CONDUCTORS AND INSULATORS
Materials can be broadly classified based on their ability to permit the flow of
charge. If electrons were placed on a metal sphere, they would quickly spread out
and cover the outside of the sphere uniformly. These electrons would be free to
flow through the metal and redistribute themselves, moving to get as far away from
one another as they could. Materials that permit the flow of excess charge are
called conductors; they conduct electricity. Metals are excellent conductors.
Metals conduct electricity because the structure of a typical metal consists of a
lattice of nuclei and electrons, with about one electron per atom not bound to its
nucleus. Electrons are free to move about the lattice, creating a sort of sea of
mobile electrons. This freedom allows excess charge to flow freely.
Insulators, however, closely guard their electrons, and even extra ones that might
be added. Electrons are not free to roam throughout the atomic lattice. Examples of
insulators are glass, wood, rubber, and plastic. If excess charge is placed on an
insulator, it stays put.
Midway between conductors and insulators is a class of materials known as
semiconductors. As the name indicates, they’re less conducting than most metals,
but more conducting than most insulators. Examples of semiconducting materials
are silicon and germanium.
An extreme example of a conductor is the superconductor. This is a material that
offers absolutely no resistance to the flow of charge; it is a perfect conductor of
electric charge. Many metals and ceramics become superconducting when they are
brought to extremely low temperatures.
- A solid sphere of copper is given a negative charge. Discuss the
electric field inside and outside the sphere.
Here’s How to Crack It
The excess electrons that are deposited on the sphere move quickly to the outer
surface (copper is a great conductor). Any excess charge on a conductor resides
entirely on the outer surface.