We often use the
tools that are made of paper, plastic, rubber, wax, wood, aluminum, even a
material made of iron and steel in electric tools in everyday life. The atomic structure of matter affects
how easily charges, i.e., electrons, move through a substance
and hence how it is used electrically. Electrically, materials are classified
as conductors, insulators, or semiconductors.
Conductors
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| the example of conductors |
Materials through which charges move
easily are termed conductors. The most familiar examples are metals.
Good metal conductors have large numbers of free electrons that are able to
move about easily. In particular, silver, copper, gold, and aluminum are
excellent conductors. Of these, copper is the most widely used. Not only is it
an excellent conductor, it is inexpensive and easily formed into wire, making
it suitable for a broad spectrum of applications ranging from common house
wiring to sophisticated electronic equipment. Aluminum, although it is only
about 60% as good a conductor as copper, is also used, mainly in applications where
light weight is important, such as in overhead power transmission lines. Silver
and gold are too expensive for general use. However, gold, because it oxidizes
less than other materials, is used in specialized applications; for example,
some critical electrical connectors use it because it makes a more reliable connection than other materials.
Insulators
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| the example of insulator |
Materials that do not conduct (e.g.,
glass, porcelain, plastic, rubber, and so on) are termed insulators. The
covering on electric lamp cords, for example, is an insulator. It is used to
prevent the wires from touching and to protect us from electric shock. Insulators
do not conduct because they have full or nearly full valence shells and thus
their electrons are tightly bound. However, when high enough voltage is
applied, the force is so great that electrons are literally torn from their
parent atoms, causing the insulation to break down and conduction to occur. In
air, you see this as an arc or flashover. In solids, charred insulation usually
results.
Semiconductors
Silicon and germanium (plus a few
other materials) have half-filled valence shells and are thus neither good
conductors nor good insulators. Known as semiconductors, they have
unique electrical properties that make them important to the electronics
industry. The most important material is silicon. It is used to make
transistors, diodes, integrated circuits, and other electronic devices.
Semiconductors have made possible personal computers, VCRs, portable CD
players, calculators, and a host of other electronic products. You will study
them in great detail in your electronics courses.