Speed of electricity
The word "electricity" refers generally to the movement of electrons (or other charge carriers) through a conductor in the presence of potential and an electric field. The "speed" of this flow has multiple meanings. In everyday electronics, the signals or energy travel quickly, as electromagnetic waves, while the electrons themselves move slowly.
Use of the word "speed", which expresses the velocity of an object, to describe the quantity of digital information transmitted per second, is not scientifically correct, but is often used colloquially. The correct term is "rate", expressed in bits or bytes per second, baud, etc. Baud is a count of how many states of the electric signal (also known as symbols) can be sent and received in one second to carry data.
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. This is a very large distance compared to those typically used in field measurement and application.
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.
The drift velocity deals with the average velocity that a particle, such as an electron, gets due to an electric field. In general, an electron will 'rattle around' in a conductor at the Fermi velocity randomly. 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 hour. AC voltages cause no net movement; the electrons oscillate back and forth in response to the alternating electric field.
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