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Unit of electric current. S.I. unit of electric current is ampere. It is also the practical unit of current. It is denoted by A.
1 ampere (A) = 1 coulomb (C)/1 second (s) = 1 CS-1
The current through a wire is called one ampere, if one coulomb of charge flows through the wire in one second.
Note: In a metallic conductor, free electrons are carriers of electricity and hence electrons constitute the electric current (charge on an electron = 1.6 x 10-19). 6.25 x 1018 electrons crossing per second through any section of a conductor give rise to a current of 1 A.” 44. (a) Satish K. Gupta, op.cit., p. 1264
(b)
"Distinction Between Metals, Insulators and Semiconductos Metals are good conductors of electricity, insulators do not conduct electricity, while the semi- conductors have conductivity in between those of metals and insulators. Let us make distinction between conductors, insulators and semiconductors on the basis of band theory of solids.
A solid is a large collection of atoms. The energy levels of an atom get modified due to the presence of other surrounding atoms and the energy levels in the outermost shells of all the atoms form valence band and the conduction band separated by a forbidden energy gap.
The energy band formed by a series of energy levels containing valence electrons is called valency band At 0 K, the electrons start filling the energy levels in valence band starting from the lowest one. The highet energy level, which an electron can occupy in the valence band at 0 K is called Fermi level.
The lowest unfilled energy band formed just above the valence band is called conduction band.
At 0 K, the Fermi level as well as all the lower energy levels are completely occupied by the electrons. As the temperature rises, the electrons absorb energy and get excited. The electrons jump to the higher energy levels. These electrons in the higher energy levels are comparatively at larger distance from the nucleus and are more free as compared to the electrons in the lower energy levels. Depending on the energy gap between valence band and the conduction band, the solids behave as conductors, insulators and semiconductors."
Text-book of Physics (xii), part II, pp 192, 193—
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