Conductors and Insulators

"What is the use of a new-born child ?"
Benjamin Franklin
(when asked what was the use of a new invention)

Moving electric charges constitute what is know as an electric current. It is the electric currents in semi-conductor devices which are responsible for the electronic technology in today's society.

Conductors are materials which allow the free movement of electric charge. Examples include,

Metals
Some liquids
Gas plasmas

Insulators (or non conductors) are materials which provide significant resistance to the flow of electric charge. Examples include,

Non metals - plastic, wood, glass, rubber etc.
Gases

Semi-conductors are materials whose resistance to current flow falls between conductors and insulators. There are very few such materials, but their importance in electronic technology cannot be emphasized enough. Examples,

Silicon
Germanium

Mechanisms of conduction:

Metals (solid)

Each atom in the solid is "fixed", forming a lattice.
Outer electrons in a metal are weakly bound to the atomic nucleus.
When an external electric field is applied these outer electrons move through the material creating an electric current.

Liquid conductors and gas plasmas

Conducting liquids and gases are comprised of positive and negative ions (charged particles).
Both positive and negative ions move when an external electric field is applied, thus creating the current.
A positive charge moving to the right creates the same current as an equal negative charge moving to the left.

Insulators

All electrons in these materials are tightly bound to the atomic nuclei. External electric fields are typically not large enough to cause any flow of charge.

Semi-conductors

These materials have a small number of weakly bound electrons, the number of which is very dependent on the temperature and potential difference applied across the material.

It is important to realise that because sustained electric currents only occur when a potential difference is maintained in a closed circuit, as many charge carriers enter as leave any part of the circuit. In other words electric current is not "used up"; it has the same value everywhere in the circuit.

Marilyn Monroe suggests to Einstein: What do you say, professor, shouldn't we marry and have a little baby together: what a baby it would be - my looks and your intelligence!
Einstein: I'm afraid, dear lady, it might be the other way around...
Albert Einstein

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu