Physics for Geologists, Second edition

8 Electricity and magnetism
Electricity
We tend to think of electricity in terms of the lights, heat or music that come
on at the flick of a switch. Here we are not so much concerned with these,
but rather with a general understanding of the nature of electricity, and
some specific applications. We are very much aware of electricity during
a thunderstorm; and may become more aware of it in dry climates when
static electricity is sufficient to give us a small shock when touching a door
handle or putting a key in a lock.
Electricity is all around us, and in everything in our lives. Atoms consist
of charged particles, negatively charged electrons and positively charged
protons. Atoms remain intact because unlike charges attract and like charges
repel each other. Ampkre and Faraday showed that electricity and magnetism
are intimately connected.
The force of attraction (unlike charges, with different signs) or repulsion
(like charges, with the same signs) between two point charges in a vacuum
is given by Coulomb's Law:
where Q1 and Q2 are the point charges (in units of coulombs) which may be
positive or negative, r is the distance between them, and c is the proportionality
constant. When the medium between the point charges is not a vacuum,
the proportionality constant equals 1/4nkso N m2 cP2. In this expression
k is called the dielectric constant or relative permittivity (dimensionless),
and EO is the permittivity of free space or of vacuum.
Electric charges are transportable and transferable, as in batteries and
along wires. If you rub some materials with silk, both the silk and the material
acquire a charge that can be taken somewhere else with but slow dissipation.
You can discharge an object by touching it, provided you are not insulated
from the Earth. So there are materials that conduct electricity with ease,
and others that do not. All metals are good conductors, as are some nonmetals.
Good conductors of electricity are also good conductors of heat, and
Copyright 2002 by Richard E. Chapman