The word electricity refers generally to the movement of electrons (or other charge carriers) through a conductor in the presence of a potential difference or an electric field. The speed of this flow has multiple meanings. In everyday electrical and electronic devices, the signals travel as electromagnetic waves typically at 50%–99% of the speed of light in vacuum, while the electrons themselves move much more slowly; see drift velocity and electron mobility. Electromagnetic waves The speed at which energy or signals travel down a cable is actually the speed of the electromagnetic wave traveling along (guided by) the cable. I.e., a cable is a form of a waveguide. The propagation of the wave is affected by the interaction with the material(s) in and surrounding the cable, caused by the presence of electric charge carriers (interacting with the electric field component) and magnetic dipoles (interacting with the magnetic field component). These interactions are typically described using mean field theory by the permeability and the permittivity of the materials involved. The energy/signal usually flows overwhelmingly outside the electric conductor of a cable; the purpose of the conductor is thus not to conduct energy, but to guide the energy-carrying wave. Speed of electromagnetic waves in good dielectrics The speed of electromagnetic waves in a low-loss dielectric is given by where = speed of light in vacuum. = the permeability of free space = 4π x 10−7 H/m. = relative magnetic permeability of the material. Usually in good dielectrics, eg. vacuum, air, Teflon, . . = the permitivity of free space = 8.854 x 10−12 F/m. = relative permitivity of the material. Usually in good conductors eg. copper, silver, gold, . . Speed of electromagnetic waves in good conductors The speed of electromagnetic waves in a good conductor is given by where = frequency. = angular frequency = 2f. = conductivity of annealed copper = . = conductivity of the material relative to the conductivity of copper. For hard drawn copper may be as low as 0.97. . and permeability is defined as above in = the permeability of free space = 4π x 10−7 H/m. = relative magnetic permeability of the material. Magnetically conductive materials such as copper typically have a near 1. . In copper at 60Hz, 3.2m/s. As a consequence of Snell's Law and the extremely low speed, electromagnetic waves always enter good conductors in a direction that is within a