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# 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