Tài liệu Tiếng anh chuyên ngành điện - điện tử

Unit 1
Conductors, insulators and
semiconductors
I. Reading and comprehension:
If we connect a battery across a body, there is a movement of free electrons
towards the positive end. This movement of electrons is an electric current.
All materials can be classified into three groups according to how readily they
permit an electric current to flow. These are: conductors, insulators and
semiconductors.
In the first category are substances which provide an easy path for an electric
current. All metals are conductors, however some metals do not conduct well.
Manganin, for example, is a poor conductor. Copper is a good conductor,
therefore it is widely used for cables. A non-metal which conducts well is
carbon. Salt water is an example of a liquid conductor.
A material which does not easily release electrons is called an insulator.
Rubber, nylon, porcelain and air are all insulator. There are no perfect
insulators. All insulators will allow some flows of electrons, however this can
usually be ignored because the flow they permit is so sm