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Electrical resistivity and conductivity

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+#REDIRECT [[Electrical resistivity and conductivity]]
-{{Unreferenced|date=October 2007}}
-{{mergeto|Electrical conductivity|date=December 2010}}
-{{electromagnetism|cTopic=[[Electrical network|Electrical Network]]}}
-'''Electrical conduction''' is the movement of [[electric charge|electrically charged]] particles through a [[transmission medium]] ([[electrical conductor]]). The movement of charge constitutes an [[Current (electricity)|electric current]]. This charge transport may reflect a potential difference due to an [[electric field]], or a concentration gradient in carrier density.   The latter reflects [[Fick's law|diffusion]] of the charge carriers. The physical parameters governing this transport  depend upon the material.
-Conduction in [[metal]]s and [[resistor]]s follows [[Ohm's Law]].  This states that the current is proportional to the applied electric field. Current density (current per unit area) ''J'' in a material is measured by the [[electrical conductivity|conductivity]] ''σ'', defined as:
-:'''J''' = ''σ'' '''[[electric field|E]]'''
-or its reciprocal [[electrical resistivity|resistivity]] ''ρ'':
-:'''J''' = '''[[electric field|E]]''' / ''ρ''
-Conduction in [[semiconductor devices]] may occur by a combination of electric field (drift) and diffusion. The current density is then:
-:'''J''' = ''σ'' '''[[electric field|E]]''' +  '''[[diffusion constant|D]] [[Gradient#Definition|∇]]'''[[charge density|qn]]'''
-with ''q'' the [[elementary charge]] and ''n'' the electron density. The carriers move in the direction of decreasing concentration, so for electrons a positive current results for a positive density gradient. If the carriers are holes, replace electron density ''n'' by the negative of the [[electron hole|hole]] density ''p''.
-In linear [[anisotropy|anisotropic]] materials, ''σ'', ''ρ'' and ''D'' are [[tensor]]s.
-==Electrolytes==
-Electric currents in [[electrolyte]]s are flows of electrically charged [[atom]]s ([[ion]]s). For example, if an electric field is placed across a solution of [[sodium|Na]]<sup>+</sup> and [[chlorine|Cl]]<sup>&ndash;</sup>, the sodium ions move towards the negative electrode (cathode), while the chloride ions move towards the positive electrode (anode). If conditions are right, reactions take place at the electrode surfaces which release electrons from the chloride and transfer electrons to the sodium.{{Dubious|date=September 2010}}
-Water-ice and certain solid electrolytes called [[proton conductor]]s contain positive hydrogen ions or "[[protons]]" which are mobile.  In these materials, electric currents are composed of moving protons, as opposed to the moving electrons found in [[metal]]s.
-In certain olyte mixtures, populations of brightly-colored ions form the moving electric charges.  The slow migration of these ions during an electric current is one example of a situation where a current is directly visible to human eyes.
-==Gases and plasmas==
-In air, and other ordinary [[gas]]es below the breakdown field, the dominant source of electrical conduction is via a relatively small number of mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electrical conductivity is extremely low, gases are [[dielectric]]s or [[Electrical insulation|insulator]]s. 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 a significant number of mobile electrons and positive ions, causing it to behave as 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.
-==Vacuum==
-Since a "[[free space|perfect vacuum]]" contains no charged particles, vacuums normally behave as perfect insulators (they would be the greatest insulators known). However, metal electrode surfaces can cause a region of the vacuum to become conductive by injecting [[free electron]]s 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 heated [[cathode]] of [[vacuum tubes]]. [[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 flow. 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|sprytron]]s are some of the electronic switching and amplifying devices based on vacuum conductivity.
-==References==
-{{DEFAULTSORT:Electrical Conduction}}
-[[Category:Electrical phenomena]]
-[[ar:توصيل كهربائي]]
-[[ca:Conducció elèctrica]]
-[[cy:Dargludiad trydan]]
-[[es:Conducción eléctrica]]
-[[eo:Konduktado (elektro)]]
-[[hi:विद्युत चालन]]
-[[nl:Elektrische geleiding]]
-[[ja:電気伝導]]
-[[pl:Przewodnictwo elektryczne]]
-[[sl:Električni prevodnik]]
-[[ta:மின்கடத்தி]]
-[[uk:Електропровідність]]
-[[ur:برقی ایصال]]
-[[vi:Dẫn điện]]
-[[zh:電傳導]]