The resistivity r is an intrinsic property of a material, like density or specific heat. For wire of cross section A, the resistance is:

where L is the length of the material and A is the cross-sectional area. This is analagous 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 in units of ohm-meters. See tables 20.1 and 20.2 from your text 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 and silicon is an example of a semi-conductor. The resistivity is temperature-dependent, just like density can be temperature dependent. This is illustrated in class with the blow torch demo.

Examples     Resistor's index