Materials in general have a characteristic behavior of resisting the flow
of electric charge. This physical property, or ability to resist current, is
known as resistance and is represented by the symbol R. The resistance
of any material with a uniform cross-sectional area A depends on
A and its length , as shown in Fig. 2.1(a). We can represent resistance
(as measured in the laboratory), in mathematical form,
(2.1)
where is known as the resistivity of the material in ohm-meters. Good
conductors, such as copper and aluminum, have low resistivities, while
insulators, such as mica and paper, have high resistivities. Table 2.1
presents the values of for some common materials and shows which
materials are used for conductors, insulators, and semiconductors.
The circuit element used to model the current-resisting behavior of a
material is the resistor. For the purpose of constructing circuits, resistors
are usually made from metallic alloys and carbon compounds. The circuit