When voltage is applied to a piece of metal wire, as shown in
figure 1.2 (a), the current *I* flowing through the wire is
proportional to the voltage *V* across two points in the wire. This
property is known as Ohm's law, which reads

where *R* is called resistance, and *G* is called
conductance. The resistance *R* and the conductance *G* of the same
piece of wire is related by *R* = 1/*G*.
Resistance is measured in *ohms* ()
and conductance in *siemens* (S or ).

**Figure 1.2:** Ohm's law. (a) Metal wire; (b) circuit symbol

Any apparatus/device that has this property is called a *
resistor.* Study of the
physics of resistance shows that it is proportional to the
length of the metal wire, *l*, and inversely proportional to the
cross-sectional area, *A*, i.e.,
where the proportionality constant is known as the
*resistivity* of the metal.

We may calculate the power required to
pass current *I* through a resistor of resistance *R* using the
previously derived formula, i.e.,

Using the Ohm's law equation, we get
The last inequality defines a property called *passivity.*

Tue Mar 10 13:15:28 HKT 1998