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Not all substances are good conductors of electricity. As a general
rule, metals are good conductors whereas nonmetals are poor
conductors. The poorest of conductors are commonly called
"insulators," or "nonconductors." Aluminum, copper, gold, iron,
mercury, nickel, platinum, and silver are examples of good
conductors. Amber, glass, mica, paper, porcelain, rubber, silk, and
sulfur are all nonconductors. The difference between a conductor and
an insulator is that in a conductor, there are free electrons,
whereas in an insulator, all of the electrons are tightly bound to
their respective atoms. In an uncharged body, there are an equal
number of positive and negative charges. In metals, a few of the
electrons are free to move from atom to atom, so that when a
negatively charged rod is brought to the end of the conductor, it
repels nearby free electrons in the conductor, causing them to move.
They in turn repel free electrons in front of them, giving rise to a
flow of electrons all along the conductor. There are a large number
of substances that are neither good conductors of electricity nor
good insulators. These substances are called "semi-conductors." In
them, electrons are capable of being moved only with some difficulty,
i.e., with considerable force.
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