Introduction
Conduction is the property of a material, associated with flow of electric current. Electrical conductivity , is the reciprocal of resistivity, and is expressed in -1m-1. Conductivity is related to electrical resistance R as l /A, where l is the length and A is the area of cross section of the material. Though all substances contain electrons; only a few of them are good conductors. Only those electrons which are comparatively loosely bound to an isolated atom and which behave like free electrons in a solid contribute to electrical conduction.
Conducting materials are classified into three major categories, viz.,
High resistivity materials:
Materials like tungsten, nichrome etc., (resistivity in the order, x10-6 ohm-m) are used in the manufacturing resistors, heating elements etc.
Low resistivity materials:
Materials like copper, aluminium etc. (resistivity in the order, x10-8 ohm-m) are used as conductors in electrical power transmission and distribution, winding wires in motors and transformers.
Zero resistivity materials:
Materials like alloys of aluminium, Zinc, gallium etc., below their transition temperature, conduct electricity with zero resistance. Such materials are used for superconducting magnets, memory storage etc.
Free electron theory
In the year 1900, Drude and Lorentz developed the classical free electron theory which assumes that metals contain free electrons obeying the laws of classical mechanics.
Postulates
• A solid metal is made up of atoms with nucleus at centre around which electrons are revolving.
• In a metal, the valance electrons of atoms are free to move about the entire volume of the metal like the molecule of a perfect gas in a container.
• In the absence of electric field the electrons move in random directions, making collisions with positive ions fixed in the lattice and the collisions are elastic.
• When an electric field is applied, the free electrons are accelerated and move in the opposite direction of the applied electric field.
• The free electrons in the metal move freely everywhere without any mutual interaction between them.
• The free electrons are moving in a completely uniform potential field due to ions which are fixed in the lattice.
Conductivities:
Basically conductivities can be classified as thermal conductivity and electrical conductivity.
Thermal conductivity (K)
Thermal conductivity of a material is defined as the amount of heat flowing per unit time though unit cross section of the material maintained at unit temperature gradient and is expressed in W/m/K.
Conduction is the property of a material, associated with flow of electric current. Electrical conductivity , is the reciprocal of resistivity, and is expressed in -1m-1. Conductivity is related to electrical resistance R as l /A, where l is the length and A is the area of cross section of the material. Though all substances contain electrons; only a few of them are good conductors. Only those electrons which are comparatively loosely bound to an isolated atom and which behave like free electrons in a solid contribute to electrical conduction.
Conducting materials are classified into three major categories, viz.,
High resistivity materials:
Materials like tungsten, nichrome etc., (resistivity in the order, x10-6 ohm-m) are used in the manufacturing resistors, heating elements etc.
Low resistivity materials:
Materials like copper, aluminium etc. (resistivity in the order, x10-8 ohm-m) are used as conductors in electrical power transmission and distribution, winding wires in motors and transformers.
Zero resistivity materials:
Materials like alloys of aluminium, Zinc, gallium etc., below their transition temperature, conduct electricity with zero resistance. Such materials are used for superconducting magnets, memory storage etc.
Free electron theory
In the year 1900, Drude and Lorentz developed the classical free electron theory which assumes that metals contain free electrons obeying the laws of classical mechanics.
Postulates
• A solid metal is made up of atoms with nucleus at centre around which electrons are revolving.
• In a metal, the valance electrons of atoms are free to move about the entire volume of the metal like the molecule of a perfect gas in a container.
• In the absence of electric field the electrons move in random directions, making collisions with positive ions fixed in the lattice and the collisions are elastic.
• When an electric field is applied, the free electrons are accelerated and move in the opposite direction of the applied electric field.
• The free electrons in the metal move freely everywhere without any mutual interaction between them.
• The free electrons are moving in a completely uniform potential field due to ions which are fixed in the lattice.
Conductivities:
Basically conductivities can be classified as thermal conductivity and electrical conductivity.
Thermal conductivity (K)
Thermal conductivity of a material is defined as the amount of heat flowing per unit time though unit cross section of the material maintained at unit temperature gradient and is expressed in W/m/K.
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