View source for Electrical resistivity and conductivity

{{About|electrical conductivity in general|other types of conductivity|Conductivity (disambiguation){{!}}Conductivity|specific applications in electrical elements|Electrical resistance and conductance}}
{{short description|A measure of a substance's ability to resist or conduct electric current}}
{{Use British English|date = October 2018}}

'''Electrical resistivity''' (also called '''specific electrical resistance''' or '''volume resistivity''') and its inverse, electrical conductivity, is a fundamental property of  a material that quantifies how strongly it resists or conducts [[electric current]]. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the [[Greek alphabet|Greek letter]] ''ρ'' ([[Rho (letter)|rho]]). The [[SI]] unit of electrical resistivity is the [[ohm]]-[[meter]] (Ω⋅m).<ref>{{cite book | last = Lowrie | first = William |  title = Fundamentals of Geophysics  | url = https://books.google.com/books?id=h2-NjUg4RtEC&pg=PA254  | publisher = Cambridge University Press | isbn = 978-05-2185-902-8  | pages = 254–55 | date = 2007 | access-date = March 24, 2019 }}</ref><ref>{{cite book | first = Narinder | last = Kumar | title = Comprehensive Physics for Class XII  | url = https://books.google.com/books?id=IryMtwHHngIC&pg=PA282  | date = 2003  | publisher = Laxmi Publications  | location = New Delhi | isbn = 978-81-7008-592-8 | pages = 280–84 | access-date = March 24, 2019 }}</ref><ref>{{cite book | first = Eric | last = Bogatin  | title = Signal Integrity: Simplified  | url = https://books.google.com/books?id=_IiONSphoB4C&lpg=PP1&dq=Signal%20integrity&pg=PA114#v=onepage&q=Signal%20integrity&f=false | year = 2004 | publisher = Prentice Hall Professional | isbn = 978-0-13-066946-9 | page = 114 | access-date = March 24, 2019 }}</ref>  For example, if a {{nowrap|1 m × 1 m × 1 m}} solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1&nbsp;Ω, then the resistivity of the material is 1&nbsp;Ω⋅m.

'''Electrical conductivity''' or '''specific conductance''' is the reciprocal of electrical resistivity. It represents a material's ability to conduct  electric current. It is commonly signified by the Greek letter σ ([[Sigma (letter)|sigma]]), but κ ([[kappa]]) (especially in electrical engineering) and γ ([[gamma]]) are sometimes used. The SI unit of electrical conductivity is [[Siemens (unit)|siemens]] per [[metre]] (S/m).

== Definition ==
===Ideal case===
[[Image:Resistivity geometry.png|thumb|A piece of resistive material with electrical contacts on both ends.]]
In an ideal case, cross-section and physical composition of the examined material are uniform across the sample, and the electric field and current density are both parallel and constant everywhere. Many [[resistor]]s and [[electrical conductor|conductors]] do in fact have a uniform cross section with a uniform flow of electric current, and are made of a single material, so that this is a good model. (See the adjacent diagram.) When this is the case, the electrical resistivity ''ρ'' (Greek: [[Rho (letter)|rho]]) can be calculated by:

:<math>\rho = R \frac{A}{\ell}, \,\!</math>

where

:<math>R</math> is the [[electrical resistance]] of a uniform specimen of the material
:''<math>\ell</math>'' is the [[length]] of the specimen
:<math>A</math> is the [[Cross section (geometry)|cross-sectional area]] of the specimen

Both ''resistance'' and ''resistivity'' describe how difficult it is to make electrical current flow through a material, but unlike resistance, resistivity is an ''[[intrinsic property]]''. This means that all pure copper wires (which have not been subjected to distortion of their crystalline structure etc.), irrespective of their shape and size, have the same ''resistivity'', but a long, thin copper wire has a much larger ''resistance'' than a thick, short copper wire. Every material has its own characteristic resistivity. For example, rubber has a far larger resistivity than copper.

In a [[hydraulic analogy]], passing current through a high-resistivity material is like pushing water through a pipe full of sand—while passing current through a low-resistivity material is like pushing water through an empty pipe. If the pipes are the same size and shape, the pipe full of sand has higher resistance to flow. Resistance, however, is not ''solely'' determined by the presence or absence of sand. It also depends on the length and width of the pipe: short or wide pipes have lower resistance than narrow or long pipes.

{{anchor|Pouillet's law}} <!--Pouillet's law redirects here-->
The above equation can be transposed to get '''Pouillet's law''' (named after [[Claude Pouillet]]):

:<math>R = \rho \frac{\ell}{A}. \,\!</math>

The resistance of a given material is proportional to the length, but inversely proportional to the cross-sectional area. Thus resistivity can be expressed using the [[SI]] unit "[[ohm]] [[metre]]" (Ω⋅m) — ''i.e.'' ohms divided by metres (for the length) and then multiplied by square metres (for the cross-sectional area).

For example, if ''A'' = {{val|1|u=m2}} <math>\ell</math> = {{val|1|u=m}} (forming a cube with perfectly conductive contacts on opposite faces), then the resistance of this element in ohms is numerically equal to the resistivity of the material it is made of in Ω⋅m.

Conductivity, σ, is the inverse of resistivity:

:<math>\sigma = \frac{1}{\rho}. \,\!</math>

Conductivity has SI units of "[[Siemens (unit)|siemens]] per metre" (S/m).

===General scalar case===
For less ideal cases, such as more complicated geometry, or when the current and electric field vary in different parts of the material, it is necessary to use a more general expression in which the resistivity at a particular point is defined as the ratio of the [[electric field]] to the [[current density|density]] of the current it creates at that point:

:<math>\rho=\frac{E}{J}, \,\!</math>

where
:<math>\rho</math> is the resistivity of the conductor material,
:<math>E</math> is the [[Magnitude (mathematics)|magnitude]] of the electric field,
:<math>J</math> is the magnitude of the [[current density]],

in which <math>E</math> and <math>J</math> are inside the conductor.

Conductivity is the inverse (reciprocal) of resistivity. Here, it is given by:

:<math>\sigma=\frac{1}{\rho} = \frac{J}{E}. \,\!</math>

For example, rubber is a material with large ''ρ'' and small ''σ''—because even a very large electric field in rubber makes almost no current flow through it. On the other hand, copper is a material with small ''ρ'' and large ''σ''—because even a small electric field pulls a lot of current through it.

As shown below, this expression simplifies to a single number when the electric field and current density are constant in the material.

:{| class="toccolours collapsible collapsed" width="80%" style="text-align:left"
!Derivation from general definition of resistivity
|-
|There are three equations to be combined here. The first is the resistivity for parallel current and electric field:

:<math>\rho=\frac{E}{J}, \,\!</math>

If the electric field is constant, the electric field is given by the total voltage ''V'' across the conductor divided by length ''ℓ'' of the conductor:

:<math>E=V/\ell</math>

If the current density is constant, it is equal to the total current divided by the cross sectional area:

:<math>J=I/A</math>

Plugging in the values of ''E'' and ''J'' into the first expression, we obtain:

:<math>\rho=\frac{V A}{I\ell}</math>

Finally, we apply Ohm's law, ''V''/''I''&nbsp;=&nbsp;''R''.

:<math>\rho=R\frac A\ell</math>

|}

=== Tensor resistivity ===
When the resistivity of a material has a directional component, the most general definition of resistivity must be used. It starts from the tensor-vector form of [[Ohm's law]], which relates the electric field inside a material to the electric current flow. This equation is completely general, meaning it is valid in all cases, including those mentioned above. However, this definition is the most complicated, so it is only directly used in [[Anisotropy|anisotropic]] cases, where the more simple definitions cannot be applied. If the material is not anisotropic, it is safe to ignore the tensor-vector definition, and use a simpler expression instead.

Here, [[Anisotropy|anisotropic]] means that the material has different properties in different directions. For example, a crystal of [[graphite]] consists microscopically of a stack of sheets, and current flows very easily through each sheet, but much less easily from one sheet to the adjacent one.<ref name=Pierson/> In such cases, the current does not flow in exactly the same direction as the electric field. Thus, the appropriate equations are generalized to the three-dimensional tensor form:<ref>J.R. Tyldesley (1975) ''An introduction to Tensor Analysis: For Engineers and Applied Scientists'', Longman, {{ISBN|0-582-44355-5}}</ref><ref>G. Woan (2010) ''The Cambridge Handbook of Physics Formulas'', Cambridge University Press, {{ISBN|978-0-521-57507-2}}</ref>

:<math> \mathbf{J} = \boldsymbol\sigma \mathbf{E} \,\, \rightleftharpoons \,\, \mathbf{E} = \boldsymbol\rho \mathbf{J} \,\!</math>

where the conductivity '''''σ''''' and resistivity '''''ρ''''' are rank-2 [[tensor]]s, and electric field '''''E''''' and current density '''''J''''' are vectors. These tensors can be represented by 3×3 matrices, the vectors with 3x1 matrices, with [[matrix multiplication]] used on the right side of these equations. In matrix form, the resistivity relation is given by:

:<math>\begin{bmatrix}E_x \\ E_y \\ E_z \end{bmatrix} = \begin{bmatrix}\rho_{xx} & \rho_{xy} & \rho_{xz} \\ \rho_{yx} & \rho_{yy} & \rho_{yz} \\ \rho_{zx} & \rho_{zy} & \rho_{zz} \end{bmatrix}\begin{bmatrix}J_x \\ J_y \\ J_z \end{bmatrix}</math>

where
:<math>\mathbf{E}</math> is the electric field vector, with components (''E''<sub>''x''</sub>, ''E''<sub>''y''</sub>, ''E''<sub>''z''</sub>).
:<math>\boldsymbol{\rho}</math> is the resistivity tensor, in general a three by three matrix.
:<math>\mathbf{J}</math> is the electric current density vector, with components (''J''<sub>''x''</sub>, ''J''<sub>''y''</sub>, ''J''<sub>''z''</sub>)

Equivalently, resistivity can be given in the more compact [[Einstein notation]]:

:<math>\mathbf{E}_i = \boldsymbol\rho_{ij} \mathbf{J}_j</math>

In either case, the resulting expression for each electric field component is:

:<math> E_x = \rho_{xx} J_x + \rho_{xy} J_y + \rho_{xz} J_z.</math>
:<math> E_y = \rho_{yx} J_x + \rho_{yy} J_y + \rho_{yz} J_z.</math>
:<math> E_z = \rho_{zx} J_x + \rho_{zy} J_y + \rho_{zz} J_z.</math>

Since the choice of the coordinate system is free, the usual convention is to simplify the expression by choosing an ''x''-axis parallel to the current direction, so ''J''<sub>''y''</sub>&nbsp;=&nbsp;''J''<sub>''z''</sub>&nbsp;=&nbsp;0. This leaves:

:<math>\rho_{xx}=\frac{E_x}{J_x}, \quad \rho_{yx}=\frac{E_y}{J_x}, \text{ and }\rho_{zx}=\frac{E_z}{J_x}.</math>

Conductivity is defined similarly:<ref>{{cite journal|title=Finite‐difference modelling of magnetotelluric fields in two‐dimensional anisotropic media|journal = Geophysical Journal International|volume = 128|issue = 3|pages = 505–521|author=Josef Pek, Tomas Verner|date=3 Apr 2007|doi=10.1111/j.1365-246X.1997.tb05314.x}}</ref>

:<math>\begin{bmatrix}J_x \\ J_y \\ J_z \end{bmatrix} = \begin{bmatrix}\sigma_{xx} & \sigma_{xy} & \sigma_{xz} \\ \sigma_{yx} & \sigma_{yy} & \sigma_{yz} \\ \sigma_{zx} & \sigma_{zy} & \sigma_{zz} \end{bmatrix}\begin{bmatrix}E_x \\ E_y \\ E_z \end{bmatrix}</math>

or

:<math>\mathbf{J}_i = \boldsymbol{\sigma}_{ij} \mathbf{E}_{j}</math>

Both resulting in:

:<math> J_x = \sigma_{xx} E_x + \sigma_{xy} E_y + \sigma_{xz} E_z</math>
:<math> J_y = \sigma_{yx} E_x + \sigma_{yy} E_y + \sigma_{yz} E_z</math>
:<math> J_z = \sigma_{zx} E_x + \sigma_{zy} E_y + \sigma_{zz} E_z</math>

Looking at the two expressions, <math>\boldsymbol{\rho}</math> and <math>\boldsymbol{\sigma}</math> are the [[Invertible matrix|matrix inverse]] of each other. However, in the most general case, the individual matrix elements are not necessarily reciprocals of one another; for example, ''σ<sub>xx</sub>'' may not be equal to 1/''ρ<sub>xx</sub>''. This can be seen in the [[Hall effect]], where <math>\rho_{xy}</math> is nonzero. In the Hall effect, due to rotational invariance about the ''z''-axis, <math> \rho_{yy}=\rho_{xx} </math> and <math> \rho_{yx}=-\rho_{xy}</math>, so the relation between resistivity and conductivity simplifies to:<ref>{{cite web|url=http://www.damtp.cam.ac.uk/user/tong/qhe/qhe.pdf|title=The Quantum Hall Effect: TIFR Infosys Lectures|author=David Tong|date=Jan 2016|accessdate=14 Sep 2018}}</ref>

:<math>\sigma_{xx}=\frac{\rho_{xx}}{\rho_{xx}^2 + \rho_{xy}^2}, \quad \sigma_{xy} = \frac{-\rho_{xy}}{\rho_{xx}^2 + \rho_{xy}^2}</math>

If the electric field is parallel to the applied current, <math>\rho_{xy}</math> and <math>\rho_{xz}</math> are zero. When they are zero, one number, <math>\rho_{xx}</math>, is enough to describe the electrical resistivity. It is then written as simply <math>\rho</math>, and this reduces to the simpler expression.

==Causes of conductivity==

===Band theory simplified===
{{see also|Band theory}}
{{Band structure filling diagram}}

According to elementary [[quantum mechanics]], an electron in an atom or crystal can only have certain precise energy levels; energies between these levels are impossible. When a large number of such allowed levels have close-spaced energy values – i.e. have energies that differ only minutely – those close energy levels in combination are called an "energy band". There can be many such energy bands in a material, depending on the atomic number of the constituent atoms<ref group=lower-alpha>The atomic number is the count of electrons in an atom that is electrically neutral – has no net electric charge.</ref> and their distribution within the crystal.<ref group=lower-alpha>Other relevant factors that are specifically not considered are the size of the whole crystal and external factors of the surrounding environment that modify the energy bands, such as imposed electric or magnetic fields.</ref>

The material's electrons seek to minimize the total energy in the material by settling into low energy states; however, the [[Pauli exclusion principle]] means that only one can exist in each such state. So the electrons "fill up" the band structure starting from the bottom. The characteristic energy level up to which the electrons have filled is called the [[Fermi level]]. The position of the Fermi level with respect to the band structure is very important for electrical conduction: Only electrons in energy levels near or above the [[Fermi level]] are free to move within the broader material structure, since the electrons can easily jump among the partially occupied states in that region. In contrast, the low energy states are completely filled with a fixed limit on the number of electrons at all times, and the high energy states are empty of electrons at all times.

Electric current consists of a flow of electrons. In metals there are many electron energy levels near the Fermi level, so there are many electrons available to move. This is what causes the high electronic conductivity of metals.

An important part of band theory is that there may be forbidden bands of energy: energy intervals that contain no energy levels. In insulators and semiconductors, the number of electrons is just the right amount to fill a certain integer number of low energy bands, exactly to the boundary. In this case, the Fermi level falls within a band gap. Since there are no available states near the Fermi level, and the electrons are not freely movable, the electronic conductivity is very low.

===In metals===
{{main|Free electron model}}
[[Image:Newtons cradle animation book 2.gif|right|200px|thumb|Like balls in a [[Newton's cradle]], electrons in a metal quickly transfer energy from one terminal to another, despite their own negligible movement.]]

A [[metal]] consists of a [[Crystal lattice|lattice]] of [[atom]]s, each with an outer shell of electrons that freely dissociate from their parent atoms and travel through the lattice. This is also known as a positive ionic lattice.<ref>[https://web.archive.org/web/20120416024619/http://ibchem.com/IB/ibnotes/brief/bon-sl.htm Bonding (sl)]. ibchem.com</ref> This 'sea' of dissociable electrons allows the metal to conduct electric current. When an electrical potential difference (a [[voltage]]) is applied across the metal, the resulting electric field causes electrons to drift towards the positive terminal. The actual [[drift velocity]] of electrons is typically small, on the order of magnitude of meters per hour. However, due to the sheer number of moving electrons, even a slow drift velocity results in a large [[current density]].<ref name=classroom>{{cite web|url=http://www.physicsclassroom.com/Class/circuits/u9l2c.cfm#p6 |title=Current versus Drift Speed |publisher=The physics classroom |accessdate=20 August 2014}}</ref> The mechanism is similar to transfer of momentum of balls in a [[Newton's cradle]]<ref>{{cite book|url=http://www.dummies.com/how-to/content/electronics-basics-direct-and-alternating-current.html |title=Electronics All-in-One For Dummies |last=Lowe |first=Doug |isbn=978-0-470-14704-7 |publisher=John Wiley & Sons |year=2012}}</ref> but the rapid propagation of an electric energy along a wire is not due to the mechanical forces, but the propagation of an energy-carrying electromagnetic field guided by the wire.

Most metals have electrical resistance. In simpler models (non quantum mechanical models) this can be explained by replacing electrons and the crystal lattice by a wave-like structure. When the electron wave travels through the lattice, the waves [[wave interference|interfere]], which causes resistance. The more regular the lattice is, the less disturbance happens and thus the less resistance. The amount of resistance is thus mainly caused by two factors. First, it is caused by the temperature and thus amount of vibration of the crystal lattice. Higher temperatures cause bigger vibrations, which act as irregularities in the lattice. Second, the purity of the metal is relevant as a mixture of different ions is also an irregularity.<ref>{{cite web|url=http://education.jlab.org/qa/current_02.html |title=Questions & Answers – How do you explain electrical resistance?|author=Keith Welch|accessdate=28 April 2017 |publisher=[[Thomas Jefferson National Accelerator Facility]]}}</ref><ref>{{cite web|title=Electromigration : What is electromigration?|url=http://www.csl.mete.metu.edu.tr/Electromigration/emig.htm|publisher=Middle East Technical University|accessdate=31 July 2017 |quote=When electrons are conducted through a metal, they interact with imperfections in the lattice and scatter. […] Thermal energy produces scattering by causing atoms to vibrate. This is the source of resistance of metals.}}</ref>

===In semiconductors and insulators===
{{main|Semiconductor|Insulator (electricity)}}
In metals, the [[Fermi level]] lies in the [[conduction band]] (see Band Theory, above) giving rise to free conduction electrons. However, in [[semiconductors]] the position of the Fermi level is within the band gap, about halfway between the conduction band minimum (the bottom of the first band of unfilled electron energy levels) and the valence band maximum (the top of the band below the conduction band, of filled electron energy levels). That applies for intrinsic (undoped) semiconductors. This means that at absolute zero temperature, there would be no free conduction electrons, and the resistance is infinite. However, the resistance decreases as the charge carrier density (i.e., without introducing further complications, the density of electrons) in the conduction band increases. In extrinsic (doped) semiconductors, [[dopant]] atoms increase the majority charge carrier concentration by donating electrons to the conduction band or producing holes in the valence band. (A "hole" is a position where an electron is missing; such holes can behave in a similar way to electrons.) For both types of donor or acceptor atoms, increasing dopant density reduces resistance. Hence, highly doped semiconductors behave metallically. At very high temperatures, the contribution of thermally generated carriers dominates over the contribution from dopant atoms, and the resistance decreases exponentially with temperature.

===In ionic liquids/electrolytes===
{{main|Conductivity (electrolytic)}}
In [[electrolyte]]s, electrical conduction happens not by band electrons or holes, but by full atomic species ([[ion]]s) traveling, each carrying an electrical charge. The resistivity of ionic solutions (electrolytes) varies tremendously with concentration – while distilled water is almost an insulator, [[Saline water|salt water]] is a reasonable electrical conductor. Conduction in [[ionic liquid]]s is also controlled by the movement of ions, but here we are talking about molten salts rather than solvated ions. In [[cell membrane|biological membranes]], currents are carried by ionic salts. Small holes in cell membranes, called [[ion channel]]s, are selective to specific ions and determine the membrane resistance.

===Superconductivity===
{{main|Superconductivity}}
The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as [[copper]] or [[silver]], this decrease is limited by impurities and other defects. Even near [[absolute zero]], a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing in a loop of [[superconducting wire]] can persist indefinitely with no power source.<ref name="Gallop">
{{cite book
 |author=John C. Gallop
 |year=1990
 |title=SQUIDS, the Josephson Effects and Superconducting Electronics
 |url=https://books.google.com/?id=ad8_JsfCdKQC&printsec=frontcover
 |publisher=[[CRC Press]]
 |pages=3, 20
 |isbn=978-0-7503-0051-3
}}</ref>

In 1986, researchers discovered that some [[cuprate superconductor|cuprate]]-[[perovskite (structure)|perovskite]] [[ceramic]] materials have much higher critical temperatures, and in 1987 one was produced with a critical temperature above {{convert|90|K|°C|abbr=on|0}}.<ref>{{cite web|title=The History of Superconductors |url=http://www.superconductors.org/history.htm |accessdate=23 February 2016 |url-status=dead |archiveurl=https://web.archive.org/web/20160303170753/http://www.superconductors.org/History.htm |archivedate=3 March 2016 }}</ref> Such a high transition temperature is theoretically impossible for a [[conventional superconductor]], so the researchers named these conductors ''[[high-temperature superconductors]]''. [[Liquid nitrogen]] boils at 77&nbsp;K, cold enough to activate high-temperature superconductors, but not nearly cold enough for conventional superconductors.  In conventional superconductors, electrons are held together in pairs by an attraction mediated by lattice [[phonon]]s.{{clarify|date=August 2016}}<!-- how pairing is related to resisitivty--> The best available model of high-temperature superconductivity is still somewhat crude. There is a hypothesis that electron pairing in high-temperature superconductors is mediated by short-range spin waves known as [[paramagnon]]s.<ref>{{Cite book
 | author = D. Pines
 | contribution = The Spin Fluctuation Model for High Temperature Superconductivity: Progress and Prospects
 | year = 2002
 | title = The Gap Symmetry and Fluctuations in High-Tc Superconductors
 | pages = 111–142
 | place = New York
 | publisher = Kluwer Academic
 | isbn = 978-0-306-45934-4
 | doi = 10.1007/0-306-47081-0_7
 | series = NATO Science Series: B
 | volume = 371
}}</ref>{{dubious|Dubious phrase|Dubious date=August 2016|date=September 2018}}

===Plasma===
{{main|Plasma (physics)}}
[[File:Lightning over Oradea Romania 3.jpg|thumb|right|[[Lightning]] is an example of plasma present at Earth's surface. Typically, lightning discharges 30,000 amperes at up to 100 million volts, and emits light, radio waves, and X-rays.<ref>See [http://www.nasa.gov/vision/universe/solarsystem/rhessi_tgf.html Flashes in the Sky: Earth's Gamma-Ray Bursts Triggered by Lightning]</ref> Plasma temperatures in lightning might approach 30,000 Kelvin (29,727&nbsp;°C) (53,540&nbsp;°F), or five times hotter than the temperature at the sun surface, and electron densities may exceed 10<sup>24</sup> m<sup>−3</sup>.]]

Plasmas are very good conductors and electric potentials play an important role.

The potential as it exists on average in the space between charged particles, independent of the question of how it can be measured, is called the ''plasma potential'', or ''space potential''. If an electrode is inserted into a plasma, its potential generally lies considerably below the plasma potential, due to what is termed a [[Debye sheath]]. The good electrical conductivity of plasmas makes their electric fields very small. This results in the important concept of ''quasineutrality'', which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma (''n''<sub>e</sub>&nbsp;=&nbsp;<Z>''n''<sub>i</sub>), but on the scale of the [[Debye length]] there can be charge imbalance. In the special case that ''[[Double layer (plasma)|double layers]]'' are formed, the charge separation can extend some tens of Debye lengths.

The magnitude of the potentials and electric fields must be determined by means other than simply finding the net [[charge density]]. A common example is to assume that the electrons satisfy the [[Boltzmann relation]]:
:<math>n_\text{e} \propto e^{e\Phi/k_\text{B} T_\text{e}}.</math>

Differentiating this relation provides a means to calculate the electric field from the density:
:<math>\mathbf{E} = -\frac{k_\text{B} T_\text{e}}{e}\frac{\nabla n_\text{e}}{n_\text{e}}.</math>

(The "downward pointing triangle" is a vector gradient; see [[nabla symbol]] and [[gradient]] for more information.)

It is possible to produce a plasma that is not quasineutral. An electron beam, for example, has only negative charges. The density of a non-neutral plasma must generally be very low, or it must be very small. Otherwise, the repulsive [[electrostatic force]] dissipates it.

In [[astrophysical]] plasmas, [[electric field screening|Debye screening]] prevents electric fields from directly affecting the plasma over large distances, i.e., greater than the [[Debye length]]. However, the existence of charged particles causes the plasma to generate, and be affected by, [[magnetic field]]s. This can and does cause extremely complex behavior, such as the generation of plasma double layers, an object that separates charge over a few tens of [[Debye length]]s. The dynamics of plasmas interacting with external and self-generated magnetic fields are studied in the academic discipline of [[magnetohydrodynamics]].

Plasma is often called the ''fourth [[state of matter]]'' after solid, liquids and gases.<ref>Yaffa Eliezer, Shalom Eliezer, ''The Fourth State of Matter: An Introduction to the Physics of Plasma'', Publisher: Adam Hilger, 1989, {{ISBN|978-0-85274-164-1}}, 226 pages, page 5</ref><ref>{{cite book|author=Bittencourt, J.A.|title=Fundamentals of Plasma Physics|publisher=Springer|year=2004|isbn=9780387209753|page=1|url=https://books.google.com/books?id=qCA64ys-5bUC&pg=PA1}}</ref> It is distinct from these and other lower-energy [[states of matter]]. Although it is closely related to the gas phase in that it also has no definite form or volume, it differs in a number of ways, including the following:

{| class="wikitable"
|-
! Property !! Gas !! Plasma
|-
! Electrical conductivity
| Very low: air is an excellent insulator until it breaks down into plasma at electric field strengths above 30 kilovolts per centimeter.<ref>{{cite web|url=http://hypertextbook.com/facts/2000/AliceHong.shtml|title=Dielectric Strength of Air|year=2000|first=Alice|last=Hong|work=The Physics Factbook}}</ref>
| Usually very high: for many purposes, the conductivity of a plasma may be treated as infinite.
|-
! Independently acting species
| One: all gas particles behave in a similar way, influenced by [[gravity]] and by [[collision]]s with one another.
| Two or three: [[electron]]s, [[ion]]s, [[proton]]s and [[neutron]]s can be distinguished by the sign and value of their [[electric charge|charge]] so that they behave independently in many circumstances, with different bulk velocities and temperatures, allowing phenomena such as new types of [[waves in plasma|waves]] and [[Instability|instabilities]].
|-
! Velocity distribution
| [[Maxwell–Boltzmann distribution|Maxwellian]]: collisions usually lead to a Maxwellian velocity distribution of all gas particles, with very few relatively fast particles.
| Often non-Maxwellian: collisional interactions are often weak in hot plasmas and external forcing can drive the plasma far from local equilibrium and lead to a significant population of unusually fast particles.
|-
! Interactions
| Binary: two-particle collisions are the rule, three-body collisions extremely rare.
| Collective: waves, or organized motion of plasma, are very important because the particles can interact at long ranges through the electric and magnetic forces.
|}

==Resistivity and conductivity of various materials== <!-- [[Supercapacitor]] links here -->
{{Main|Electrical resistivities of the elements (data page)}}
* A conductor such as a metal has high conductivity and a low resistivity.
* An [[Electrical insulation|insulator]] like [[glass]] has low conductivity and a high resistivity.
* The conductivity of a [[semiconductor]] is generally intermediate, but varies widely under different conditions, such as exposure of the material to electric fields or specific frequencies of [[light]], and, most important, with [[temperature]] and composition of the semiconductor material.

The degree of [[Doping (semiconductor)|semiconductors doping]] makes a large difference in conductivity. To a point, more doping leads to higher conductivity. The conductivity of a [[Solution (chemistry)|solution]] of [[Water (molecule)|water]] is highly dependent on its [[concentration]] of dissolved [[salts]], and other chemical species that [[Ionization|ionize]] in the solution. Electrical conductivity of water samples is used as an indicator of how salt-free, ion-free, or impurity-free the sample is; the purer the water, the lower the conductivity (the higher the resistivity). Conductivity measurements in water are often reported as ''specific conductance'', relative to the conductivity of pure water at {{val|25|u=°C}}. An [[EC meter]] is normally used to measure conductivity in a solution. A rough summary is as follows:

{|class="wikitable"
|-
! Material
! Resistivity, ρ (Ω·m)
|-
|[[Superconductors]]
| 0
|-
|[[Metal]]s
| 10<sup>−8</sup>
|-
|[[Semiconductor]]s
| Variable
|-
|[[Electrolyte]]s
| Variable
|-
|[[Electrical insulation|Insulators]]
| 10<sup>16</sup>
|-
|[[Superinsulator]]s
|∞
|}

This table shows the resistivity (''ρ''), conductivity and [[temperature coefficient]] of various materials at 20&nbsp;[[Celsius|°C]] (68 [[Fahrenheit|°F]], 293 [[Kelvin|K]])

{| class="wikitable sortable"
|-
! Material
! data-sort-type="number" | Resistivity<br>ρ (Ω·m) at {{val|20|u=°C}}
! data-sort-type="number" | Conductivity<br>σ (S/m) at {{val|20|u=°C}}
! data-sort-type="number" | Temperature<br /> coefficient<ref group=lower-alpha>The numbers in this column increase or decrease the [[significand]] portion of the resistivity. For example, at {{convert|30|C|K|abbr=on}}, the resistivity of silver is {{val|1.65|e=-8}}. This is calculated as {{nowrap|1=Δρ = α ΔT ρ<sub>o</sub>}} where ρ<sub>o</sub> is the resistivity at {{val|20|u=°C}} (in this case) and α is the temperature coefficient.</ref><br /> (K<sup>−1</sup>)
! class="unsortable"| Reference
|-
|[[Silver]]<ref group=lower-alpha>The conductivity of metallic silver is not significantly better than metallic copper for most practical purposes – the difference between the two can be easily compensated for by thickening the copper wire by only 3%. However silver is preferred for exposed electrical contact points because ''corroded'' silver is a tolerable conductor, but corroded copper is a fairly good insulator, like most corroded metals.</ref> || data-sort-value="0.0000000159"|{{val|1.59|e=-8}} || data-sort-value="6.30E7"|{{val|6.30|e=7}}||0.00380||<ref name="serway">{{cite book |author=Raymond A. Serway |title=Principles of Physics |edition=2nd |year=1998 |publisher=Saunders College Pub |location=Fort Worth, Texas; London |isbn=978-0-03-020457-9 |page=602 |url=https://archive.org/details/principlesofphys00serw }}</ref><ref name="Griffiths">{{cite book|title=Introduction to Electrodynamics|url=https://archive.org/details/introductiontoel00grif_0|url-access=registration|author=David Griffiths|publisher=[[Prentice Hall]]|year=1999|isbn=978-0-13-805326-0|editor=Alison Reeves|edition=3rd|location=Upper Saddle River, New Jersey|page=[https://archive.org/details/introductiontoel00grif_0/page/286 286]|chapter=7&nbsp;Electrodynamics|oclc=40251748|authorlink=David Griffiths (physicist)|origyear=1981}}<!-- |accessdate = 2006-01-29 --></ref>
|-
|[[Copper]]<ref group=lower-alpha>Copper is widely used in electrical equipment, building wiring, and telecommunication cables.</ref>||data-sort-value="0.0000000168"|{{val|1.68|e=-8}}||data-sort-value="5.96E7"|{{val|5.96|e=7}}||0.00404||<ref>{{cite journal |last1=Matula |first1=R.A. |title=Electrical resistivity of copper, gold, palladium, and silver |journal=Journal of Physical and Chemical Reference Data |date=1979 |volume=8 |issue=4 |page=1147 |doi=10.1063/1.555614 |bibcode=1979JPCRD...8.1147M}}</ref><ref name="Giancoli">{{cite book|title=Physics for Scientists and Engineers with Modern Physics|author=Douglas Giancoli|publisher=[[Prentice Hall]]|year=2009|isbn=978-0-13-149508-1|editor=Jocelyn Phillips|edition=4th|location=Upper Saddle River, New Jersey|page=658|chapter=25&nbsp;Electric Currents and Resistance|origyear=1984}}<!--| accessdate = 2013-03-04 --></ref>
|-
|[[Annealing (metallurgy)|Annealed]] [[copper]]<ref group=lower-alpha>Referred to as 100% IACS or ''International Annealed Copper Standard''. The unit for expressing the conductivity of nonmagnetic materials by testing using the [[eddy current]] method. Generally used for temper and alloy verification of aluminium.</ref> ||data-sort-value="0.0000000172"|{{val|1.72|e=-8}} ||data-sort-value="5.80E7"| {{val|5.80|e=7}} || 0.00393 || <ref>{{cite web |url=https://archive.org/details/copperwiretables31unituoft |title=Copper wire tables |publisher=United States National Bureau of Standards |via=Internet Archive - archive.org (archived 2001-03-10) |access-date=2014-02-03 |df=dmy-all}}</ref>
|-
|[[Gold]]<ref group=lower-alpha>Despite being less conductive than copper, gold is commonly used in [[electrical contacts]] because it does not easily corrode.</ref>||data-sort-value="2.44E-8"|{{val|2.44|e=-8}}||data-sort-value="4.11E7"|{{val|4.11|e=7}}||0.00340||<ref name="serway"/>
|-
|[[Aluminium]]<ref group=lower-alpha>Commonly used for [[overhead power line]] with steel reinforced [[Aluminium conductor steel-reinforced cable|(ACSR)]]</ref> ||data-sort-value="0.0000000265"|{{val|2.65|e=-8}}||data-sort-value="3.8E7"|{{val|3.77|e=7}}||0.00390||<ref name="serway"/>
|-
|[[Calcium]]||data-sort-value="0.0000000336"|{{val|3.36|e=-8}}||data-sort-value="2.98E7"|{{val|2.98|e=7}}||0.00410||
|-
|[[Tungsten]]||data-sort-value="0.0000000560"|{{val|5.60|e=-8}}||data-sort-value="1.79E7"|{{val|1.79|e=7}}||0.00450||<ref name="serway"/>
|-
|[[Zinc]]||data-sort-value="0.0000000590"|{{val|5.90|e=-8}}||data-sort-value="1.69E7"|{{val|1.69|e=7}}||0.00370||<ref>[http://physics.mipt.ru/S_III/t Physical constants]. (PDF format; see page&nbsp;2, table in the right lower corner). Retrieved on 2011-12-17.</ref>
|-
|[[Nickel]]||data-sort-value="0.0000000699"|{{val|6.99|e=-8}}||data-sort-value="1.43E7"|{{val|1.43|e=7}}||0.006||
|-
|[[Lithium]]||data-sort-value="9.28E-8"|{{val|9.28|e=-8}}||data-sort-value="1.08E7"|{{val|1.08|e=7}}||0.006||
|-
|[[Iron]]||data-sort-value="0.000000097"|{{val|9.70|e=-8}}||data-sort-value="1E7"|{{val|e=7}}||0.005||<ref name="serway"/>
|-
|[[Platinum]]||data-sort-value="0.000000106"|{{val|1.06|e=-7}}||data-sort-value="9.43E6"|{{val|9.43|e=6}}||0.00392||<ref name="serway"/>
|-
|[[Tin]]||data-sort-value="0.000000109"|{{val|1.09|e=-7}}||data-sort-value="9.17E6"|{{val|9.17|e=6}}||0.00450||
|-
|[[Gallium]]||data-sort-value="0.000000140"|{{val|1.40|e=-7}}||data-sort-value="7.10E6"|{{val|7.10|e=6}}||0.004||
|-
|[[Niobium]] || data-sort-value="0.000000140"|{{val|1.40|e=-7}}||data-sort-value="7.00E6"|{{val|7.00|e=6}}||||<ref>
[https://www.plansee.com/en/materials/niobium.html Material properties of niobium.]</ref> 
|-
|[[Carbon steel]] (1010)||data-sort-value="0.000000143"|{{val|1.43|e=-7}}||data-sort-value="6.99E6"|{{val|6.99|e=6}}||||<ref>[http://www.matweb.com/search/DataSheet.aspx?MatGUID=025d4a04c2c640c9b0eaaef28318d761 AISI 1010 Steel, cold drawn]. Matweb</ref>
|-
|[[Lead]]||data-sort-value="0.000000220"|{{val|2.20|e=-7}}||data-sort-value="4.55E6"|{{val|4.55|e=6}}||0.0039||<ref name="serway"/>
|-
|[[Galinstan]]
|{{val|2.89|e=-7}}
|3.46×10<sup>6</sup> 
|
|<ref>{{Cite journal|last=Karcher|first=Ch.|last2=Kocourek|first2=V.|date=December 2007|title=Free-surface instabilities during electromagnetic shaping of liquid metals|url=https://doi.org/10.1002/pamm.200700645|journal=PAMM|volume=7|issue=1|pages=4140009–4140010|doi=10.1002/pamm.200700645|issn=1617-7061}}</ref>
|-
|[[Titanium]]||data-sort-value="0.000000420"|{{val|4.20|e=-7}}||data-sort-value="2.38E6"|{{val|2.38|e=6}}||0.0038||
|-
|Grain oriented [[electrical steel]]||data-sort-value="0.000000460"|{{val|4.60|e=-7}}||data-sort-value="2.17E6"|{{val|2.17|e=6}}||||<ref>{{cite web|url=http://www.jfe-steel.co.jp/en/products/electrical/catalog/f1e-001.pdf |title=JFE steel |accessdate=2012-10-20}}</ref>
|-
|[[Manganin]]||data-sort-value="0.000000482"|{{val|4.82|e=-7}}||data-sort-value="2.07E6"|{{val|2.07|e=6}}||0.000002||<ref name="giancoli">{{cite book | author=Douglas C. Giancoli | title=Physics: Principles with Applications | edition=4th | year=1995 | publisher=Prentice Hall | location=London | isbn=978-0-13-102153-2 | url=https://archive.org/details/physicsprinciple00gian_0 }}<br>(see also [http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/rstiv.html Table of Resistivity]. hyperphysics.phy-astr.gsu.edu)
</ref>
|-
|[[Constantan]]||data-sort-value="0.000000490"|{{val|4.90|e=-7}}||data-sort-value="2.04E6"|{{val|2.04|e=6}}||0.000008||<ref>John O'Malley (1992) ''Schaum's outline of theory and problems of basic circuit analysis'', p. 19, McGraw-Hill Professional, {{ISBN|0-07-047824-4}}</ref>
|-
|[[Stainless steel]]<ref group=lower-alpha>18% chromium and 8% nickel austenitic stainless steel</ref>||data-sort-value="0.000000690"|{{val|6.90|e=-7}}||data-sort-value="1.45E6"|{{val|1.45|e=6}}||0.00094||<ref>Glenn Elert (ed.), [http://hypertextbook.com/facts/2006/UmranUgur.shtml "Resistivity of steel"], ''The Physics Factbook'', retrieved and [https://web.archive.org/web/20110606042043/http://hypertextbook.com/facts/2006/UmranUgur.shtml archived] 16 June 2011.</ref>
|-
|[[Mercury (element)|Mercury]]||data-sort-value="0.000000980"|{{val|9.80|e=-7}}||data-sort-value="1.02E6"|{{val|1.02|e=6}}||0.00090||<ref name="giancoli"/>
|-
|[[Manganese]] ||data-sort-value="0.00000144"|{{val|1.44|e=-6}}||data-sort-value="6.94E5"|{{val|6.94|e=5}}||  ||  
|-
|[[Nichrome]]<ref group=lower-alpha>Nickel-iron-chromium alloy commonly used in heating elements.</ref>||data-sort-value="0.0000011"|{{val|1.10|e=-6}}||data-sort-value="6.70E5"|{{val|6.70|e=5}}<br>{{citation needed|date=September 2018}}||{{val|0.0004}}||<ref name="serway"/>
|-
|[[Amorphous carbon|Carbon (amorphous)]]||data-sort-value="5E-4"|{{val|5|e=-4}} to {{val|8|e=-4}} <!-- 3.5×10<sup>−5</sup> Serway figure removed because unclear what form of carbon is being referenced-->||data-sort-value="1.25E3"|{{val|1.25|e=3}} to {{val|2.00|e=3}}|| −0.0005||<ref name="serway"/><ref>Y. Pauleau, Péter B. Barna, P. B. Barna (1997) ''Protective coatings and thin films: synthesis, characterization, and applications'', p. 215, Springer, {{ISBN|0-7923-4380-8}}.</ref>
|-
|[[Graphite|Carbon (graphite)]]<br>parallel to<br>[[basal plane]]<ref group=lower-alpha>Graphite is strongly anisotropic.</ref>||data-sort-value="2.5E-6"|{{val|2.5|e=-6}} to {{val|5.0|e=-6}} ||data-sort-value="2E5"|{{val|2|e=5}} to {{val|3|e=5}}<br>{{citation needed|date=September 2018}}|| ||<ref name=Pierson>Hugh O. Pierson, ''Handbook of carbon, graphite, diamond, and fullerenes: properties, processing, and applications'', p. 61, William Andrew, 1993 {{ISBN|0-8155-1339-9}}.</ref>
|-
|[[Graphite|Carbon (graphite)]]<br>perpendicular to<br>basal plane||data-sort-value="3E-3"|{{val|3|e=-3}}|| data-sort-value="3.3E2" |{{val|3.3|e=2}}|| ||<ref name=Pierson/>
|-
|[[GaAs]]||data-sort-value="0.001"|{{val|e=-3}} to {{val|e=8}}||data-sort-value="1E-8"|{{val|e=-8}} to {{val|e=3}}|| ||<ref name="Ohring">{{cite book | author = Milton Ohring | title = Engineering materials science, Volume 1| edition = 3rd | year = 1995 | page = 561|isbn=978-0125249959|publisher=Academic Press}}</ref>
|-
|[[Germanium]]<ref name="semi" group=lower-alpha>The resistivity of [[semiconductor]]s depends strongly on the presence of [[Impurity|impurities]] in the material.</ref>||data-sort-value="4.6E-1"|{{val|4.6|e=-1}}||2.17|| −0.048||<ref name="serway"/><ref name="Griffiths"/>
|-
|[[Sea water]]<ref group=lower-alpha>Corresponds to an average salinity of 35&nbsp;g/kg at {{val|20|u=°C}}.</ref>||data-sort-value="2E-1"|{{val|2.0|e=-1}}|| data-sort-value="4.8" |{{val|4.8}}|| ||<ref>[http://www.kayelaby.npl.co.uk/general_physics/2_7/2_7_9.html Physical properties of sea water] {{Webarchive|url=https://web.archive.org/web/20180118072121/http://www.kayelaby.npl.co.uk/general_physics/2_7/2_7_9.html |date=2018-01-18 }}. Kayelaby.npl.co.uk. Retrieved on 2011-12-17.</ref>
|-
|Swimming pool water<ref group=lower-alpha>The pH should be around 8.4 and the conductivity in the range of 2.5–3&nbsp;mS/cm. The lower value is appropriate for freshly prepared water. The conductivity is used for the determination of TDS (total dissolved particles).</ref>||data-sort-value="4E-1"|{{val|3.3|e=-1}} to {{val|4.0|e=-1}}||data-sort-value="0.25"|{{val|0.25}} to {{val|0.30}}||||<ref>[http://chemistry.stackexchange.com/questions/28333/electrical-conductivity-of-pool-water]. chemistry.stackexchange.com</ref>
|-
|[[Drinking water]]<ref group=lower-alpha>This value range is typical of high quality drinking water and not an indicator of water quality</ref>||data-sort-value="2E1"|{{val|2|e=1}} to {{val|2|e=3}}||data-sort-value="5E-4"|{{val|5|e=-4}} to {{val|5|e=-2}}|| ||{{Citation needed|date=January 2011}}
|-
|[[Silicon]]<ref name="semi" group=lower-alpha/>||data-sort-value="6.4E2"|{{val|6.4|e=2}}||data-sort-value="1.56E-3"|{{val|1.56|e=-3}}||{{val|-0.075}}||<ref name="serway"/>
|-
|[[Wood|Wood (damp)]]||data-sort-value="1E3"|{{val|e=3}} to {{val|e=4}}||data-sort-value="1E-4"|{{val|e=-4}} to {{val|e=-3}}||||<ref name="Transmission Lines data">[http://www.transmission-line.net/2011/07/electrical-properties-of-wood-poles.html Transmission Lines data]. Transmission-line.net. Retrieved on 2014-02-03.</ref>
|-
|[[Deionized water]]<ref group=lower-alpha>Conductivity is lowest with monatomic gases present; changes to {{val|12|e=-5}} upon complete de-gassing, or to {{val|7.5|e=-5}} upon equilibration to the atmosphere due to dissolved CO<sub>2</sub></ref> ||data-sort-value="1.8E5"|{{val|1.8|e=5}}||data-sort-value="5.5E-6"|{{val|5.5|e=-6}}|| ||<ref>{{cite journal|doi=10.1021/jp045975a|title=De-Gassed Water is a Better Cleaning Agent|year=2005|author1=R. M. Pashley |author2=M. Rzechowicz |author3=L. R. Pashley |author4=M. J. Francis |journal=The Journal of Physical Chemistry B|volume=109|pmid=16851085|issue=3|pages=1231–8}}</ref>
|-
|[[Glass]]||data-sort-value="1E11"|{{val|e=11}} to {{val|e=15}}||data-sort-value="1E-11"|{{val|e=-15}} to {{val|e=-11}} ||?||<ref name="serway"/><ref name="Griffiths"/>
|-
|[[Diamond|Carbon (diamond)]]||data-sort-value="1E12"|{{val|e=12}}||data-sort-value="1E-13"|~{{val|e=-13}}|| ||<ref>Lawrence S. Pan, Don R. Kania, ''Diamond: electronic properties and applications'', p. 140, Springer, 1994 {{ISBN|0-7923-9524-7}}.</ref>
|-
|[[Hard rubber]]||data-sort-value="1E13"|{{val|e=13}}||data-sort-value="1E-14"|{{val|e=-14}}||?||<ref name="serway"/>
|-
|[[Air]]||data-sort-value="1E9"|{{val|e=9}} to {{val|e=15}}|| data-sort-value="1E-15" |~{{val|e=-15}} to {{val|e=-9}}|| ||<ref>{{cite journal|doi=10.1029/2007JD009716|title=Effect of relative humidity and sea level pressure on electrical conductivity of air over Indian Ocean|year=2009|author1=S. D. Pawar |author2=P. Murugavel |author3=D. M. Lal |journal=Journal of Geophysical Research|volume=114|issue=D2|pages=D02205|bibcode=2009JGRD..114.2205P}}</ref><ref>{{cite journal|doi=10.1002/2016EA000241|title=What we can learn from measurements of air electric conductivity in 222Rn ‐ rich atmosphere|journal=Earth and Space Science|volume=4|issue=2|pages=91–106|year=2016|author1=E. Seran|author2=M. Godefroy|author3=E. Pili|bibcode=2017E&SS....4...91S}}</ref>
|-
|[[Wood|Wood (oven dry)]]||data-sort-value="1E14"|{{val|e=14}} to {{val|e=16}}||data-sort-value="1E-16"|{{val|e=-16}} to {{val|e=-14}}||||<ref name="Transmission Lines data"/>
|-
|[[Sulfur]]||data-sort-value="1E15"|{{val|e=15}}||data-sort-value="1E-16"|{{val|e=-16}}||?||<ref name="serway"/>
|-
|[[Fused quartz]]||data-sort-value="7.5E17"|{{val|7.5|e=17}}||data-sort-value="1.3E-18"|{{val|1.3|e=-18}}||?||<ref name="serway"/>
|-
|[[Polyethylene terephthalate|PET]]||data-sort-value="1E21"|{{val|e=21}}||data-sort-value="1E-21"|{{val|e=-21}}||?||
|-
|[[PTFE|Teflon]]||data-sort-value="1E23"|{{val|e=23}} to {{val|e=25}}||data-sort-value="1E-25"|{{val|e=-25}} to {{val|e=-23}}||?||
|}

The effective temperature coefficient varies with temperature and purity level of the material. The 20&nbsp;°C value is only an approximation when used at other temperatures. For example, the coefficient becomes lower at higher temperatures for copper, and the value 0.00427 is commonly specified at {{val|0|u=°C}}.<ref>[http://library.bldrdoc.gov/docs/nbshb100.pdf Copper Wire Tables] {{Webarchive|url=https://web.archive.org/web/20100821071645/http://library.bldrdoc.gov/docs/nbshb100.pdf |date=2010-08-21 }}. US Dep. of Commerce. National Bureau of Standards Handbook. February 21, 1966</ref>

The extremely low resistivity (high conductivity) of silver is characteristic of metals. [[George Gamow]] tidily summed up the nature of the metals' dealings with electrons in his popular science book ''One, Two, Three...Infinity'' (1947):
{{quotation|
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.}}
More technically, the [[free electron model]] gives a basic description of electron flow in metals.

Wood is widely regarded as an extremely good insulator, but its resistivity is sensitively dependent on moisture content, with damp wood being a factor of at least {{val|e=10}} worse insulator than oven-dry.<ref name="Transmission Lines data"/> In any case, a sufficiently high voltage – such as that in lightning strikes or some high-tension power lines – can lead to insulation breakdown and electrocution risk even with apparently dry wood.

==Temperature dependence==

===Linear approximation===
The electrical resistivity of most materials changes with temperature. If the temperature ''T'' does not vary too much, a [[linear approximation]] is typically used:
:<math>\rho(T) = \rho_0[1+\alpha (T - T_0)]</math>
where <math>\alpha</math> is called the ''temperature coefficient of resistivity'', <math>T_0</math> is a fixed reference temperature (usually room temperature), and <math>\rho_0</math> is the resistivity at temperature <math>T_0</math>. The parameter <math>\alpha</math> is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, <math>\alpha</math> is different for different reference temperatures. For this reason it is usual to specify the temperature that <math>\alpha</math> was measured at with a suffix, such as <math>\alpha_{15}</math>, and the relationship only holds in a range of temperatures around the reference.<ref>M.R. Ward (1971) ''Electrical Engineering Science'', pp. 36–40, McGraw-Hill.</ref> When the temperature varies over a large temperature range, the [[linear approximation]] is inadequate and a more detailed analysis and understanding should be used.

===Metals===
{{see also|Free electron model#Mean free path}}
[[Image:Resistivity Cu-Ag-Au.svg|thumb|Temperature dependence of the resistivity of gold, copper and silver.]]
In general, electrical resistivity of metals increases with temperature. Electron–[[phonon]] interactions can play a key role. At high temperatures, the resistance of a metal increases linearly with temperature. As the temperature of a metal is reduced, the temperature dependence of resistivity follows a power law function of temperature. Mathematically the temperature dependence of the resistivity ρ of a metal is given by the Bloch–Grüneisen formula:

:<math> \rho(T)=\rho(0)+A\left(\frac T {\Theta_R}\right)^n \int_0^{\Theta_R/T} \frac{x^n}{(e^x-1)(1-e^{-x})} \, dx </math>

where <math>\rho(0)</math> is the residual resistivity due to defect scattering, A is a constant that depends on the velocity of electrons at the [[Fermi surface]], the [[Debye radius]] and the number density of electrons in the metal. <math>\Theta_R</math> is the [[Debye temperature]] as obtained from resistivity measurements and matches very closely with the values of Debye temperature obtained from specific heat measurements. n is an integer that depends upon the nature of interaction:

* ''n''&nbsp;=&nbsp;5 implies that the resistance is due to scattering of electrons by phonons (as it is for simple metals)
* ''n''&nbsp;=&nbsp;3 implies that the resistance is due to s-d electron scattering (as is the case for transition metals)
* ''n''&nbsp;=&nbsp;2 implies that the resistance is due to electron–electron interaction.

If more than one source of scattering is simultaneously present, Matthiessen's Rule (first formulated by [[Augustus Matthiessen]] in the 1860s)<ref>A. Matthiessen, Rep. Brit. Ass. 32, 144 (1862)</ref><ref>A. Matthiessen, Progg. Anallen, 122, 47 (1864)</ref> states that the total resistance can be approximated by adding up several different terms, each with the appropriate value of&nbsp;''n''.

As the temperature of the metal is sufficiently reduced (so as to 'freeze' all the phonons), the resistivity usually reaches a constant value, known as the '''residual resistivity'''. This value depends not only on the type of metal, but on its purity and thermal history. The value of the residual resistivity of a metal is decided by its impurity concentration. Some materials lose all electrical resistivity at sufficiently low temperatures, due to an effect known as [[superconductivity]].

An investigation of the low-temperature resistivity of metals was the motivation to [[Heike Kamerlingh Onnes]]'s experiments that led in 1911 to discovery of [[superconductivity]]. For details see [[History of superconductivity]].

===Semiconductors===
{{main|Semiconductor}}
In general, [[intrinsic semiconductor]] resistivity decreases with increasing temperature. The electrons are bumped to the [[conduction band|conduction energy band]] by thermal energy, where they flow freely, and in doing so leave behind [[electron hole|holes]] in the [[valence band]], which also flow freely. The electric resistance of a typical [[intrinsic semiconductor|intrinsic]] (non doped) [[semiconductor]] decreases [[exponential decay|exponentially]] with temperature:

:<math>\rho= \rho_0 e^{-aT}\,</math>

An even better approximation of the temperature dependence of the resistivity of a semiconductor is given by the [[Steinhart–Hart equation]]:

:<math>\frac{1}{T} = A + B \ln\rho + C (\ln\rho)^3 \,</math>

where ''A'', ''B'' and ''C'' are the so-called '''Steinhart–Hart coefficients'''.

This equation is used to calibrate [[thermistor]]s.

[[Extrinsic semiconductor|Extrinsic (doped) semiconductors]] have a far more complicated temperature profile. As temperature increases starting from absolute zero they first decrease steeply in resistance as the carriers leave the donors or acceptors. After most of the donors or acceptors have lost their carriers, the resistance starts to increase again slightly due to the reducing mobility of carriers (much as in a metal). At higher temperatures, they behave like intrinsic semiconductors as the carriers from the donors/acceptors become insignificant compared to the thermally generated carriers.<ref>J. Seymour (1972)  ''Physical Electronics'', chapter 2, Pitman</ref>

In non-crystalline semiconductors, conduction can occur by charges [[quantum tunnelling]] from one localised site to another. This is known as [[variable range hopping]] and has the characteristic form of
:<math>\rho = A\exp\left(T^{-1/n}\right),</math>

where ''n'' = 2, 3, 4, depending on the dimensionality of the system.

==Complex resistivity and conductivity==
When analyzing the response of materials to alternating electric fields ([[dielectric spectroscopy]]),<ref>{{cite journal |last1=Stephenson |first1=C. |last2= Hubler |first2=A. |title= Stability and conductivity of self-assembled wires in a transverse electric field |journal=Sci. Rep. |volume=5 |date=2015 |page= 15044 |doi= 10.1038/srep15044 |pmid=26463476 |pmc=4604515 |bibcode= 2015NatSR...515044S }}</ref> in applications such as [[electrical impedance tomography]],<ref>Otto H. Schmitt, University of Minnesota [https://web.archive.org/web/20101013215436/http://www.otto-schmitt.org/OttoPagesFinalForm/Sounds/Speeches/MutualImpedivity.htm Mutual Impedivity Spectrometry and the Feasibility of its Incorporation into Tissue-Diagnostic Anatomical Reconstruction and Multivariate Time-Coherent Physiological Measurements]. otto-schmitt.org. Retrieved on 2011-12-17.</ref> it is convenient to replace resistivity with a [[complex number|complex]] quantity called '''impedivity ''' (in analogy to [[electrical impedance]]). Impedivity is the sum of a real component, the resistivity, and an imaginary component, the '''reactivity''' (in analogy to [[Reactance (electronics)|reactance]]). The magnitude of impedivity is the square root of sum of squares of magnitudes of resistivity and reactivity.

Conversely, in such cases the conductivity must be expressed as a [[complex number]] (or even as a matrix of complex numbers, in the case of [[anisotropic]] materials) called the ''[[Admittance|admittivity]]''. Admittivity is the sum of a real component called the conductivity and an imaginary component called the [[Susceptance|susceptivity]].

An alternative description of the response to alternating currents uses a real (but frequency-dependent) conductivity, along with a real [[permittivity]]. The larger the conductivity is, the more quickly the alternating-current signal is absorbed by the material (i.e., the more [[opacity (optics)|opaque]] the material is). For details, see [[Mathematical descriptions of opacity]].

==Resistance versus resistivity in complicated geometries==
Even if the material's resistivity is known, calculating the resistance of something made from it may, in some cases, be much more complicated than the formula <math>R = \rho \ell /A </math> above. One example is [[spreading resistance profiling]], where the material is inhomogeneous (different resistivity in different places), and the exact paths of current flow are not obvious.

In cases like this, the formulas
:<math> J = \sigma E \,\, \rightleftharpoons \,\, E = \rho J \,\!</math>
must be replaced with
:<math> \mathbf{J}(\mathbf{r}) = \sigma(\mathbf{r}) \mathbf{E}(\mathbf{r}) \,\, \rightleftharpoons \,\, \mathbf{E}(\mathbf{r}) = \rho(\mathbf{r}) \mathbf{J}(\mathbf{r}), \,\!</math>
where '''''E''''' and '''''J''''' are now [[vector field]]s. This equation, along with the [[continuity equation]] for '''''J''''' and the [[Poisson's equation]] for '''''E''''', form a set of [[partial differential equation]]s. In special cases, an exact or approximate solution to these equations can be worked out by hand, but for very accurate answers in complex cases, computer methods like [[Finite element method|finite element analysis]] may be required.

==Resistivity-density product==
In some applications where the weight of an item is very important, the product of resistivity and density is more important than absolute low resistivity – it is often possible to make the conductor thicker to make up for a higher resistivity; and then a low-resistivity-density-product material (or equivalently a high conductivity-to-density ratio) is desirable. For example, for long-distance [[overhead power line]]s, aluminium is frequently used rather than copper (Cu) because it is lighter for the same conductance.

Silver, although it is the least resistive metal known, has a high density and performs similarly to copper by this measure, but is much more expensive. Calcium and the alkali metals have the best resistivity-density products, but are rarely used for conductors due to their high reactivity with water and oxygen (and lack of physical strength). Aluminium is far more stable. Toxicity excludes the choice of beryllium.<ref>https://www.lenntech.com/periodic/elements/be.htm</ref> (Pure beryllium is also brittle.) Thus, aluminium is usually the metal of choice when the weight or cost of a conductor is the driving consideration.

{| class="wikitable sortable"
|-
! rowspan=2 | Material
! rowspan=2 data-sort-type=number | Resistivity <br/>(nΩ·m)
! rowspan=2 data-sort-type=number | [[Density]] <br/>(g/cm<sup>3</sup>)
! colspan=2 | Resistivity × density<br>product
! rowspan=2 | Relative volume<br>required<br/>to give same<br>conductance as Cu
! rowspan=2 | Relative weight<br>(mass)<br>required<br/>to give same<br>conductance as Cu
! rowspan=2| Approx price<br>(USD per kg)<br>(12/9/2018)
! rowspan=2|Relative cost<br>versus Cu
|-
! data-sort-type=number | (g·mΩ/m<sup>2</sup>)
! data-sort-type=number | Relative<br>to Cu
|-
| [[Sodium]]
| 47.7
| 0.97
| 46
| 31%
| 2.843
|0.31
|
|
|-
| [[Lithium]]
| 92.8
| 0.53
| 49
| 33%
| 5.531
|0.33
|
|
|-
| [[Calcium]]
| 33.6
| 1.55
| 52
| 35%
| 2.002
|0.35
|
|
|-
| [[Potassium]]
| 72.0
| 0.89
| 64
| 43%
| 4.291
|0.43
|
|
|-
| [[Beryllium]]
| 35.6
| 1.85
| 66
| 44%
| 2.122
|0.44
|
|
|-
| [[Aluminium]]
| 26.50
| 2.70
| 72
| 48%
| 1.5792
|0.48
|2.0
|0.16
|-
| [[Magnesium]]
| 43.90
| 1.74
| 76
| 51%
| 2.616
|0.51
|
|
|-
| [[Copper]]
| 16.78
| 8.96
| 150
| 100%
| 1
|1
|6.0
|1
|-
| [[Silver]]
| 15.87
| 10.49
| 166
| 111%
| 0.946
|1.11
|456
|84
|-
| [[Gold]]
| 22.14
| 19.30
| 427
| 285%
| 1.319
|2.85
|39,000
|19,000
|-
| [[Iron]]
| 96.1
| 7.874
| 757
| 505%
| 5.727
|5.05
|
|
|}

==See also==
{{colbegin}}
* [[Charge transport mechanisms]]
* [[Chemiresistor]]
* [[Permittivity#Classification of materials|Classification of materials based on permittivity]]
* [[Conductivity near the percolation threshold]]
* [[Contact resistance]]
* [[Electrical resistivities of the elements (data page)]]
* [[Electrical resistivity tomography]]
* [[Sheet resistance]]
* [[SI electromagnetism units]]
* [[Skin effect]]
* [[Spitzer resistivity]]
{{colend}}

== Notes ==
{{notelist}}

==References==
{{Reflist|25em}}

==Further reading==
* {{cite book | author= Paul Tipler| title=Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics |edition=5th | publisher=W. H. Freeman | year=2004 | isbn=978-0-7167-0810-0}}
* [https://www.academia.edu/29112469/Electrical_Conductivity_and_Resistivity Measuring Electrical Resistivity and Conductivity]

==External links==
{{wikibooks |A-level_Physics_(Advancing_Physics)/Resistivity_and_Conductivity}}
* {{cite web|title=Electrical Conductivity|url=http://www.sixtysymbols.com/videos/conductivity.htm|work=Sixty Symbols|year=2010|publisher=[[Brady Haran]] for the [[University of Nottingham]]}}
*[https://www.youtube.com/watch?v=gcSAomwrLD4 Electrical Resistivity and conductivity-Wikipedia article spoken for the blind] [https://www.youtube.com/channel/UCV63FScHnShoobfaENC8R_Q/about?&ab_channel=Audiopedia "Audiopedia as of July 17, 2014"]
*[http://www.wolframalpha.com/input/?i=conductivity+sulfur%2C+silicon%2C+copper&lk=3 Comparison of the electrical conductivity of various elements in WolframAlpha]

{{DEFAULTSORT:Electrical Resistivity And Conductivity}}
[[Category:Electrical resistance and conductance| ]]
[[Category:Physical quantities]]
[[Category:Materials science]]