Editing Electric current (section)

==Conduction mechanisms in various media==
{{Main|Electrical conductivity|Charge transport mechanisms}}
In metallic solids, electric charge flows by means of [[electron]]s, from lower to higher [[electrical 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 a [[vacuum]], 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 to [[proton conductor|positive charge flow]]. For example, the electric currents in [[electrolyte]]s are flows of positively and negatively charged ions. In a common lead-acid [[electrochemistry|electrochemical]] cell, electric currents are composed of positive [[hydronium]] ions flowing in one direction, and negative sulfate ions flowing in the other. Electric currents in [[electric spark|sparks]] or [[Plasma physics|plasma]] 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 a [[metal]], some of the outer electrons in each atom are not bound to the individual molecules as they are in [[molecular solid]]s, or in full bands as they are in insulating materials, but are free to move within the [[Metal#Definition|metal lattice]]. These [[conduction electron]]s serve as [[charge carrier]]s 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 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 10<sup>6</sup> metres per second.<ref>{{cite web|url=http://library.thinkquest.org/C0111709/English/DC-Circuts/mechanism.html|title=The Mechanism Of Conduction In Metals|archive-url=https://web.archive.org/web/20121025004809/http://library.thinkquest.org/C0111709/English/DC-Circuts/mechanism.html|archive-date=2012-10-25|website=Think Quest}}</ref> Given a surface through which a metal wire passes, electrons move in both directions across the surface at an equal rate. As [[George Gamow]] wrote in his [[popular 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 a [[Direct current|DC]] [[voltage source]] such as a [[battery (electricity)|battery]], 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 the [[Positive (electrical polarity)|positive]] terminal under the influence of this field. The free electrons are therefore the [[charge carrier]] in a typical solid conductor.

For a steady flow of charge through a surface, the current ''I'' (in amperes) can be calculated with the following equation:
<math display=block>I = {Q \over t} \, ,</math>
where ''Q'' is the electric charge transferred through the surface over a [[time]] ''t''.  If ''Q'' and ''t'' are measured in [[coulomb]]s 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:
<math display=block>I = \frac{\mathrm{d}Q}{\mathrm{d}t} \, .</math>

===Electrolytes===
{{Main|Conductivity (electrolytic)}}
[[File:Superionic ice conducting.svg|thumb|A [[proton conductor]] in a static [[electric field]]]]

Electric currents in [[electrolyte]]s are flows of electrically charged particles ([[ion]]s). For example, if an electric field is placed across a solution of [[sodium|Na]]<sup>+</sup> and [[chlorine|Cl]]<sup>−</sup> (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 called ''[[proton conductor]]s'' contain positive hydrogen ions ("[[proton]]s") 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.<ref>{{cite book|first=Rudolf|last=Holze|url=https://books.google.com/books?id=TbcDvDcDFB0C&pg=PA44|title=Experimental Electrochemistry: A Laboratory Textbook|page=44|publisher=John Wiley & Sons|year=2009|isbn=978-3527310982}}</ref>

===Gases and plasmas===
In air and other ordinary [[gas]]es 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 are [[dielectric]]s or [[Electrical insulation|insulators]]. However, once the applied [[electric field]] approaches the [[dielectric breakdown|breakdown]] value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and [[ionizing]], neutral gas atoms or molecules in a process called [[avalanche breakdown]]. The breakdown process forms a [[Plasma (physics)|plasma]] 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 a [[Electrostatic discharge|spark]], [[electric arc|arc]] or [[lightning]].

[[Plasma (physics)|Plasma]] is the state of matter where some of the electrons in a gas are stripped or "ionized" from their [[molecule]]s or atoms. A plasma can be formed by high [[temperature]], 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 [O<sub>2</sub> → 2O], which then recombine creating [[ozone]] [O<sub>3</sub>]).<ref>{{cite web | title = Lab Note #106 ''Environmental Impact of Arc Suppression'' | publisher = Arc Suppression Technologies | date = April 2011 | url = http://www.arcsuppressiontechnologies.com/arc-suppression-facts/lab-app-notes/ | access-date = March 15, 2012}}</ref>

===Vacuum===
Since a "[[free space|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 or [[ion]]s through either [[field electron emission]] or [[thermionic emission]]. Thermionic emission occurs when the thermal energy exceeds the metal's [[work function]], while [[field electron emission]] occurs when the electric field at the surface of the metal is high enough to cause [[quantum tunneling|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 [[electrical filament|filament]] or indirectly [[hot cathode|heated cathode]] of [[vacuum tube]]s. [[cold cathode|Cold electrodes]] can also spontaneously produce electron clouds via thermionic emission when small incandescent regions (called ''cathode spots'' or ''anode spots'') are formed. These are incandescent regions of the electrode surface that are created by a localized high current. These regions may be initiated by [[field electron emission]], but are then sustained by localized thermionic emission once a [[vacuum arc]] forms. These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field. [[Vacuum tube]]s and [[Krytron|sprytrons]] are some of the electronic switching and amplifying devices based on vacuum conductivity.

===Superconductivity===
{{Main|Superconductivity}}

Superconductivity is a phenomenon of exactly zero [[Electrical resistance and conductance|electrical resistance]] and expulsion of [[magnetic field]]s occurring in certain materials when [[cryogenics|cooled]] below a characteristic [[Critical point (thermodynamics)|critical temperature]]. It was discovered by [[Heike Kamerlingh Onnes]] on April&nbsp;8, 1911 in [[Leiden]]. Like [[ferromagnetism]] and [[atomic spectral line]]s, superconductivity is a [[quantum mechanics|quantum mechanical]] phenomenon. It is characterized by the [[Meissner effect]], the complete ejection of [[magnetic field|magnetic field lines]] from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of ''[[perfect conductor|perfect conductivity]]'' in [[classical physics]].

===Semiconductor===
{{Main|Semiconductor}}

In a [[semiconductor]] it is sometimes useful to think of the current as due to the flow of positive "[[electron hole|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 has [[electrical conductivity]] intermediate in magnitude between that of a [[electrical Conductor|conductor]] and an [[insulator (electrical)|insulator]]. This means a conductivity roughly in the range of 10<sup>−2</sup>  to 10<sup>4</sup> [[siemens (unit)|siemens]] per centimeter (S⋅cm<sup>−1</sup>).

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 discrete [[quantum state]]s 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]] 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 the ''conduction 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 the [[band gap]] between the bands. The size of this energy band gap serves as an arbitrary dividing line (roughly 4 [[electronvolt|eV]]) between semiconductors and [[Electrical insulation|insulators]].

With covalent bonds, an electron moves by hopping to a neighboring bond. The [[Pauli 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{{snd}}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 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 the [[electrical conductivity]]. However, as a semiconductor's temperature rises above [[absolute 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 as ''free electrons'', though they are often simply called ''electrons'' if that is clear in context.