**Resistivity**

The resistivity *r** * (Greek letter rho) is an intrinsic property of a material,
like density or specific heat. For a wire, the resistance is

where *L* is
the length of the wire and *A *is its cross-sectional
area. This is analogous to the friction of water going through a
pipe: if the wire has a larger diameter, you get more current;
if the wire is longer, you get less current. The resistivity is
measured in units of * ohm-meters*.

See tables in your textbook for a list of resistivities of various materials. The values span a huge range. Metals like copper or silver have very low resistivities and are known as conductors (they contain some electrons that can move rather freely). Insulators like rubber have high resistivities. Silicon is an example of a semi-conductor: its conductivity is intermediate between that of a conductor and that of an insulator. Practical devices based on semiconductors have profoundly transformed our technological society during the last 50 years.

The resistivity is temperature-dependent, just as density can be. This is illustrated in lecture by applying a blowtorch to iron wire and glass.