________________
39.
Ohm = voltampere The constant R appearing in equation depends on the dimensions, nature and the temperature of the conductor. (b) Satish K. Gupta op. cit, p. 113“Conductors. Insulators and Dielectrics The carriers of current in metals are free electrons and the free electron model for the metal can explain some of the observed properties of the metals in a qualitative manner. The elements, in which the valence shell is filled less than half, are found to be good conductors. For example, in metals such as copper, aluminium, silver, etc., the valence shell contains three or less electrons. Since an atom has a tendency to have a filled valence shell, the valence electrons in the atoms of a metal leave the atoms and are free to move through the metal lattice in a random manner. They constantly collide among themselves and with the positive ions located in the metal lattice. They have practically no affinity to their parent atoms. However, the average velocity of free electrons in a metal is zero. When an external electric field is applied across the two ends of a metal, the free electrons experience force and get accelerated. There is a net flow of electrons through the metal. It is found that as the strength of the applied electric field is increased, more and more free electrons cross through a section of the metal. The material which do not have free electrons in them are unable to conduct electricity and are termed as insulators. In fact, the same material may possess the following two properties: (i) It may not conduct electricity through it and as such it is
called insulator. (ii) It may not conduct electricity through it but on applying
electric field, induced charges are produced on its faces. Such, a material is called dielectric. The valence electrons in atoms of a dielectric are tightly bound to their nuclei
and ordinarily cannot detach themselves. Kakani, Saxena, Chhajer & Lodha, op.cit. p.2 - "Certain materials, such as silver, copper, aluminium and nickel contains relatively large number of free electrons are called conductors; materials such as glass, dry wood, silk and porcelain,
(c)
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