Electric current is the ordered movement of
electrically-charged particles. and their distribution within the crystal. 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 . Like the balls in 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
lattice of
atoms, 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. 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 metres per hour. However, due to the sheer number of moving electrons, even a slow drift velocity results in a large
current density. The mechanism is similar to transfer of momentum of balls in a
Newton's cradle 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
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.
In semiconductors and insulators 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 In
electrolytes, electrical conduction happens not by band electrons or holes, but by full atomic species (
ions) travelling, each carrying an electrical charge. The resistivity of ionic solutions (electrolytes) varies tremendously with concentration – while distilled water is almost an insulator,
salt water is a reasonable electrical conductor. Conduction in
ionic liquids is also controlled by the movement of ions, but here we are talking about molten salts rather than solvated ions. In
biological membranes, currents are carried by ionic salts. Small holes in cell membranes, called
ion channels, are selective to specific ions and determine the membrane resistance. 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 \alpha, which is the ratio of the concentration of ions N to the concentration of molecules of the dissolved substance N_0: N = \alpha N_0 ~. The specific electrical conductivity (\sigma) of a solution is equal to: \sigma = q\left(b^+ + b^-\right)\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, \alpha: the coefficient of dissociation.
Superconductivity showing the resistance of a mercury wire as a function of temperature. The abrupt drop in resistance is the superconducting transition. The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In normal (that is, non-superconducting) 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. In a normal conductor, the current is driven by a voltage gradient, whereas in a superconductor, there is no voltage gradient and the current is instead related to the phase gradient of the superconducting order parameter. A consequence of this is that an electric current flowing in a loop of
superconducting wire can persist indefinitely with no power source. In a class of superconductors known as
type II superconductors, including all known
high-temperature 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
magnetic vortices 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 non-superconducting 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.
Plasma 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. Plasma temperatures in lightning might approach 30,000 kelvin (29,727 °C) (53,540 °F), and electron densities may exceed 1024 m−3. 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 (), 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_\text{e} \propto \exp\left(e\Phi/k_\text{B} T_\text{e}\right). Differentiating this relation provides a means to calculate the electric field from the density: \mathbf{E} = -\frac{k_\text{B} T_\text{e}}{e}\frac{\nabla n_\text{e}}{n_\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 non-neutral 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,
magnetic fields. This can and does cause extremely complex behaviour, such as the generation of plasma double layers, an object that separates charge over a few tens of Debye lengths. 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. 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: ==Resistivity and conductivity of various materials== • A conductor such as a metal has high conductivity and a low resistivity. • An
insulator such as
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
semiconductors doping makes a large difference in conductivity. To a point, more doping leads to higher conductivity. The conductivity of a
water/
aqueous solution is highly dependent on its
concentration of dissolved
salts and other chemical species that
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 . An
EC meter is normally used to measure conductivity in a solution. A rough summary is as follows:
Table This table shows the resistivity (), conductivity and
temperature coefficient of various materials at . 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 . 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 oven-dry. 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==