Electrical conduction: Difference between revisions
Content deleted Content added
Embrittled (talk | contribs) move material to electrical conductivity |
m Reverted edits by Embrittled (talk) to last revision by Wtshymanski (HG) |
||
| (2 intermediate revisions by one other user not shown) | |||
| Line 1: | Line 1: | ||
#REDIRECT [[Electrical resistivity and conductivity]] |
|||
{{Unreferenced|date=October 2007}} |
|||
{{electromagnetism|cTopic=[[Electrical network|Electrical Network]]}} |
|||
'''Electrical conduction''' is the movement of [[electric charge|electrically charged]] particles through an [[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. |
|||
==Solids (including insulating solids)== |
|||
In crystalline solids, '''atoms''' interact with their neighbors, and the energy levels of the electrons in isolated atoms turn into '''bands'''. Whether a material conducts or not is determined by its [[band structure]] and the occupancy of these bands as determined by the [[Fermi energy#Fermi level|Fermi level]]. Electrons, being [[fermion]]s, follow the [[Pauli exclusion principle]], meaning that two electrons in the same interacting system cannot occupy the same state, which further means that at least one of their four quantum numbers have to be different. Thus electrons in a solid fill up the energy bands up to a certain level, called the [[Fermi energy]]. Bands which are completely full of electrons cannot conduct electricity, because there is no state of nearby energy to which the electrons can jump. Materials in which all bands are full (i.e. the Fermi energy is between two bands) are [[Electrical insulation|insulator]]s. In some cases, however, the band theory breaks down and materials that are predicted to be conductors by band theory turn out to be insulators. [[Mott insulators]] and [[charge transfer insulators]] are two such classes of insulators. |
|||
===Metals=== |
|||
[[Metals]] are good conductors of electricity and heat because they have unfilled space in the valence energy band. (The Fermi level dictates only partial occupancy of the band.) In the absence of an electric field, ''conduction electrons'' travel in all directions at very high velocities. Even at the coldest possible temperature — [[absolute zero]] — conduction electrons can still travel at the ''Fermi velocity'' (the velocity of electrons at the [[Fermi energy]]). When an electric field is applied, a slight imbalance develops and mobile electrons flow. Electrons in this band can be accelerated by the ''field'' because there are plenty of nearby unfilled states in the band. |
|||
Resistance comes about in a metal because of the [[scattering]] of electrons from defects in the lattice or by [[phonons]]. A crude classical theory of conduction in simple metals is the [[Drude model]], in which scattering is characterized by a relaxation time ''τ''. The conductivity is then given by the formula |
|||
:<math>\sigma = \frac{ne^2 \tau}{m}</math> |
|||
where ''n'' is the density of conduction electrons, ''e'' is the electron charge, and ''m'' is the electron mass. A better model is the so-called semi-classical theory, in which the effect of the periodic potential of the lattice on the electrons gives them an [[effective mass]] (ref. [[band theory]] ). |
|||
===Semiconductors=== |
|||
The Fermi level in a [[semiconductor]] is placed so all bands are either full or empty. A solid with no partially filled bands is an insulator, but at finite temperature, electrons can be thermally excited from the filled [[valence band]] to the next highest, the empty [[conduction band]]. The fraction of electrons excited in this way depends on the temperature and the [[band gap]], the energy difference between the two bands. Exciting these electrons into the conduction band leaves behind positively charged [[electron hole|hole]]s in the valence band, which also can conduct electricity. |
|||
In semiconductors, impurities greatly affect the concentration and type of charge carriers. Donor (n-type) impurities have extra valence electrons with energies very close to the conduction band which can be easily thermally excited to the conduction band. Acceptor (p-type) impurities capture electrons from the valence band, allowing the easy formation of holes. If an insulator is doped with enough impurities, a [[Mott transition]] can occur, and the insulator turns into a conductor. |
|||
===Superconductors=== |
|||
Superconductors are [[perfect conductor]]s below a certain material-specific critical temperature and external magnetic field. In metals and certain other materials, a transition to the [[superconductivity|superconducting]] state occurs at low (sub-[[cryogenic]]) temperature. By an interaction mediated by some other part of the system (in metals, [[phonons]]), the electrons pair up into '''[[Cooper pair]]s'''. The [[boson|bosonic]] Cooper pairs form a [[superfluid]] which has zero resistance. See [[BCS theory]]. |
|||
==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>–</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. |
|||
==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]] |
|||
Latest revision as of 01:08, 15 December 2012
Redirect to: