It may be defmed as the property of a substance due to which it opposes (or restricts) the flow of electricity (i.e., electrons) through it.
Metals (as a class), acids and salts solutions are good conductors of electricity. Amongst pure metals,silver, copper and aluminium are very good conductors in the given order. This, as discussed earlier, is due to the presence of a large number of free or loosely-attached electrons in their atoms. Thesevagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide and other atoms and electrons, thereby producing heat.
Those substances which offer relatively greater difficulty or hindrance to the passage of these electrons are said to be relatively poor conductors of electricity like bakelite, mica, glass, rubber, p.v.c. (polyvinyl chloride) and dry wood etc. Amongst good insulators can be included fibrous substances such as paper and cotton when dry, mineral oils free from acids and water, ceramics like hard porcelain and asbestos and many other plastics besides p.v.c. It is helpful to remember that electric friction is similar to friction in Mechanics.
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