Insulators

Most solid materials are classified as insulators because they offer very largeresistance to the flow of electric current. Metals are classified as conductorsbecause their outer electrons are not tightly bound, but in most materials even the outermost electrons are so tightly bound that there is essentially zero electron flow through them with ordinary voltages. Some materials are particularly good insulators and can be characterized by their high resistivities:

	Resistivity (ohm m)
Glass	1012
Mica	9 x 1013
Quartz (fused)	5 x 1016

This is compared to the resistivity of copper:

	Resistivity (ohm m)
Copper	1.7 x 10-8

Energy band model

Dielectric insulators