Speed of electricity
Encyclopedia
The speed of electricity refers to the movement of electrons (or ion
s) 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.
), 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.
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 voltage
s cause no net movement; the electrons oscillate back and forth in response to the alternating electric field.
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.
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.
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s) through a conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
in the presence of potential and an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. 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. In simplified systems, the speed of electricity is given as the electromagnetic wave which conveys information (dataData
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
), 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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
and almost that fast in air. Propagation speed
Velocity of propagation
The velocity factor , also called wave propagation speed or velocity of propagation , of a transmission medium is the speed at which a wavefront passes through the medium, relative to the...
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
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
it is about 66% of the speed of light.
Electric drift
The drift velocityDrift velocity
The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. It can also be referred to as Axial Drift Velocity since particles defined are assumed to be moving along a plane. In general, an electron will 'rattle around' in a conductor at the...
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
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
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 voltage
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
s 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 |
|
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 LightSpeed of lightThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
- Drift velocityDrift velocityThe drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. It can also be referred to as Axial Drift Velocity since particles defined are assumed to be moving along a plane. In general, an electron will 'rattle around' in a conductor at the...
- Velocity of propagationVelocity of propagationThe velocity factor , also called wave propagation speed or velocity of propagation , of a transmission medium is the speed at which a wavefront passes through the medium, relative to the...
- Telegrapher's equations
- Reflections of signals on conducting linesReflections of signals on conducting linesA signal travelling along an electrical transmission line will be partly, or wholly, reflected back in the opposite direction when the travelling signal encounters a discontinuity in the transmission parameters of the line, or at the far end of the line if the line is not correctly terminated in...
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