electric current


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Related to electric current: potential difference, resistance, voltage

electric current:

see electricityelectricity,
class of phenomena arising from the existence of charge. The basic unit of charge is that on the proton or electron—the proton's charge is designated as positive while the electron's is negative.
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.

current, electric,

net movement or flow of electric charge from one point to another or across some boundary. See alternating currentalternating current,
abbr. AC, a flow of electric charge that undergoes periodic reverses in direction. In North America ordinary household current alternates at a frequency of 60 times per second. See electricity; generator.
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; direct currentdirect current,
abbr. DC, a movement of electric charge across an arbitrarily defined surface in one direction only. See electricity; generator.
..... Click the link for more information.
; electricityelectricity,
class of phenomena arising from the existence of charge. The basic unit of charge is that on the proton or electron—the proton's charge is designated as positive while the electron's is negative.
..... Click the link for more information.
.

Electric current

The net transfer of electric charge per unit time. It is usually measured in amperes. The passage of electric current involves a transfer of energy. Except in the case of superconductivity, a current always heats the medium through which it passes.

On the other hand, a stream of electrons or ions in a vacuum, which also may be regarded as an electric current, produces no local heating. Measurable currents range in magnitude from the nearly instantaneous 105 or so amperes in lightning strokes to values of the order of 10-16 ampere, which occur in research applications.

All matter may be classified as conducting, semiconducting, or insulating, depending upon the ease with which electric current is transmitted through it. Most metals, electrolytic solutions, and highly ionized gases are conductors. Transition elements, such as silicon and germanium, are semiconductors, while most other substances are insulators. See Conduction (electricity), Displacement current, Electric insulator, Semiconductor, Superconductivity

Electric current

The amount of electricity flowing in a wire or other conductor measured in amperes. Alternating current reverses its direction alternating at 60 cycles per second, abbreviated AC. Direct current flows in one direction only through a circuit from a power source, such as a battery.

Electric Current

 

an ordered (directed) motion of electrically charged particles or charged macroscopic bodies. The direction of current is assumed to be the same as the direction of motion of positively charged particles. If a current is generated by the motion of negatively charged particles, such as electrons, the direction of current is assumed to be opposite to that of the motion of particles.

A distinction is made between conduction current and convection current. Conduction current is associated with the motion of charged particles with respect to some medium (as within macroscopic bodies). Convection current is the motion of charged macroscopic bodies as a whole (such as the motion of charged rain drops). (SeeCONVECTION CURRENT.)

The existence of electric currents in conductors can be deduced from the effects produced by the current, for example, from the heating of conductors, changes in the chemical composition of the conductors, and the generation of a magnetic field. The magnetic effects of current manifest themselves in all types of conductors, without exception; in superconductors heat is not liberated, while the chemical effects of current are observed primarily in electrolytes. A magnetic field is generated not only by conduction currents or convection currents but also by variable electric fields arising in dielectrics or in a vacuum. J. C. Maxwell designated as displacement current (seeDISPLACEMENT CURRENT) a quantity that is proportional to the rate of change of an electric field with time. Displacement current is included in Maxwell’s equations on an equal footing with the current resulting from the motion of charges. Therefore, the total electric current, equal to the sum of the conduction current and the displacement current, can be defined as the quantity that determines the strength of a magnetic field.

Quantitatively, electric current is characterized by a scalar quantity—the current strength I—and by a vector quantity—the current density j. If the current density in the cross section of the conductor is uniformly distributed, then the current strength

I = jS = q0nv̄S

where q0 is the particle’s charge, n is the concentration of particles (the number of particles per unit volume), is the average velocity of the directed motion of the particles, and S is the transverse cross section of the conductor.

A necessary prerequisite for the appearance and existence of electric currents is the presence of free charged particles (that is, of positively or negatively charged particles that are not bound together in a unified electrically neutral system) and of a force that initiates and supports their ordered motion. Generally, the force causing such motion is a force supplied by the electric field within the conductor. The magnitude of this field is determined by the voltage at the ends of the conductor. If the voltage does not change with time, the current established in the conductor will be a direct current, and if the voltage does change with time, an alternating current will result. The most important characteristic of a conductor is the dependence of current strength on voltage (the voltampere characteristic). This characteristic is simplest for metallic conductors and electrolytes, in which the current strength is directly proportional to the voltage (Ohm’s law).

Materials are classified as conductors, dielectrics, or semiconductors according to their capability to conduct an electric current. Conductors contain a great many free charged particles, while dielectrics contain few. Therefore, the current strength in dielectrics is very low, even at high voltages, and consequently dielectrics are good insulators. Semiconductors represent an intermediate group, ranking between conductors and dielectrics.

In metals, the free charged particles (the current carriers) are conduction electrons, the concentration of which is practically independent of temperature and constitutes 1022–1023 cm –3. An aggregation of these electrons can be considered to be an “electron gas.” Electron gas in metals is in a state of degeneracy; that is, it clearly exhibits its quantum properties. The quantum theory of metals explains the dependence of the electric resistance of metals on temperature (linear increase with increasing temperature) and the direct proportionality between current strength and voltage.

Electric current in electrolytes is caused by the directed motion of positive and negative ions. Ions are formed in electrolytes as a result of electrolytic dissociation. With increasing temperature, the number of molecules of the dissolved substance, which disintegrate into ions, increases, and consequently the resistance of the electrolyte decreases. As the current passes through the electrolyte, ions approach the electrodes and become neutralized. The mass of the substance precipitated on the electrodes is defined by Faraday’s laws of electrolysis.

Gases consisting of neutral molecules are dielectrics. Electric current is conducted only by ionized gases, called plasma. In a plasma, the role of current carriers is played both by negative ions (as in electrolytes) and by free electrons (as in metals). Ions and free electrons are formed in gases as a result of strong heating or of external effects, such as ultra-violet radiation, X rays, and collisions of fast electrons with neutral atoms or molecules.

Electric current in electron-vacuum devices, such as vacuum tubes and cathode-ray tubes, is produced by electron fluxes emitted by a heated electrode—the cathode. Electrons are accelerated by an electric field and arrive at another electrode—the anode.

Current carriers in semiconductors consist of electrons and holes.

REFERENCES

Tamm I. E. Osnovy teorii elektrichestva, 9th ed. Moscow, 1976. Chapters 3 and 6.
Kalashnikov S. G. Elektrichestvo, 4th ed. Moscow, 1977. (Obshchii kurs fiziki.) Chapters 6,14–16, and 18.

G. IA. MIAKISHEV

electric current

[i¦lek·trik ′kə·rənt]
(electricity)
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