Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek alphabet (rho). The SI unit of electrical resistivity is the ohmmeter (Ω⋅m).
For example, if a solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is , then the resistivity of the material is .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 (sigma), but (kappa) (especially in electrical engineering) and (gamma) are sometimes used. The SI unit of electrical conductivity is siemens per metre (S/m).
$$\backslash rho\; =\; R\; \backslash frac\{A\}\{\backslash ell\},$$
where
The resistivity can be expressed using the SI unit ohm metre (Ω⋅m) — i.e. ohms multiplied by square metres (for the crosssectional area) then divided by metres (for the length).
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 highresistivity material is like pushing water through a pipe full of sand — while passing current through a lowresistivity 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.
The above equation can be transposed to get Pouillet's law (named after Claude Pouillet):
$$R\; =\; \backslash rho\; \backslash frac\{\backslash ell\}\{A\}.$$The resistance of a given element is proportional to the length, but inversely proportional to the crosssectional area. For example, if = , $\backslash ell$ = (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, , is the inverse of resistivity:
$$\backslash sigma\; =\; \backslash frac\{1\}\{\backslash rho\}.$$
Conductivity has SI units of siemens per metre (S/m).
$$\backslash rho=\backslash frac\{E\}\{J\},$$
where
in which $E$ and $J$ are inside the conductor.
Conductivity is the inverse (reciprocal) of resistivity. Here, it is given by:
$$\backslash sigma\; =\; \backslash frac\{1\}\{\backslash rho\}\; =\; \backslash frac\{J\}\{E\}.$$
For example, rubber is a material with large and small — 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 and large — 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.
! 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:
$$\backslash rho=\backslash frac\{E\}\{J\},$$ If the electric field is constant, the electric field is given by the total voltage across the conductor divided by length of the conductor: $$E\; =\; \backslash frac\{V\}\{\backslash ell\}$$ If the current density is constant, it is equal to the total current divided by the cross sectional area: $$J\; =\; \backslash frac\{I\}\{A\}$$ Plugging in the values of and into the first expression, we obtain: $$\backslash rho\; =\; \backslash frac\{V\; A\}\{I\backslash ell\}$$ Finally, we apply Ohm's law, . $$\backslash rho\; =\; R\backslash frac\{A\}\{\backslash ell\}$$ 
Here, anisotropy 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. 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 threedimensional tensor form:J.R. Tyldesley (1975) An introduction to Tensor Analysis: For Engineers and Applied Scientists, Longman, G. Woan (2010) The Cambridge Handbook of Physics Formulas, Cambridge University Press,
$$\backslash mathbf\{J\}\; =\; \backslash boldsymbol\backslash sigma\; \backslash mathbf\{E\}\; \backslash ,\backslash ,\; \backslash rightleftharpoons\; \backslash ,\backslash ,\; \backslash mathbf\{E\}\; =\; \backslash boldsymbol\backslash rho\; \backslash mathbf\{J\}$$
where the conductivity and resistivity are rank2 , and electric field and current density are vectors. These tensors can be represented by 3×3 matrices, the vectors with 3×1 matrices, with matrix multiplication used on the right side of these equations. In matrix form, the resistivity relation is given by:
$$$$
\begin{bmatrix} E_x \\ E_y \\ E_z \end{bmatrix} = \begin{bmatrix} \rho_{xx} & \rho_{xy} & \rho_{xz} \\ \rho_{yx} & \rho_{yy} & \rho_{yz} \\ \rho_{zx} & \rho_{zy} & \rho_{zz} \end{bmatrix}\begin{bmatrix} J_x \\ J_y \\ J_z \end{bmatrix}
where
Equivalently, resistivity can be given in the more compact Einstein notation:
$$\backslash mathbf\{E\}\_i\; =\; \backslash boldsymbol\backslash rho\_\{ij\}\; \backslash mathbf\{J\}\_j\; ~.$$
In either case, the resulting expression for each electric field component is:
$$\backslash begin\{align\}\; E\_x\; \&=\; \backslash rho\_\{xx\}\; J\_x\; +\; \backslash rho\_\{xy\}\; J\_y\; +\; \backslash rho\_\{xz\}\; J\_z\; \backslash \backslash \; E\_y\; \&=\; \backslash rho\_\{yx\}\; J\_x\; +\; \backslash rho\_\{yy\}\; J\_y\; +\; \backslash rho\_\{yz\}\; J\_z\; \backslash \backslash \; E\_z\; \&=\; \backslash rho\_\{zx\}\; J\_x\; +\; \backslash rho\_\{zy\}\; J\_y\; +\; \backslash rho\_\{zz\}\; J\_z\; \backslash end\{align\}$$
Since the choice of the coordinate system is free, the usual convention is to simplify the expression by choosing an axis parallel to the current direction, so . This leaves:
$$\backslash rho\_\{xx\}=\backslash frac\{E\_x\}\{J\_x\},\; \backslash quad\; \backslash rho\_\{yx\}=\backslash frac\{E\_y\}\{J\_x\},\; \backslash text\{\; and\; \}\backslash rho\_\{zx\}=\backslash frac\{E\_z\}\{J\_x\}.$$
Conductivity is defined similarly:
$$$$
\begin{bmatrix} J_x \\ J_y \\ J_z \end{bmatrix} = \begin{bmatrix} \sigma_{xx} & \sigma_{xy} & \sigma_{xz} \\ \sigma_{yx} & \sigma_{yy} & \sigma_{yz} \\ \sigma_{zx} & \sigma_{zy} & \sigma_{zz} \end{bmatrix}\begin{bmatrix} E_x \\ E_y \\ E_z \end{bmatrix}
or
$$\backslash mathbf\{J\}\_i\; =\; \backslash boldsymbol\{\backslash sigma\}\_\{ij\}\; \backslash mathbf\{E\}\_\{j\}$$
Both resulting in:
$$\backslash begin\{align\}\; J\_x\; =\; \backslash sigma\_\{xx\}\; E\_x\; +\; \backslash sigma\_\{xy\}\; E\_y\; +\; \backslash sigma\_\{xz\}\; E\_z\; \backslash \backslash \; J\_y\; =\; \backslash sigma\_\{yx\}\; E\_x\; +\; \backslash sigma\_\{yy\}\; E\_y\; +\; \backslash sigma\_\{yz\}\; E\_z\; \backslash \backslash \; J\_z\; =\; \backslash sigma\_\{zx\}\; E\_x\; +\; \backslash sigma\_\{zy\}\; E\_y\; +\; \backslash sigma\_\{zz\}\; E\_z\; \backslash end\{align\}$$
Looking at the two expressions, $\backslash boldsymbol\{\backslash rho\}$ and $\backslash boldsymbol\{\backslash sigma\}$ are the matrix inverse of each other. However, in the most general case, the individual matrix elements are not necessarily reciprocals of one another; for example, may not be equal to . This can be seen in the Hall effect, where $\backslash rho\_\{xy\}$ is nonzero. In the Hall effect, due to rotational invariance about the axis, $\backslash rho\_\{yy\}=\backslash rho\_\{xx\}$ and $\backslash rho\_\{yx\}=\backslash rho\_\{xy\}$, so the relation between resistivity and conductivity simplifies to:
$$\backslash sigma\_\{xx\}=\backslash frac\{\backslash rho\_\{xx\}\}\{\backslash rho\_\{xx\}^2\; +\; \backslash rho\_\{xy\}^2\},\; \backslash quad\; \backslash sigma\_\{xy\}\; =\; \backslash frac\{\backslash rho\_\{xy\}\}\{\backslash rho\_\{xx\}^2\; +\; \backslash rho\_\{xy\}^2\}$$
If the electric field is parallel to the applied current, $\backslash rho\_\{xy\}$ and $\backslash rho\_\{xz\}$ are zero. When they are zero, one number, $\backslash rho\_\{xx\}$, is enough to describe the electrical resistivity. It is then written as simply $\backslash rho$, and this reduces to the simpler expression.
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.
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 wavelike structure. When the electron wave travels through the lattice, the waves 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. The small decrease in conductivity on melting of pure metals is due to the loss of long range crystalline order. The short range order remains and strong correlation between positions of ions results in coherence between waves diffracted by adjacent ions.
The concentration of ions in a liquid ( e.g., in an aqueous solution) depends on the degree of dissociation of the dissolved substance, characterized by a dissociation coefficient $\backslash alpha$, which is the ratio of the concentration of ions $N$ to the concentration of molecules of the dissolved substance $N\_0$:
$$N\; =\; \backslash alpha\; N\_0\; ~.$$
The specific electrical conductivity ($\backslash sigma$) of a solution is equal to:
$$\backslash sigma\; =\; q\backslash left(b^+\; +\; b^\backslash right)\backslash alpha\; N\_0\; ~,$$
where $q$: module of the ion charge, $b^+$ and $b^$: mobility of positively and negatively charged ions, $N\_0$: concentration of molecules of the dissolved substance, $\backslash alpha$: the coefficient of dissociation.
In a class of superconductors known as type II superconductors, including all known hightemperature superconductors, an extremely low but nonzero resistivity appears at temperatures not too far below the nominal superconducting transition when an electric current is applied in conjunction with a strong magnetic field, which may be caused by the electric current. This is due to the motion of Abrikosov vortex in the electronic superfluid, which dissipates some of the energy carried by the current. The resistance due to this effect is tiny compared with that of nonsuperconducting materials, but must be taken into account in sensitive experiments. However, as the temperature decreases far enough below the nominal superconducting transition, these vortices can become frozen so that the resistance of the material becomes truly zero.
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 (), but on the scale of the Debye length there can be charge imbalance. In the special case that 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: $$n\_\backslash text\{e\}\; \backslash propto\; e^\{e\backslash Phi/k\_\backslash text\{B\}\; T\_\backslash text\{e\}\}.$$
Differentiating this relation provides a means to calculate the electric field from the density: $$\backslash mathbf\{E\}\; =\; \backslash frac\{k\_\backslash text\{B\}\; T\_\backslash text\{e\}\}\{e\}\backslash frac\{\backslash nabla\; n\_\backslash text\{e\}\}\{n\_\backslash text\{e\}\}.$$
(∇ is the vector gradient operator; 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 nonneutral plasma must generally be very low, or it must be very small. Otherwise, the repulsive electrostatic force dissipates it.
In astrophysical plasmas, 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, . 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 . The dynamics of plasmas interacting with external and selfgenerated magnetic fields are studied in the academic discipline of magnetohydrodynamics.
Plasma is often called the fourth state of matter after solid, liquids and gases.Yaffa Eliezer, Shalom Eliezer, The Fourth State of Matter: An Introduction to the Physics of Plasma, Publisher: Adam Hilger, 1989, , 226 pages, page 5
It is distinct from these and other lowerenergy 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:
The degree of semiconductors doping makes a large difference in conductivity. To a point, more doping leads to higher conductivity. The conductivity of a solution of water is highly dependent on its concentration of dissolved salts, and other chemical species that Ionization in the solution. Electrical conductivity of water samples is used as an indicator of how saltfree, ionfree, or impurityfree 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 . An EC meter is normally used to measure conductivity in a solution. A rough summary is as follows:
+ Resistivity of classes of materials 
This table shows the resistivity (), conductivity and temperature coefficient of various materials at .
+ Resistivity, conductivity, and temperature coefficient for several materials 
The effective temperature coefficient varies with temperature and purity level of the material. The 20 °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 . Copper Wire Tables . US Dep. of Commerce. National Bureau of Standards Handbook. February 21, 1966
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):
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 worse insulator than ovendry. In any case, a sufficiently high voltage – such as that in lightning strikes or some hightension power lines – can lead to insulation breakdown and electrocution risk even with apparently dry wood.
where $\backslash alpha$ is called the temperature coefficient of resistivity, $T\_0$ is a fixed reference temperature (usually room temperature), and $\backslash rho\_0$ is the resistivity at temperature $T\_0$. The parameter $\backslash alpha$ is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, $\backslash alpha$ is different for different reference temperatures. For this reason it is usual to specify the temperature that $\backslash alpha$ was measured at with a suffix, such as $\backslash alpha\_\{15\}$, and the relationship only holds in a range of temperatures around the reference.
where = 2, 3, 4, depending on the dimensionality of the system.
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 is the sum of a real component called the conductivity and an imaginary component called the Susceptance.
An alternative description of the response to alternating currents uses a real (but frequencydependent) conductivity, along with a real permittivity. The larger the conductivity is, the more quickly the alternatingcurrent signal is absorbed by the material (i.e., the more opaque the material is). For details, see Mathematical descriptions of opacity.
In cases like this, the formulas $$J\; =\; \backslash sigma\; E\; \backslash ,\backslash ,\; \backslash rightleftharpoons\; \backslash ,\backslash ,\; E\; =\; \backslash rho\; J$$
must be replaced with $$\backslash mathbf\{J\}(\backslash mathbf\{r\})\; =\; \backslash sigma(\backslash mathbf\{r\})\; \backslash mathbf\{E\}(\backslash mathbf\{r\})\; \backslash ,\backslash ,\; \backslash rightleftharpoons\; \backslash ,\backslash ,\; \backslash mathbf\{E\}(\backslash mathbf\{r\})\; =\; \backslash rho(\backslash mathbf\{r\})\; \backslash mathbf\{J\}(\backslash mathbf\{r\}),$$
where and are now . This equation, along with the continuity equation for and the Poisson's equation for , form a set of partial differential equations. 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 analysis may be required.
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 resistivitydensity 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. (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.
+ Resistivity, density, and resistivitydensity products of selected materials 

