conduction

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conduction

conduction, transfer of heat or electricity through a substance, resulting from a difference in temperature between different parts of the substance, in the case of heat, or from a difference in electric potential, in the case of electricity. Since heat is energy associated with the motions of the particles making up the substance, it is transferred by such motions, shifting from regions of higher temperature, where the particles are more energetic, to regions of lower temperature. The rate of heat flow between two regions is proportional to the temperature difference between them and the heat conductivity of the substance. In solids, the molecules themselves are bound and contribute to conduction of heat mainly by vibrating against neighboring molecules; a more important mechanism, however, is the migration of energetic free electrons through the solid. Metals, which have a high free-electron density, are good conductors of heat, while nonmetals, such as wood or glass, have few free electrons and do not conduct as well. Especially poor conductors, such as asbestos, have been used as insulators to impede heat flow (see insulation). Liquids and gases have their molecules farther apart and are generally poor conductors of heat. Conduction of electricity consists of the flow of charges as a result of an electromotive force, or potential difference. The rate of flow, i.e., the electric current, is proportional to the potential difference and to the electrical conductivity of the substance, which in turn depends on the nature of the substance, its cross-sectional area, and its temperature. In solids, electric current consists of a flow of electrons; as in the case of heat conduction, metals are better conductors of electricity because of their greater free-electron density, while nonmetals, such as rubber, are poor conductors and may be used as electrical insulators, or dielectrics. Increasing the cross-sectional area of a given conductor will increase the current because more electrons will be available for conduction. Increasing the temperature will inhibit conduction in a metal because the increased thermal motions of the electrons will tend to interfere with their regular flow in an electric current; in a nonmetal, however, an increase in temperature improves conduction because it frees more electrons. In liquids and gases, current consists not only in the flow of electrons but also in that of ions. A highly ionized liquid solution, e.g., saltwater, is a good conductor. Gases at high temperatures tend to become ionized and thus become good conductors (see plasma), although at ordinary temperatures they tend to be poor conductors. See electrochemistry; electrolysis; superconductivity.
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Conduction (electricity)

The passage of electric charges due to a force exerted on them by an electric field. Conductivity is the measure of the ability of a conductor to carry electric current; it is defined as the ratio of the amount of charge passing through unit area of the conductor (perpendicular to the current direction) per second divided by the electric field intensity (the force on a unit charge). Conductivity is the reciprocal of resistivity and is therefore commonly expressed in units of siemens per meter, abbreviated S/m. See Electrical resistivity

In metals and semiconductors (such as silicon, of which transistors are made) the charges that are responsible for current are free electrons and holes (which, as missing electrons, act like positive charges). These are electrons or holes not bound to any particular atom and therefore able to move freely in the field. Conductivity due to electrons is known as n-type conductivity; that due to holes is known as p-type. See Hole states in solids, Semiconductor

The conductivity of metals is much higher than that of semiconductors because they have many more free electrons or holes. The free electrons or holes come from the metal atoms. Semiconductors differ from metals in two important respects. First, the semiconductor atoms do not contribute free electrons or holes unless thermally excited, and second, free electrons or holes can also arise from impurities or defects.

An exception to some of the rules stated above has been found in conjugated polymers. Polyacetylene, for example, although a semiconductor with extremely high resistance when undoped, can be doped so heavily with certain nonmetallic impurities (iodine, for example) that it attains a conductivity comparable to that of copper.

In metals, although the number of free carriers does not vary with temperature, an increase in temperature decreases conductivity. The reason is that increasing temperature causes the lattice atoms to vibrate more strongly, impeding the motion of the free carriers in the field. This effect also occurs in semiconductors, but the increase in number of free carriers with temperature is usually a stronger effect. At low temperatures the thermal vibrations are weak, and the impediment to the motion of free carriers in the field comes from imperfections and impurities, which in metals usually does not vary with temperature. At the lowest temperatures, close to absolute zero, certain metals become superconductors, possessing infinite conductivity. See Superconductivity

Electrolytes conduct electricity by means of the positive and negative ions in solution. In ionic crystals, conduction may also take place by the motion of ions. This motion is much affected by the presence of lattice defects such as interstitial ions, vacancies, and foreign ions. See Ionic crystals

Electric current can flow through an evacuated region if electrons or ions are supplied. In a vacuum tube the current carriers are electrons emitted by a heated filament. The conductivity is low because only a small number of electrons can be “boiled off” at the normal temperatures of electron-emitting filaments. See Electron emission


Conduction (heat)

The flow of thermal energy through a substance from a higher- to a lower-temperature region. Heat conduction occurs by atomic or molecular interactions. Conduction is one of the three basic methods of heat transfer, the other two being convection and radiation. See Convection (heat), Heat radiation, Heat transfer

Steady-state conduction is said to exist when the temperature at all locations in a substance is constant with time, as in the case of heat flow through a uniform wall. Examples of essentially pure transient or periodic heat conduction and simple or complex combinations of the two are encountered in the heat-treating of metals, air conditioning, food processing, and the pouring and curing of large concrete structures. Also, the daily and yearly temperature variations near the surface of the Earth can be predicted reasonably well by assuming a simple sinusoidal temperature variation at the surface and treating the Earth as a semi-infinite solid. The widespread importance of transient heat flow in particular has stimulated the development of a large variety of analytical solutions to many problems. The use of many of these has been facilitated by presentation in graphical form.

For an example of the conduction process, consider a gas such as nitrogen which normally consists of diatomic molecules. The temperature at any location can be interpreted as a quantitative specification of the mean kinetic and potential energy stored in the molecules or atoms at this location. This stored energy will be partly kinetic because of the random translational and rotational velocities of the molecules, partly potential because of internal vibrations, and partly ionic if the temperature (energy) level is high enough to cause dissociation. The flow of energy results from the random travel of high-temperature molecules into low-temperature regions and vice versa. In colliding with molecules in the low-temperature region, the high temperature molecules give up some of their energy. The reverse occurs in the high-temperature region. These processes take place almost instantaneously in infinitesimal distances, the result being a quasi-equilibrium state with energy transfer. The mechanism for energy flow in liquids and solids is similar to that in gases in principle, but different in detail.

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

Conduction

Movement of heat through a material. R-value is a measure of resistance to conductive heat flow.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved

conduction

[kən′dək·shən]
(electricity)
The passage of electric charge, which can occur by a variety of processes, such as the passage of electrons or ionized atoms. Also known as electrical conduction.
(physics)
Transmission of energy by a medium which does not involve movement of the medium itself.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Conduction (heat)

The flow of thermal energy through a substance from a higher- to a lower-temperature region. Heat conduction occurs by atomic or molecular interactions. Conduction is one of the three basic methods of heat transfer, the other two being convection and radiation. See Convection (heat), Heat transfer

Steady-state conduction is said to exist when the temperature at all locations in a substance is constant with time, as in the case of heat flow through a uniform wall. Examples of essentially pure transient or periodic heat conduction and simple or complex combinations of the two are encountered in the heat-treating of metals, air conditioning, food processing, and the pouring and curing of large concrete structures. Also, the daily and yearly temperature variations near the surface of the Earth can be predicted reasonably well by assuming a simple sinusoidal temperature variation at the surface and treating the Earth as a semi-infinite solid. The widespread importance of transient heat flow in particular has stimulated the development of a large variety of analytical solutions to many problems. The use of many of these has been facilitated by presentation in graphical form.

For an example of the conduction process, consider a gas such as nitrogen which normally consists of diatomic molecules. The temperature at any location can be interpreted as a quantitative specification of the mean kinetic and potential energy stored in the molecules or atoms at this location. This stored energy will be partly kinetic because of the random translational and rotational velocities of the molecules, partly potential because of internal vibrations, and partly ionic if the temperature (energy) level is high enough to cause dissociation. The flow of energy results from the random travel of high-temperature molecules into low-temperature regions and vice versa. In colliding with molecules in the low-temperature region, the high temperature molecules give up some of their energy. The reverse occurs in the high-temperature region. These processes take place almost instantaneously in infinitesimal distances, the result being a quasi-equilibrium state with energy transfer. The mechanism for energy flow in liquids and solids is similar to that in gases in principle, but different in detail.

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.

thermal conduction

The process of heat transfer through a material medium in which kinetic energy is transmitted by particles of the material from particle to particle without gross displacement of the particles.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

conduction

1. the transfer of energy by a medium without bulk movement of the medium itself
2. the transmission of an electrical or chemical impulse along a nerve fibre
3. Physics another name for conductivity
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005