Speed of electricity

The speed of electricity refers to the movement of electrons (or ions) through a conductor in the presence of potential and an electric field. The finite velocity of the electric field thus requires consideration, and may even become the dominating factor in the electrical phenomena: (a) In the conduction of very high frequency currents, of hundred thousands of cycles. (b) In the action, propagation and dissipation of high frequency disturbances in electric circuits. (c) In flattening steep wave fronts and rounding the wave shape of complex waves and sudden impulses. (d) In circuits having no return circuit or no well-defined return circuit, such as the lightning stroke, the discharge path and ground circuit of the lightning arrester, the wireless antenna, etc. (e) Where the electric field at considerable distance from the conductor is of importance, as in radio-telegraphy and telephony.

Electromagnetic waves

The velocity and the electrical resistance outside of conduction is often assumed by the propagation speed of an electromagnetic wave.^{[citation needed]} In simplified systems, the speed of electricity is given as the electromagnetic wave which conveys information (data), 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, such that in an unshielded copper conductor range 95 to 97% that of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light.

Electric drift

The drift velocity deals with the average velocity that a particle, such as an electron, attains 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.

Potentials and electrics

Electricity's speed
Illustrating effects of the finite velocity of the electric field
The inductance of a finite length of an infinitely long conductor without return conductor, self-inductance as well as mutual inductance. The inductance of a finite length of an infinitely long conductor with return conductor, self-inductance, as well as mutual inductance. The capacity of a finite section of an infinitely long conductor. The mutual inductance between two finite conductors at considerable distance from each other. The capacity of a sphere in space.

Investigation of electric circuits

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. 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.

Defined return circuit

For an infinite distance of the return conductor or a conductor without return conductor, a finite length of an infinitely long conductor without return conductor would have an infinite inductance and inversely, zero capacity. The magnetic field is assumed as instantaneous, that is, the velocity of propagation of the magnetic field is neglected.

Considering, however, the finite velocity of the magnetic field, the magnetic field at a distance from the conductor and at a time which corresponds to the current in the conductor at the time, the time required for the electric field to travel the distance; or, the magnetic field at distance and time corresponds to the current in the conductor at the time.

See also

• Speed of Light
• Drift velocity
• Velocity of propagation
• Telegrapher's equations
• Reflections of signals on conducting lines

Further reading

• Alfvén, H. (1950). Cosmical electrodynamics. Oxford: Clarendon Press
• Alfvén, H. (1981). Cosmic plasma. Taylor & Francis US.
• General Electric review, Volume 15 By General Electric. "Velocity of Propagation of Electric Field", Charles Proteus Steinmetz.
• Fleming, J. A. (1911). Propagation of electric currents in telephone & telegraph conductors. New York: Van Nostrand

External links

• Propagation Times
• Speed of Electricity