Page 31 - Teach Yourself Electricity and Electronics

Insulators 11

Sometimes oxygen atoms are by themselves; then we denote the molecule simply as O.
Sometimes there are three atoms of oxygen grouped together. This is the gas called
ozone, that has received much attention lately in environmental news. It is written O 3 .
All matter, whether it is solid, liquid, or gas, is made of molecules. These particles
are always moving. The speed with which they move depends on the temperature. The
hotter the temperature, the more rapidly the molecules move around. In a solid, the
molecules are interlocked in a sort of rigid pattern, although they vibrate continuously
(Fig. 1-4A). In a liquid, they slither and slide around (Fig. 1-4B). In a gas, they are lit-
erally whizzing all over the place, bumping into each other and into solids and liquids
adjacent to the gas (Fig. 1-4C).

Conductors

In some materials, electrons move easily from atom to atom. In others, the electrons
move with difficulty. And in some materials, it is almost impossible to get them to move.
An electrical conductor is a substance in which the electrons are mobile.
The best conductor at room temperature is pure elemental silver. Copper and alu-
minum are also excellent electrical conductors. Iron, steel, and various other metals are
fair to good conductors of electricity.
In most electrical circuits and systems, copper or aluminum wire is used. Silver is
impractical because of its high cost.
Some liquids are good electrical conductors. Mercury is one example. Salt water is
a fair conductor.
Gases are, in general, poor conductors of electricity. This is because the atoms or
molecules are usually too far apart to allow a free exchange of electrons. But if a gas be-
comes ionized, it is a fair conductor of electricity.
Electrons in a conductor do not move in a steady stream, like molecules of water
through a garden hose. Instead, they are passed from one atom to another right next to
it (Fig. 1-5). This happens to countless atoms all the time. As a result, literally trillions
of electrons pass a given point each second in a typical electrical circuit.
You might imagine a long line of people, each one constantly passing a ball to the
neighbor on the right. If there are plenty of balls all along the line, and if everyone keeps
passing balls along as they come, the result will be a steady stream of balls moving along
the line. This represents a good conductor.
If the people become tired or lazy, and do not feel much like passing the balls along,
the rate of flow will decrease. The conductor is no longer very good.

Insulators

If the people refuse to pass balls along the line in the previous example, the line repre-
sents an electrical insulator. Such substances prevent electrical currents from flowing,
except possibly in very small amounts.
Most gases are good electrical insulators. Glass, dry wood, paper, and plastics are
other examples. Pure water is a good electrical insulator, although it conducts some
current with even the slightest impurity. Metal oxides can be good insulators, even
though the metal in pure form is a good conductor.