Resistivity
The resistivity r is an intrinsic property of a material, like density or specific heat. For a wire of cross section A, 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 more cross section, you get more current. If the wire is longer, you get less current. The resistivity is measured in units of ohm-meters.
See tables 20.1 and 20.2 in your textbook for a list of resistivities of various materials. Metals like copper or silver have low resistivities and are known as conductors. 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.
The resistivity is temperature-dependent, just as density can be. This is illustrated in lecture by applying a blow torch to iron wire and glass.