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Conduction is the movement of
electric charge particles through a transmission medium (electrical conductor). The movement can form an
Current (electricity) in response to an
electric field. The underlying mechanism for this movement depends on the material.
Conduction in metals and resistors 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 unit area)
j appears in a material is measured by the
electrical conductivity σ, defined as:
Current (electricity) =
σ electric field
or its reciprocal
electrical resistivity ρ:
Current (electricity) =
electric field /
ρ
In linear anisotropic materials,
σ and
ρ are
tensors.
Solids (including insulating solids)
In crystalline solids,
atoms interact with their neighbors, and the energy levels of the electrons in isolated atoms turn into
bands. Whether a material conducts or not is determined by its
band structure. Electrons, being
fermions, follow the Pauli exclusion principle, meaning that two electrons in the same interacting system cannot occupy the same state, which further means that their four quantum numbers have to be different. Thus electrons in a solid fill up the energy bands up to a certain level, called the Fermi energy. Bands which are completely full of electrons cannot conduct electricity, because there is no state of nearby energy to which the electrons can jump. Materials in which all bands are full (i.e. the Fermi energy is between two bands) are Electrical insulations. In some cases, however, the band theory breaks down and materials that are predicted to be conductors by band theory turn out to be insulators. Mott insulators and
charge transfer insulators are two such classes of insulators.
Metals
Metals are good conductors of electricity and heat because they have unfilled space in the valence energy band. In the absence of an electric field,
conduction electrons travel in all directions at very high velocities. Even at the coldest possible temperature — absolute zero — conduction electrons can still travel at the
Fermi velocity (the velocity of electrons at the Fermi energy). When an electric field is applied, a slight imbalance develops and mobile electrons flow. Electrons in this band can be accelerated by the
field because there are plenty of nearby unfilled states in the band.
Resistance comes about in a metal because of the
scattering of electrons from defects in the lattice or by phonons. A crude 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
\sigma = \frac{ne^2 \tau}{m}
where
n is the density of conduction electrons,
e is the electron charge, and
m is the electron mass. A better model is the so-called semi-classical theory, in which the effect of the periodic potential of the lattice on the electrons gives them an
effective mass.
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
electron 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
Superconductors are those materials which are extreme conductors. They are totally dependent on the temperature. As the temperature changes their properties also change.In metals and certain other materials, a transition to the
superconductivity state occurs at low (sub-cryogenic) temperature. By an interaction mediated by some other part of the system (in metals,
phonons), the electrons pair up into
Cooper pairs. The
boson 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 across a solution of sodium+ and
chlorine–, the sodium ions will move constantly towards the negative electrode (anode), while the chloride ions will move towards the positive electrode (cathode). If the conditions are right, redox reactions will take place at the electrode surfaces, releasing electrons from the chloride, and allow electrons to be absorbed into the sodium.
Water-ice and certain solid electrolytes called proton conductors contain positive hydrogen ions which are free to move. In these materials, currents of electricity are composed of moving protons (as opposed to the moving electrons found in metals).
In certain electrolyte mixtures, populations of brightly-colored ions form the moving electric charges. The slow migration of these ions during an electric current is one example of a situation where a current is directly visible to human eyes.
Gases and plasmas
In air, and other ordinary gases below the breakdown field, the dominant source of electrical conduction is via a relatively small number of mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electrical conductivity is extremely low, gases are dielectrics or
Electrical insulations. However, once the applied electric field approaches the dielectric breakdown value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and ionizing, neutral gas atoms or molecules in a process called
avalanche breakdown. The breakdown process forms a
Plasma (physics) that contains a significant number of mobile electrons and positive ions, causing it to behave as an electrical conductor. In the process, it forms a light emitting conductive path, such as a
spark,
electric arc or lightning.
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 a high electric or alternating magnetic field as noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions, and hence carry the bulk of the current.
Vacuum
Since a "
free space" contains no charged particles, vacuums normally behave as perfect insulators (they would be the greatest insulators known to man) However, metal electrode surfaces can cause a region of the vacuum to become conductive by injecting free electrons or
ions through either
field emission or
thermionic emission. Thermionic emission occurs when the thermal energy exceeds the metal's work function, while field emission occurs when the electric field at the surface of the metal is high enough to cause quantum tunneling, which results in the ejection of free electrons from the metal into the vacuum. Externally heated electrodes are often used to generate an
electron cloud as in the
electrical filament or indirectly heated cathode of vacuum tubes.
cold cathode can also spontaneously produce electron clouds via thermionic emission when small incandescent regions (called
cathode spots or
anode spots) are formed. These are incandescent regions of the electrode surface that are created by a localized high current flow. These regions may be initiated by field emission, but are then sustained by localized thermionic emission once a vacuum arc forms. These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field.
Vacuum tubes and Krytrons are some of the electronic switching and amplifying devices based on vacuum conductivity.
See also
Conduction is the movement of
electric charge particles through a transmission medium (
electrical conductor). The movement can form an
Current (electricity) in response to an
electric field. The underlying mechanism for this movement depends on the material.
Conduction in
metals and
resistors 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 unit area)
j appears in a material is measured by the electrical conductivity
σ, defined as:
Current (electricity) =
σ electric field
or its reciprocal
electrical resistivity ρ:
Current (electricity) =
electric field /
ρ
In linear anisotropic materials,
σ and
ρ are
tensors.
Solids (including insulating solids)
In crystalline solids,
atoms interact with their neighbors, and the energy levels of the electrons in isolated atoms turn into
bands. Whether a material conducts or not is determined by its band structure. Electrons, being
fermions, follow the
Pauli exclusion principle, meaning that two electrons in the same interacting system cannot occupy the same state, which further means that their four quantum numbers have to be different. Thus electrons in a solid fill up the energy bands up to a certain level, called the
Fermi energy. Bands which are completely full of electrons cannot conduct electricity, because there is no state of nearby energy to which the electrons can jump. Materials in which all bands are full (i.e. the Fermi energy is between two bands) are
Electrical insulations. In some cases, however, the band theory breaks down and materials that are predicted to be conductors by band theory turn out to be insulators. Mott insulators and charge transfer insulators are two such classes of insulators.
Metals
Metals are good conductors of electricity and heat because they have unfilled space in the valence energy band. In the absence of an electric field,
conduction electrons travel in all directions at very high velocities. Even at the coldest possible temperature —
absolute zero — conduction electrons can still travel at the
Fermi velocity (the velocity of electrons at the
Fermi energy). When an electric field is applied, a slight imbalance develops and mobile electrons flow. Electrons in this band can be accelerated by the
field because there are plenty of nearby unfilled states in the band.
Resistance comes about in a metal because of the scattering of electrons from defects in the lattice or by
phonons. A crude 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
\sigma = \frac{ne^2 \tau}{m}
where
n is the density of conduction electrons,
e is the electron charge, and
m is the electron mass. A better model is the so-called semi-classical theory, in which the effect of the periodic potential of the lattice on the electrons gives them an
effective mass.
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 electron 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
Superconductors are those materials which are extreme conductors. They are totally dependent on the temperature. As the temperature changes their properties also change.In metals and certain other materials, a transition to the
superconductivity state occurs at low (sub-cryogenic) temperature. By an interaction mediated by some other part of the system (in metals, phonons), the electrons pair up into
Cooper pairs. The boson 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 across a solution of
sodium+ and
chlorine–, the sodium ions will move constantly towards the negative electrode (anode), while the chloride ions will move towards the positive electrode (cathode). If the conditions are right,
redox reactions will take place at the electrode surfaces, releasing electrons from the chloride, and allow electrons to be absorbed into the sodium.
Water-ice and certain solid electrolytes called
proton conductors contain positive hydrogen ions which are free to move. In these materials, currents of electricity are composed of moving protons (as opposed to the moving electrons found in
metals).
In certain electrolyte mixtures, populations of brightly-colored ions form the moving electric charges. The slow migration of these ions during an electric current is one example of a situation where a current is directly visible to human eyes.
Gases and plasmas
In air, and other ordinary gases below the breakdown field, the dominant source of electrical conduction is via a relatively small number of mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electrical conductivity is extremely low, gases are dielectrics or
Electrical insulations. However, once the applied electric field approaches the dielectric breakdown value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and
ionizing, neutral gas atoms or molecules in a process called
avalanche breakdown. The breakdown process forms a Plasma (physics) that contains a significant number of mobile electrons and positive ions, causing it to behave as an electrical conductor. In the process, it forms a light emitting conductive path, such as a
spark, electric arc or
lightning.
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 a high electric or alternating magnetic field as noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions, and hence carry the bulk of the current.
Vacuum
Since a "free space" contains no charged particles, vacuums normally behave as perfect insulators (they would be the greatest insulators known to man) However, metal electrode surfaces can cause a region of the vacuum to become conductive by injecting free electrons or ions through either field emission or
thermionic emission. Thermionic emission occurs when the thermal energy exceeds the metal's work function, while field emission occurs when the electric field at the surface of the metal is high enough to cause quantum tunneling, which results in the ejection of free electrons from the metal into the vacuum. Externally heated electrodes are often used to generate an
electron cloud as in the electrical filament or indirectly heated
cathode of
vacuum tubes. cold cathode can also spontaneously produce electron clouds via thermionic emission when small incandescent regions (called
cathode spots or
anode spots) are formed. These are incandescent regions of the electrode surface that are created by a localized high current flow. These regions may be initiated by field emission, but are then sustained by localized thermionic emission once a
vacuum arc forms. These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field.
Vacuum tubes and
Krytrons are some of the electronic switching and amplifying devices based on vacuum conductivity.
See also
Electrical conduction - Wikipedia, the free encyclopedia
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