Speed of Electricity - Electromagnetic Waves

Electromagnetic Waves

The speed at which energy or signals travel down a cable is actually the speed of the electromagnetic wave, not the movement of electrons. Electromagnetic wave propagation is fast 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 is affected by insulation, so that in an unshielded copper conductor ranges 95 to 97% that of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light.

In the theoretical investigation of electric circuits, the velocity of propagation of the electric field through space is usually not considered; the electric field is assumed, as a precondition, to be present throughout space. That is, the electromagnetic component of the field is considered to be in phase with the current, and the electrostatic component is considered to be in phase with the voltage. In reality, however, the electric field starts at the conductor, and propagates through space at the velocity of light (which depends on the material it is traveling through). At any point in space, the electric field corresponds not to the condition of the electric energy flow at that moment, but to that of the flow at a moment earlier. The latency is determined by the time required for the field to propagate from the conductor to the point under consideration. In other words, the greater the distance from the conductor, the more the electric field lags.

Since the velocity of propagation is very high — about 300,000 kilometers per second — the wave of an alternating or oscillating current, even of high frequency, is of considerable length. At 60 cycles per second, the wavelength is 5000 kilometers, and even at a hundred thousand Hertz, the wavelength is 3 kilometers. That is very great compared to the distance to which electric fields usually extend.

The important part of the electric field of a conductor extends to the return conductor, which usually is only a few feet distant. At greater distance, the aggregate field can be approximated by the differential field between conductor and return conductor, which tend to cancel. Hence, the intensity of the electric field is usually inappreciable at a distance which is still small compared to the wavelength. Within the range in which an appreciable field exists, this field is practically in phase with the flow of energy in the conductor. That is, the velocity of propagation has no appreciable effect unless the return conductor is very far distant, or entirely absent, or the frequency is so high that the distance to the return conductor is an appreciable portion of the wavelength.