You have seen how well quantum physics works when we apply it to questions .involving individual atoms. In this chapter we hope to show, with a single broad example, that this theory works just as well when we apply it to questions involving assemblies of atoms in the form of solids. Every solid has an enormous range of properties that we can choose to examine. Is it transparent? Can it be hammered out into a thin sheet? At what speeds do sound waves travel through it? Is it magnetic? Is it a good heat conductor? The list goes on and on. However, we choose to focus this entire chapter on a single question:What are the ‘JIechanisms by which a solid conducts, or does not conduct, electricity?
As you wine, quantum physics provides the answer.