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Electrical conduction is the movement of charged particles through a material (current) in response to an electric field.
The underlying mechanism for this movement depends on the material.
In most situations, conduction is well-described by Ohm's Law, which states
that the current is proportional to the applied electric field. The ease with which current density (current per area) j
flows in a material is measured by the conductivity
σ, defined as:
- j = σ
E
or its reciprocal resistivity ρ:
- j = E / ρ
In anisotropic materials, σ and ρ are tensors.
Solids
In crystalline solids, atoms interact with their neighbors, and the energy levels of the electrons in isolated atoms turn into
bands. Whether a material conducts or not is determined by its band structure. Electrons, being fermions, follow the Pauli exclusion principle, meaning that two electrons
cannot occupy the same state. Thus electrons in a solid fill up the energy bands up to a certain level, called the Fermi energy. Bands which are completely full of electrons cannot conduct
electricity, because there is no state of nearby energy to which the electrons can jump. Materials in which all bands are full
(i.e. the Fermi energy is between two bands) are insulators.
Metals
Metals are good conductors because they have unfilled space in the valence energy
band. In the absence of an electric field, there exist electrons travelling in all directions and many different velocities up to
the Fermi velocity (the velocity of electrons at the Fermi energy). When an
electric field is applied, a slight imbalance develops and a current flows. Electrons in this band can be accelerated by the
field because there are plenty of nearby states on the band.
Resistance comes about in a metal because of scattering of the electrons
from defects in the lattice or by phonons. A crude theory of conduction in simple
metals is the Drude model, in which scattering is characterized by a
relaxation time τ. The conductivity is then given by the formula
-
where n is the density of conduction electrons, e is the electron charge, and m is the electron
mass. A better model is the so-called semiclassical theory, in which the effect of the periodic potential of the lattice on the
electrons gives them an effective mass.
Semiconductors
A solid with filled bands is an insulator, but at finite temperature, electrons can be thermally excited from the valence band to the next highest, the conduction band. The fraction of electrons excited in this way depends on the temperature and the band gap, the energy difference between the two bands. Exciting these electrons into the
conduction band leaves behind positively charged holes in the valence
band, which can also conduct electricity. See semiconductor for more
details.
In semiconductors, impurities greatly affect the concentration and type of charge carriers. Donor (n-type) impurities have
extra valence electrons with energies very close to the conduction band which can be easily thermally excited to the conduction
band. Acceptor (p-type) impurities capture electrons from the valence band, allowing the easy formation of holes. If an insulator
is doped with enough impurities, a Mott transition can occur, and the insulator turns into a conductor.
Superconductors
In metals and certain other materials, a transition occurs at low temperature to the superconducting state. By an interaction mediated by some other part of the system (in metals, phonons),
the electrons pair up into Cooper pairs. The bosonic Cooper pairs form a
superfluid which has zero resistance.
Electrolytes
Electric currents in electrolytes are flows of electrically charged
atoms (ions). For example, if an electric field is
placed on a solution of Na+ and Cl–, the sodium ions will move towards the negative electrode (anode), and the chlorine ions will
move towards the positive electrode (cathode). If the conditions are right, redox
reactions will take place, which release electrons from the chlorine, and allow electrons to be absorbed into the sodium. In
water ice and in certain solid electrolytes, flowing protons constitute the electric current.
Gases and plasmas
In neutral gases, electrical conductivity is very low. They act as a dielectric or insulator, up until the
electric field reaches a breakdown value, stripping the electrons from the atoms thus forming a plasma. This plasma allows the conduction of electricity, forming a spark, arc or
lightning. In ordinary air below the breakdown field, the dominant source of
electrical conduction is via mobile particles of water, which shuttle electric charge,
forming a current.
Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their
molecules or atoms. A plasma can be formed by high temperature, or by application of an electric field as noted above. Electrical conduction in a plasma is
due to the motion of both the electrons and the positively-charged ions.
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