A simple electric circuit, where current is represented by the letteri. The relationship between the voltage (V), resistance (R), and current (i orI) isV=IR; this is known asOhm's law.
The conventional symbol for current isI, which originates from the French phraseintensité du courant (current intensity).[5][6] Current intensity is often referred to simply ascurrent.[7] TheI symbol was used byAndré-Marie Ampère, after whom the unit of electric current is named, in formulatingAmpère's force law (1820).[8] The notation travelled from France to Great Britain, where it became standard, although at least one journal did not change from usingC toI until 1896.[9]
The conventional direction of current, also known asconventional current,[10][11] is arbitrarily defined as the direction in whichpositive charges flow. In aconductive material, the moving charged particles that constitute the electric current are calledcharge carriers. In metals, which make up the wires and other conductors in mostelectrical circuits, the positively chargedatomic nuclei of the atoms are held in a fixed position, and the negatively chargedelectrons are the charge carriers, free to move about in the metal. In other materials, notably thesemiconductors, the charge carriers can be positiveor negative, depending on thedopant used. Positive and negative charge carriers may even be present at the same time, as happens in anelectrolyte in anelectrochemical cell.
A flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention is needed for the direction of current that is independent of the type ofcharge carriers. Negatively charged carriers, such as the electrons (the charge carriers in metal wires and many other electronic circuit components), therefore flow in the opposite direction of conventional current flow in an electrical circuit.[10][11]
Reference direction
A current in a wire orcircuit element can flow in either of two directions. When defining avariable to represent the current, the direction representing positive current must be specified, usually by an arrow on thecircuitschematic diagram.[12][13]: 13 This is called thereference direction of the current. Whenanalyzing electrical circuits, the actual direction of current through a specific circuit element is usually unknown until the analysis is completed. Consequently, the reference directions of currents are often assigned arbitrarily. When the circuit is solved, a negative value for the current implies the actual direction of current through that circuit element is opposite that of the chosen reference direction.[a]: 29
Ohm's law states that the current through a conductor between two points is directlyproportional to thepotential difference across the two points. Introducing the constant of proportionality, theresistance,[14] one arrives at the usual mathematical equation that describes this relationship:[15]
whereI is the current through the conductor in units ofamperes,V is the potential difference measuredacross the conductor in units ofvolts, andR is theresistance of the conductor in units ofohms. More specifically, Ohm's law states that theR in this relation is constant, independent of the current.[16]
Inalternating current (AC) systems, the movement ofelectric charge periodically reverses direction. AC is the form ofelectric power most commonly delivered to businesses and residences. The usualwaveform of anAC power circuit is asine wave, though certain applications use alternative waveforms, such astriangular orsquare waves.Audio andradio signals carried on electrical wires are also examples of alternating current. An important goal in these applications is recovery of information encoded (ormodulated) onto the AC signal.
In contrast,direct current (DC) refers to a system in which electric charge moves in only one direction (sometimes called unidirectional flow). Direct current is produced by sources such asbatteries,thermocouples,solar cells, andcommutator-type electric machines of thedynamo type. Alternating current can also be converted to direct current through use of arectifier. Direct current may flow in aconductor such as a wire, but can also flow throughsemiconductors,insulators, or even through avacuum as inelectron or ion beams. Anold name for direct current wasgalvanic current.[17]
Man-made occurrences of electric current include the flow of conduction electrons in metal wires such as the overhead power lines that deliverelectrical energy across long distances and the smaller wires within electrical and electronic equipment.Eddy currents are electric currents that occur in conductors exposed to changing magnetic fields. Similarly, electric currents occur, particularly in the surface, of conductors exposed toelectromagnetic waves. When oscillating electric currents flow at the correct voltages withinradio antennas,radio waves are generated.
Electric current can be directly measured with agalvanometer, but this method involves breaking theelectrical circuit, which is sometimes inconvenient.
Current can also be measured without breaking the circuit by detecting the magnetic field associated with the current. Devices, at the circuit level, use varioustechniques to measure current:
Joule heating, also known asohmic heating andresistive heating, is the process ofpower dissipation[20]: 36 by which the passage of an electric current through aconductor increases theinternal energy of the conductor,[21]: 846 convertingthermodynamic work intoheat.[21]: 846, fn. 5 The phenomenon was first studied byJames Prescott Joule in 1841. Joule immersed a length of wire in a fixedmass ofwater and measured thetemperature rise due to a known current through the wire for a 30minute period. By varying the current and the length of the wire he deduced that the heat produced wasproportional to thesquare of the current multiplied by theelectrical resistance of the wire.
This relationship is known asJoule's Law.[20]: 36 TheSI unit ofenergy was subsequently named thejoule and given the symbolJ.[4]: 20 The commonly known SI unit of power, thewatt (symbol: W), is equivalent to one joule per second.[4]: 20
A magnetic field is produced by an electric current in asolenoid.
In an electromagnet a coil of wires behaves like amagnet when an electric current flows through it. When the current is switched off, the coil loses its magnetism immediately.Electric current produces amagnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire that persists as long as there is current.
Magnetic fields can also be used to make electric currents. When a changing magnetic field is applied to a conductor, anelectromotive force (EMF) is induced,[21]: 1004 which starts an electric current, when there is a suitable path.
Alternating electric current flows through the solenoid, producing a changing magnetic field. This field causes an electric current to flow in the wire loop byelectromagnetic induction.
In metallic solids, electric charge flows by means ofelectrons, from lower to higherelectrical potential. In other media, any stream of charged objects (ions, for example) may constitute an electric current. To provide a definition of current independent of the type of charge carriers,conventional current is defined as moving in the same direction as the positive charge flow. So, in metals where the charge carriers (electrons) are negative, conventional current is in the opposite direction to the overall electron movement. In conductors where the charge carriers are positive, conventional current is in the same direction as the charge carriers.
In avacuum, a beam of ions or electrons may be formed. In other conductive materials, the electric current is due to the flow of both positively and negatively charged particles at the same time. In still others, the current is entirely due topositive charge flow. For example, the electric currents inelectrolytes are flows of positively and negatively charged ions. In a common lead-acidelectrochemical cell, electric currents are composed of positivehydronium ions flowing in one direction, and negative sulfate ions flowing in the other. Electric currents insparks orplasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, the electric current is entirely composed of flowing ions.
Metals
In ametal, some of the outer electrons in each atom are not bound to the individual molecules as they are inmolecular solids, or in full bands as they are in insulating materials, but are free to move within themetal lattice. Theseconduction electrons serve ascharge carriers that can flow through the conductor as an electric current when an electric field is present. Metals are particularly conductive because there are many of these free electrons. With no externalelectric field applied, these electrons move about randomly due tothermal 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.[22] Given a surface through which a metal wire passes, electrons move in both directions across the surface at an equal rate. AsGeorge Gamow wrote in hispopular science book,One, Two, Three...Infinity (1947), "The metallic substances differ from all other materials by the fact that the outer shells of their atoms are bound rather loosely, and often let one of their electrons go free. Thus the interior of a metal is filled up with a large number of unattached electrons that travel aimlessly around like a crowd of displaced persons. When a metal wire is subjected to electric force applied on its opposite ends, these free electrons rush in the direction of the force, thus forming what we call an electric current."
When a metal wire is connected across the two terminals of aDCvoltage source such as abattery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward thepositive terminal under the influence of this field. The free electrons are therefore thecharge carrier in a typical solid conductor.
For a steady flow of charge through a surface, the currentI (in amperes) can be calculated with the following equation:whereQ is the electric charge transferred through the surface over atimet. IfQ andt are measured incoulombs and seconds respectively,I is in amperes.
More generally, electric current can be represented as the rate at which charge flows through a given surface as:
Electric currents inelectrolytes are flows of electrically charged particles (ions). For example, if an electric field is placed across a solution ofNa+ andCl− (and conditions are right) the sodium ions move towards the negative electrode (cathode), while the chloride ions move towards the positive electrode (anode). Reactions take place at both electrode surfaces, neutralizing each ion.
Water-ice and certain solid electrolytes calledproton conductors contain positive hydrogen ions ("protons") that are mobile. In these materials, electric currents are composed of moving protons, as opposed to the moving electrons in metals.
In certain electrolyte mixtures, brightly coloured ions are the moving electric charges. The slow progress of the colour makes the current visible.[23]
Gases and plasmas
In air and other ordinarygases below the breakdown field, the dominant source of electrical conduction is via relatively few mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electrical conductivity is low, gases aredielectrics orinsulators. However, once the appliedelectric field approaches thebreakdown value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, andionizing, neutral gas atoms or molecules in a process calledavalanche breakdown. The breakdown process forms aplasma that contains enough mobile electrons and positive ions to make it an electrical conductor. In the process, it forms a light emitting conductive path, such as aspark,arc orlightning.
Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from theirmolecules or atoms. A plasma can be formed by hightemperature, or by application of a high electric or alternating magnetic field as noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions, and hence carry the bulk of the current. The free ions recombine to create new chemical compounds (for example, breaking atmospheric oxygen into single oxygen [O2 → 2O], which then recombine creatingozone [O3]).[24]
Vacuum
Since a "perfect vacuum" contains no charged particles, it normally behaves as a perfect insulator. However, metal electrode surfaces can cause a region of the vacuum to become conductive by injecting free electrons orions through eitherfield electron emission orthermionic emission. Thermionic emission occurs when the thermal energy exceeds the metal'swork function, whilefield electron emission occurs when the electric field at the surface of the metal is high enough to causetunneling, which results in the ejection of free electrons from the metal into the vacuum. Externally heated electrodes are often used to generate anelectron cloud as in thefilament or indirectlyheated cathode ofvacuum tubes.Cold electrodes can also spontaneously produce electron clouds via thermionic emission when small incandescent regions (calledcathode spots oranode spots) are formed. These are incandescent regions of the electrode surface that are created by a localized high current. These regions may be initiated byfield electron emission, but are then sustained by localized thermionic emission once avacuum arc forms. These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field.Vacuum tubes andsprytrons are some of the electronic switching and amplifying devices based on vacuum conductivity.
In asemiconductor it is sometimes useful to think of the current as due to the flow of positive "holes" (the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron). This is the case in a p-type semiconductor. A semiconductor haselectrical conductivity intermediate in magnitude between that of aconductor and aninsulator. This means a conductivity roughly in the range of 10−2 to 104siemens per centimeter (S⋅cm−1).
In the classic crystalline semiconductors, electrons can have energies only within certain bands (i.e. ranges of levels of energy). Energetically, these bands are located between the energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material. The energy bands each correspond to many discretequantum states of the electrons, and most of the states with low energy (closer to the nucleus) are occupied, up to a particular band called thevalence band. Semiconductors and insulators are distinguished frommetals because the 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 theconduction band, the band immediately above the valence band.
The ease of exciting electrons in the semiconductor from the valence band to the conduction band depends on theband gap between the bands. The size of this energy band gap serves as an arbitrary dividing line (roughly 4eV) between semiconductors andinsulators.
With covalent bonds, an electron moves by hopping to a neighboring bond. ThePauli exclusion principle requires that the electron be lifted into the higher anti-bonding state of that bond. For delocalized states, for example in one dimension – that is in ananowire, 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 the other direction must be occupied. For this to occur, energy is required, as in the semiconductor the next higher states lie above the band gap. Often this is stated as: full bands do not contribute to theelectrical conductivity. However, as a semiconductor's temperature rises aboveabsolute zero, there is more energy in the semiconductor to spend on lattice vibration and on exciting electrons into the conduction band. The current-carrying electrons in the conduction band are known asfree electrons, though they are often simply calledelectrons if that is clear in context.
Current density is the rate at which charge passes through a chosen unit area.[25]: 31 It is defined as avector whose magnitude is the current per unit cross-sectional area.[2]: 749 As discussed inReference direction, the direction is arbitrary. Conventionally, if the moving charges are positive, then the current density has the same sign as the velocity of the charges. For negative charges, the sign of the current density is opposite to the velocity of the charges.[2]: 749 InSI units, current density (symbol: j) is expressed in the SI base units of amperes per square metre.[4]: 22
In linear materials such as metals, and under low frequencies, the current density across the conductor surface is uniform. In such conditions,Ohm's law states that the current is directly proportional to the potential difference between two ends (across) of that metal (ideal)resistor (or otherohmic device):
where is the current, measured in amperes; is thepotential difference, measured involts; and is theresistance, measured inohms. Foralternating currents, especially at higher frequencies,skin effect causes the current to spread unevenly across the conductor cross-section, with higher density near the surface, thus increasing the apparent resistance.
Drift speed
The mobile charged particles within a conductor move constantly in random directions, like the particles of agas. (More accurately, aFermi gas.) To create a net flow of charge, the particles must also move together with an average drift rate. Electrons are the charge carriers in mostmetals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the opposite direction of the electric field. The speed they drift at can be calculated from the equation:
where
is the electric current
is number of charged particles per unit volume (or charge carrier density)
Typically, electric charges in solids flow slowly. For example, in acopper wire of cross-section 0.5 mm2, carrying a current of 5 A, thedrift velocity of the electrons is on the order of a millimetre per second. To take a different example, in the near-vacuum inside acathode-ray tube, the electrons travel in near-straight lines at about a tenth of thespeed of light.
Any accelerating electric charge, and therefore any changing electric current, gives rise to anelectromagnetic wave that propagates at very high speed outside the surface of the conductor. This speed is usually a significant fraction of the speed of light, as can be deduced fromMaxwell's equations, and is therefore many times faster than the drift velocity of the electrons. For example, inAC power lines, the waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distantload, even though the electrons in the wires only move 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 thevelocity factor, and depends on the electromagnetic properties of the conductor and the insulating materials surrounding it, and on their shape and size.
The magnitudes (not the natures) of these three velocities can be illustrated by an analogy with the three similar velocities associated with gases. (See alsohydraulic analogy.)
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 (sound waves move through air much faster than large-scale motions such asconvection)
The random motion of charges is analogous to heat – the thermal velocity of randomly vibrating gas particles.
^Lowe, T. L.; Rounce, John (2002).Calculations for A-level Physics. Nelson Thornes. p. 2.ISBN0748767487.
^Berlin, Howard M.; Getz, Frank C. (1988).Principles of Electronic Instrumentation and Measurement. Merrill Pub. Co. p. 37.ISBN0675204496.
^Kumar, K. S. Suresh (2013). "Section 1.2.3".Electric Circuit Analysis. Pearson Education India.ISBN978-9332514102.'Current intensity' is usually referred to as 'current' itself.
