Electric current 1

Electric current

A simple electric circuit, where current is represented bythe letter i. The relationship between the voltage (V),

resistance (R), and current (I) is V=IR; this is known asOhm's Law.

Electric current is a flow of electric charge through aconductive medium.[1]

In electric circuits this charge is often carried by movingelectrons in a wire. It can also be carried by ions in anelectrolyte, or by both ions and electrons such as in a plasma.[2]

The SI unit for measuring the rate of flow of electric charge isthe ampere, which is charge flowing through some surface atthe rate of one coulomb per second. Electric current ismeasured using an ammeter.[1]

Symbol

The conventional symbol for current is , which originatesfrom the French phrase intensité de courant, or in Englishcurrent intensity.[3][4] This phrase is frequently used when discussing the value of an electric current, especially inolder texts; modern practice often shortens this to simply current but current intensity is still used in many recenttextbooks. The symbol was used by André-Marie Ampère, after whom the unit of electric current is named, informulating the eponymous Ampère's force law which he discovered in 1820.[5] The notation travelled from Franceto Britain, where it became standard, although at least one journal did not change from using to until 1896.[6]

Conduction mechanisms in various mediaIn metallic solids, electric charge flows by means of electrons, from lower to higher electrical potential. In othermedia, any stream of charged objects (ions, for example) may constitute an electric current. To provide a definitionof current that is independent of the type of charge carriers flowing, conventional current is defined to flow in thesame direction as positive charges. So in metals where the charge carriers (electrons) are negative, conventionalcurrent flows in the opposite direction as the electrons. In conductors where the charge carriers are positive,conventional current flows in the same direction as the charge carriers.In a vacuum, a beam of ions or electrons may be formed. In other conductive materials, the electric current is due tothe flow of both positively and negatively charged particles at the same time. In still others, the current is entirelydue to positive charge flow. For example, the electric currents in electrolytes are flows of positively and negativelycharged ions. In a common lead-acid electrochemical cell, electric currents are composed of positive hydrogen ions(protons) flowing in one direction, and negative sulfate ions flowing in the other. Electric currents in sparks orplasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, the electriccurrent is entirely composed of flowing ions.

MetalsA solid conductive metal contains mobile, or free electrons, originating in the conduction electrons. These electrons are bound to the metal lattice but no longer to an individual atom. Metals are particularly conductive because there are a large number of these free electrons, typically one per atom in the lattice. Even with no external electric field applied, these electrons move about randomly due to thermal energy but, on average, there is zero net current within the metal. At room temperature, the average speed of these random motions is 106 metres per second.[7] Given a surface through which a metal wire passes, electrons move in both directions across the surface at an equal rate. As George Gamow put in his science-popularizing book, One, Two, Three...Infinity (1947), "The metallic substances

Electric current 2

differ from all other materials by the fact that the outer shells of their atoms are bound rather loosely, and often letone of their electrons go free. Thus the interior of a metal is filled up with a large number of unattached electronsthat travel aimlessly around like a crowd of displaced persons. When a metal wire is subjected to electric forceapplied on its opposite ends, these free electrons rush in the direction of the force, thus forming what we call anelectric current."When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source placesan electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced todrift toward the positive terminal under the influence of this field. The free electrons are therefore the charge carrierin a typical solid conductor.For a steady flow of charge through a surface, the current I (in amperes) can be calculated with the followingequation:

where Q is the electric charge transferred through the surface over a time t. If Q and t are measured in coulombs andseconds respectively, I is in amperes.More generally, electric current can be represented as the rate at which charge flows through a given surface as:

ElectrolytesElectric currents in electrolytes are flows of electrically charged particles (ions). For example, if an electric field isplaced across a solution of Na+ and Cl− (and conditions are right) the sodium ions move towards the negativeelectrode (cathode), while the chloride ions move towards the positive electrode (anode). Reactions take place atboth electrode surfaces, absorbing each ion.Water-ice and certain solid electrolytes called proton conductors contain positive hydrogen ions or "protons" whichare mobile. In these materials, electric currents are composed of moving protons, as opposed to the moving electronsfound in metals.In certain electrolyte mixtures, brightly-coloured ions are the moving electric charges. The slow progress of thecolour makes the current visible.[8]

Gases and plasmasIn air and other ordinary gases below the breakdown field, the dominant source of electrical conduction is viarelatively few mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electricalconductivity is low, gases are dielectrics or insulators. However, once the applied electric field approaches thebreakdown value, free electrons become sufficiently accelerated by the electric field to create additional freeelectrons by colliding, and ionizing, neutral gas atoms or molecules in a process called avalanche breakdown. Thebreakdown process forms a plasma that contains enough mobile electrons and positive ions to make it an electricalconductor. In the process, it forms a light emitting conductive path, such as a spark, arc or lightning.Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their molecules oratoms. A plasma can be formed by high temperature, or by application of a high electric or alternating magnetic fieldas noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electricfield than the heavier positive ions, and hence carry the bulk of the current. The free ions recombine to create newchemical compounds (for example, breaking atmospheric oxygen into single oxygen [O2 → 2O], which thenrecombine creating ozone [O3]).[9]

Electric current 3

VacuumSince a "perfect vacuum" contains no charged particles, it normally behaves as a perfect insulator. However, metalelectrode surfaces can cause a region of the vacuum to become conductive by injecting free electrons or ions througheither field electron emission or thermionic emission. Thermionic emission occurs when the thermal energy exceedsthe metal's work function, while field electron emission occurs when the electric field at the surface of the metal ishigh enough to cause tunneling, which results in the ejection of free electrons from the metal into the vacuum.Externally heated electrodes are often used to generate an electron cloud as in the filament or indirectly heatedcathode of vacuum tubes. Cold electrodes can also spontaneously produce electron clouds via thermionic emissionwhen small incandescent regions (called cathode spots or anode spots) are formed. These are incandescent regionsof the electrode surface that are created by a localized high current flow. These regions may be initiated by fieldelectron emission, but are then sustained by localized thermionic emission once a vacuum arc forms. These smallelectron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electricalfield. Vacuum tubes and sprytrons are some of the electronic switching and amplifying devices based on vacuumconductivity.

SuperconductivitySuperconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurringin certain materials when cooled below a characteristic critical temperature. It was discovered by Heike KamerlinghOnnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantummechanical phenomenon. It is characterized by the Meissner effect, the complete ejection of magnetic field linesfrom the interior of the superconductor as it transitions into the superconducting state. The occurrence of theMeissner effect indicates that superconductivity cannot be understood simply as the idealization of perfectconductivity in classical physics.

SemiconductorIn a semiconductor it is sometimes useful to think of the current as due to the flow of positive "holes" (the mobilepositive charge carriers that are places where the semiconductor crystal is missing a valence electron). This is thecase in a p-type semiconductor. A semiconductor has electrical conductivity intermediate in magnitude between thatof a conductor and an insulator. This means a conductivity roughly in the range of 10-2 to 104 siemens per centimeter(S⋅cm-1).In the classic crystalline semiconductors, electrons can have energies only within certain bands (i.e. ranges of levelsof energy). Energetically, these bands are located between the energy of the ground state, the state in which electronsare tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energyrequired for an electron to escape entirely from the material. The energy bands each correspond to a large number ofdiscrete quantum states of the electrons, and most of the states with low energy (closer to the nucleus) are occupied,up to a particular band called the valence band. Semiconductors and insulators are distinguished from metals becausethe valence band in any given metal is nearly filled with electrons under usual operating conditions, while very few(semiconductor) or virtually none (insulator) of them are available in the conduction band, the band immediatelyabove the valence band.The ease with which electrons in the semiconductor can be excited from the valence band to the conduction banddepends on the band gap between the bands. The size of this energy bandgap serves as an arbitrary dividing line(roughly 4 eV) between semiconductors and insulators.With covalent bonds, an electron moves by hopping to a neighboring bond. The Pauli exclusion principle requires the electron to be lifted into the higher anti-bonding state of that bond. For delocalized states, for example in one dimension – that is in a nanowire, for every energy there is a state with electrons flowing in one direction and another state with the electrons flowing in the other. For a net current to flow, more states for one direction than for

Electric current 4

the other direction must be occupied. For this to occur, energy is required, as in the semiconductor the next higherstates lie above the band gap. Often this is stated as: full bands do not contribute to the electrical conductivity.However, as the temperature of a semiconductor rises above absolute zero, there is more energy in thesemiconductor to spend on lattice vibration and on exciting electrons into the conduction band. The current-carryingelectrons in the conduction band are known as "free electrons", although they are often simply called "electrons" ifcontext allows this usage to be clear.

Current density and Ohm's lawCurrent density is a measure of the density of an electric current. It is defined as a vector whose magnitude is theelectric current per cross-sectional area. In SI units, the current density is measured in amperes per square metre.

where I is current in the conductor, J is the current density, and A is the cross-sectional area. The dot product of thetwo vector quantities (A and J) is a scalar that represents the electric current.Current density (current per unit area) J in a material is proportional to the conductivity σ and electric field in themedium:

Instead of conductivity, a reciprocal quantity called resistivity ρ, can be used:

Conduction in semiconductor devices may occur by a combination of drift and diffusion, which is proportional todiffusion constant and charge density . The current density is then:

with being the elementary charge and the electron density. The carriers move in the direction of decreasingconcentration, so for electrons a positive current results for a positive density gradient. If the carriers are holes,replace electron density by the negative of the hole density .In linear anisotropic materials, σ, ρ and D are tensors.In linear materials such as metals, and under low frequencies, the current density across the conductor surface isuniform. In such conditions, Ohm's law states that the current is directly proportional to the potential differencebetween two ends (across) of that metal (ideal) resistor (or other ohmic device):

where is the current, measured in amperes; is the potential difference, measured in volts; and is theresistance, measured in ohms. For alternating currents, especially at higher frequencies, skin effect causes the currentto spread unevenly across the conductor cross-section, with higher density near the surface, thus increasing theapparent resistance.

Electric current 5

Drift speedThe mobile charged particles within a conductor move constantly in random directions, like the particles of a gas. Inorder for there to be a net flow of charge, the particles must also move together with an average drift rate. Electronsare the charge carriers in metals and they follow an erratic path, bouncing from atom to atom, but generally driftingin the opposite direction of the electric field. The speed at which they drift can be calculated from the equation:

whereis the electric currentis number of charged particles per unit volume (or charge carrier density)is the cross-sectional area of the conductor

is the drift velocity, and

is the charge on each particle.Typically, electric charges in solids flow slowly. For example, in a copper wire of cross-section 0.5 mm2, carrying acurrent of 5 A, the drift velocity of the electrons is on the order of a millimetre per second. To take a differentexample, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines at about a tenth ofthe speed of light.Any accelerating electric charge, and therefore any changing electric current, gives rise to an electromagnetic wavethat propagates at very high speed outside the surface of the conductor. This speed is usually a significant fraction ofthe speed of light, as can be deduced from Maxwell's Equations, and is therefore many times faster than the driftvelocity of the electrons. For example, in AC power lines, the waves of electromagnetic energy propagate throughthe space between the wires, moving from a source to a distant load, even though the electrons in the wires onlymove back and forth over a tiny distance.The ratio of the speed of the electromagnetic wave to the speed of light in free space is called the velocity factor, anddepends on the electromagnetic properties of the conductor and the insulating materials surrounding it, and on theirshape and size.The magnitudes (but, not the natures) of these three velocities can be illustrated by an analogy with the three similarvelocities associated with gases.•• The low drift velocity of charge carriers is analogous to air motion; in other words, winds.•• The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas (these waves move

through the medium much faster than any individual particles do)• The random motion of charges is analogous to heat – the thermal velocity of randomly vibrating gas particles.This analogy is extremely simplistic and incomplete: The rapid propagation of a sound wave does not impart anychange in the air molecules' drift velocity, whereas EM waves do carry the energy to propagate the actual current at arate which is much, much higher than the electrons' drift velocity. To illustrate the difference: The sound and thechange in the air's drift velocity (the force of the wind gust) cross distance at rates equaling the speeds of sound andof mechanical transmission of force (not higher than rate of drift velocity); while a change in an EM field and thechange in current (electrons' drift velocity) both propagate across distance at rates much higher than the actual driftvelocity. You can hear wind much earlier than the force of the gust reaches you, but you do not observe a change inan EM field earlier than you can observe the change of current.

Electric current 6

Electromagnetism

According to Ampère's law, an electric currentproduces a magnetic field.

Electric current produces a magnetic field. The magnetic field can bevisualized as a pattern of circular field lines surrounding the wire.

Electric current can be directly measured with a galvanometer, but thismethod involves breaking the electrical circuit, which is sometimesinconvenient. Current can also be measured without breaking the circuitby detecting the magnetic field associated with the current. Devicesused for this include Hall effect sensors, current clamps, currenttransformers, and Rogowski coils.

The theory of Special Relativity allows one to transform the magneticfield into a static electric field for an observer moving at the same speedas the charge in the diagram. The amount of current is particular to areference frame.

Conventions

The electrons, the charge carriers in an electrical circuit, flow inthe opposite direction of the conventional electric current.

The symbol for a battery ina circuit diagram.

A flow of positive charges gives the same electric current,and has the same effect in a circuit, as an equal flow ofnegative charges in the opposite direction. Since current canbe the flow of either positive or negative charges, or both, aconvention for the direction of current which is independentof the type of charge carriers is needed. The direction ofconventional current is defined arbitrarily to be thedirection of the flow of positive charges.

In metals, which make up the wires and other conductors inmost electrical circuits, the positive charges are immobile,and the charge carriers are electrons. Because the electroncarries negative charge, the electron motion in a metalconductor is in the direction opposite to that of conventional(or electric) current.

Reference direction

When analyzing electrical circuits, the actual direction ofcurrent through a specific circuit element is usuallyunknown. Consequently, each circuit element is assigned acurrent variable with an arbitrarily chosen referencedirection. When the circuit is solved, the circuit elementcurrents may have positive or negative values. A negativevalue means that the actual direction of current through thatcircuit element is opposite that of the chosen referencedirection. In electronic circuits the reference current

Electric current 7

directions are usually chosen so that all currents flow toward ground. This often matches conventional currentdirection, because in many circuits the power supply voltage is positive with respect to ground.

AC and DCThe abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current orvoltage.[10] [11]

Direct currentDirect current (DC) is the unidirectional flow of electric charge. Direct current is produced by sources such asbatteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current mayflow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuumas in electron or ion beams. The electric charge flows in a constant direction, distinguishing it from alternatingcurrent (AC). A term formerly used for direct current was galvanic current.[12]

Alternating currentIn alternating current (AC, also ac), the movement of electric charge periodically reverses direction. In direct current(DC, also dc), the flow of electric charge is only in one direction.AC is the form in which electric power is delivered to businesses and residences. The usual waveform of an ACpower circuit is a sine wave. In certain applications, different waveforms are used, such as triangular or squarewaves. Audio and radio signals carried on electrical wires are also examples of alternating current. In theseapplications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.

OccurrencesNatural examples include lightning and the solar wind, the source of the polar auroras (the aurora borealis and auroraaustralis).The human caused occurrences of electric current includes the flow of conduction electrons in metal wires, such asthe overhead power lines that deliver electrical energy across long distances and the smaller wires within electricaland electronic equipment. Eddy currents are electric currents that occur in conductors exposed to changing magneticfields. Similarly, electric currents occur, particularly in the surface, of conductors exposed to electromagnetic waves.When oscillating electric currents flow at the correct voltages within radio antennas, radio waves are generated.In electronics, other forms of electric current include the flow of electrons through resistors or through the vacuum ina vacuum tube, the flow of ions inside a battery or a neuron, and the flow of holes within a semiconductor.

Current measurementCurrent can be measured using an ammeter.At the circuit level, there are various techniques that can be used to measure current:• Shunt resistors[13]

• Hall effect current sensor transducers•• Transformers (however DC cannot be measured)• Magnetoresistive field sensors[14]

Electric current 8

References[1] Lakatos, John; Oenoki, Keiji; Judez, Hector; Oenoki, Kazushi; Hyun Kyu Cho (March 1998). "Learn Physics Today!" (http:/ / library.

thinkquest. org/ 10796/ ch13/ ch13. htm). Lima, Peru: Colegio Dr. Franklin D. Roosevelt. . Retrieved 2009-03-10.[2] Anthony C. Fischer-Cripps (2004). The electronics companion (http:/ / books. google. com/ ?id=3SsYctmvZkoC& pg=PA13). CRC Press.

p. 13. ISBN 978-0-7503-1012-3. .[3] T. L. Lowe, John Rounce, Calculations for A-level Physics, p. 2, Nelson Thornes, 2002 ISBN 0-7487-6748-7.[4] Howard M. Berlin, Frank C. Getz, Principles of Electronic Instrumentation and Measurement, p. 37, Merrill Pub. Co., 1988 ISBN

0-675-20449-6.[5] A-M Ampère, Recuil d'Observations Électro-dynamiques (http:/ / www. ampere. cnrs. fr/ textes/ recueil/ pdf/ recueilobservationsd. pdf), p.

56, Paris: Chez Crochard Libraire 1822 (in French).[6] Electric Power (http:/ / books. google. com/ books?id=BCZLAAAAYAAJ), vol. 6, p. 411, 1894.[7] "The Mechanism Of Conduction In Metals" (http:/ / library. thinkquest. org/ C0111709/ English/ DC-Circuts/ mechanism. html), Think

Quest.[8] Rudolf Holze, Experimental Electrochemistry: A Laboratory Textbook (http:/ / books. google. co. uk/ books?id=TbcDvDcDFB0C&

pg=PA44#v=onepage& q& f=true), page 44, John Wiley & Sons, 2009 ISBN 3527310983.[9] "Lab Note #106 Environmental Impact of Arc Suppression" (http:/ / www. arcsuppressiontechnologies. com/ Documents/

Office365PDFViewer. aspx?file=http:/ / www. arcsuppressiontechnologies. com/ Documents/ Lab Note 106 - APR2011 - EnvironmentalImpact. pdf). Arc Suppression Technologies. April 2011. . Retrieved March 15, 2012.

[10] N. N. Bhargava and D. C. Kulshreshtha (1983). Basic Electronics & Linear Circuits (http:/ / books. google. com/books?id=C5bt-oRuUzwC& pg=PA90). Tata McGraw-Hill Education. p. 90. ISBN 978-0-07-451965-3. .

[11] National Electric Light Association (1915). Electrical meterman's handbook (http:/ / books. google. com/ books?id=ZEpWAAAAMAAJ&pg=PA81). Trow Press. p. 81. .

[12] Andrew J. Robinson, Lynn Snyder-Mackler (2007). Clinical Electrophysiology: Electrotherapy and Electrophysiologic Testing (http:/ /books. google. com/ books?id=C2-9bcIjPBsC& pg=PA10& dq="galvanic+ current"+ "direct+ current"#v=onepage& q="galvanic current""direct current"& f=false) (3rd ed.). Lippincott Williams & Wilkins. p. 10. ISBN 978-0-7817-4484-3. .

[13] What is a Current Sensor and How is it Used? (http:/ / www. ti. com/ analog/ docs/ microsite. tsp?sectionId=560& tabId=2180&micrositeId=7). Focus.ti.com. Retrieved on 2011-12-22.

[14] Andreas P. Friedrich, Helmuth Lemme The Universal Current Sensor (http:/ / www. sensorsmag. com/ sensors/ electric-magnetic/the-universal-current-sensor-1029). Sensorsmag.com (2000-05-01). Retrieved on 2011-12-22.

External links• Allaboutcircuits.com (http:/ / www. allaboutcircuits. com), a useful site introducing electricity and electronics

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Article Sources and ContributorsElectric current  Source: http://en.wikipedia.org/w/index.php?oldid=513715285  Contributors: 123Mike456Winston789, 165.123.179.xxx, 28421u2232nfenfcenc, 2over0, 5 albert square, ABF,Abhirocks94, Acather96, Ace of Spades, Adambro, AdjustShift, Ahirwav, Ahoerstemeier, Alansohn, Alexius08, AlexiusHoratius, Alfred Centauri, Amitch, Andonic, Andres, Andy Dingley,AnkhMorpork, ArnoldReinhold, Art Carlson, Arthurmyles, Ash, Athabaska-Clearwater, AxelBoldt, Az1568, BD2412, Bart133, Bekus, Benbest, Bender235, BentzyCo, Benydogc13, Bernmaster,Bert Hickman, Big G Ursa, Bird of paradox, Bjpremore, Bmendonc, Bobo192, Bongwarrior, Brockert, Bsadowski1, Bucephalus, Bucketsofg, Burnedthru, Bvcrist, Caesura, Calvin 1998,CanisRufus, Capecodeph, Capricorn42, Carey Evans, Carlsotr, CaseInPoint, CatastrophicToad, CharlesC, Chendy, Chetvorno, Chewbacca01, ChrisGualtieri, Christian75, CinchBug, Cj3975,Clarenceos, Clementi, Cnorrisindustry, Coffee, Cometstyles, CommonsDelinker, Conversion script, Cooltoad4102, Crocodealer, Cxz111, CyclePat, Cyp, CyrilB, D, DJIndica, DMacks, DRTllbrg,DV8 2XL, Dacium, Dan Granahan, Danh, Darth Panda, Dcook32p, DeadEyeArrow, Defender of torch, Delirium, Delta1989, Deor, DerHexer, Dgw, Dicklyon, DigitalCatalyst, Discospinster,Dlohcierekim, Dmitry sychov, Dogears, Donarreiskoffer, Dr. Pathos, Dreadstar, Dspradau, ERcheck, Eaglizard, Ed g2s, EdJohnston, EdgeOfEpsilon, Edison, Editor at Large, Ehrenkater,Ekwonderland, Elkman, Enormousdude, Enviroboy, Epbr123, Epl18, Erik9, FRS, FSHero, Faradayplank, Femto, Fingers-of-Pyrex, Flekstro, FrankCostanza, Freestyle, Fresheneesz, Fribbler,FrozenMan, Fæ, GRAHAMUK, Gallagher69, Gciriani, Gdaman5, Gene Nygaard, Geomanjo, Giftlite, Gilderien, Gilliam, Gizza gander, Glane23, Glen Chagrin, Glenn, GliderMaven, GoShow,Gogo Dodo, GoingBatty, GraemeL, Graham87, Grunt, Gtstricky, H Padleckas, Haham hanuka, Hda3ku, Headbomb, HenryLi, Heron, Heyyyyah, Hillock65, Hooperbloob, Husond, Hut 6.5,Hydrargyrum, I am from south wales, IVAN3MAN, Icairns, Icep, Imnotminkus, Imperial Monarch, Indefatigable, Indon, Inter16, InvertRect, Iridescent, J.delanoy, JD554, JFreeman, JSquish,James086, Jaw959, Jeffrey Mall, Jfdwolff, Jim1138, JinJian, Jo big daddy, John, John of Reading, John254, Johnflux, Jojit fb, Jolomo, Jonkerz, JulianDelphiki, Jusdafax, Jusses2, JustinForce,Jwhamilton, Jwissick, K. 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