22
TEXT A
Read the following text and write the main idea of the text in one
sentence. Compare it with your groupmates
In electrical engineering materials
may be classified into three groups,
i.e.,
conducting
materials,
insulating
materials and semi conducting materials
depending on their ability to conduct
electricity.
The first category includes
conductors, i.e. materials which provide
an easy path for an electric current.
Conductors are materials that obey
Ohm’s law and have very low resistance.
We have already mentioned the electron
theory which states that all matter is
composed of atoms. Atoms consist of a small positive nucleus surrounded by a
cloud of electrons. Conductors are materials where some of these electrons are
free to move. These free electrons, considered identical to the outermost, or
valence electrons, are not constrained to remain in a particular atom. They are
able to move freely in matter or a vacuum when an external electric field acts
on them. The electric conductivity of the material is dependent upon the atomic
structure of the material of which the conductors are made.
Normally,
conductors have three or less valence electrons, insulators have five or more
and semiconductors usually have four valence electrons.
To end up with,
the
materials in which it is easy to get electrons to move and provide a flow of
electric current, are conductors. They are mostly metals,
such as
copper,
aluminium, silver, gold, etc.
Copper and copper-based alloys are unique in their desirable combination
of physical and mechanical properties.
Due to
their high electrical and thermal
conductivity, they are very important in electrical industry. Copper is widely
used for cables, transformer windings etc.
Although
silver is a slightly better
conductor than copper, it is too expensive for common use. Aluminium is not as
good conductor as copper, but it is cheaper and lighter. It is resistant to
atmospheric weathering and today it is the dominant metal for the transmission
lines of electrical energy.
Materials which offer high resistance to current flow are called insulators.
Even the best insulators do release an occasional free electron to serve as a
current carrier.
However,
for most practical purposes we consider an insulator
Fig.5
