrons in a conductor vibrate randomly, but without the presence of an electric field there is no net velocity. When a DC voltage is applied the electrons will increase in speed proportional to the strength of the electric field. These speeds are on the order of millimeters per second. AC voltages cause no net movement; the electrons "wiggle" back and forth in response to the alternating electric field.

In contrast, electromagnetic wave propagation is much faster, and depends on the dielectric constant of the material. In a vacuum the wave travels at the speed of light and almost that fast in air. Propagation speed in a copper conductor is about 99.9% the speed of light^{[citation needed]}.