Conduction

Conduction
Atoms are said to attract each other (remember like charges repel
and unlike charges attract). It can be seen then that some sort of
force has been created. This is found in every day life, if you unwrap
a piece of polythene and try to put it down you find it sticks to your
fingers, you call this static. A better way to describe static electricity
is electricity that is standing still, or Voltage potential with no electron
flow. It is actually the attraction of unlike charges.
We call it static electricity, demonstrated below, by rubbing a silk cloth
over a glass rod which is a charge that is stationary or at rest.

In theory we look at electrical charge as being point charges, this is
because of Coulomb’s Law, which states that:
The force between 2 point charges of a body is found to vary
inversely as the square of the distance between them, and
directly with the magnitude of charge.

We have already looked at how static electricity is formed, using this
principle and coulombs law it can be said that conduction of electricity
can occur across any medium examples of this are below:
Solid
Electrical conduction through solids occurs when a potential
difference is applied across the material, the free electrons in the
material will be attracted/ repelled along the material to try and
equalize their potential. An example of this is a metal bar.
Liquid
If a liquid solution is to conduct electricity it is called an electrolyte, it
is achieved by immersing 2 electrodes into the solution and creating a
potential difference across them, the free electrons in the liquid will
then be attracted/repelled, towards/from the electrodes, an example
of this is a cell. The liquid is being used as the medium by which the
ions flow through.
Gases
Normally most gases do not have free electrons from which
conduction can occur, therefore they are considered as a good
insulator, or dielectric, however if a high enough potential difference
is applied across the gas this will cause the electrons within the gas
to break free and become mobile, and so conduction to their opposite
polarity charge will occur. A good example of this is a lightning strike,
where there is a huge potential difference between the cloud and
earth the PD is so high that the electrons in the gas are freed up to
produce a charge.

Vacuum
Since vacuums contain no charged particles, they are normally very
good insulators, however a metal electrode present in the vacuum
can make it conductive, by adding charged particles in a cloud of free
electrons through a process known as thermoionic emission.
External to the vacuum the electrode is heated so that the electrons
are released, these electrons are then free to move through the
vacuum towards their opposite charge. An example of this is a
Cathode ray tube.

Electric current

An electric current is a flow of electric charges. The current can flow
quite easily through some materials, called conductors, and finds it
nearly impossible to flow through others, termed insulators.
Let us now think of how current flows through a conductor, most
conductors are metals such as copper, silver and gold (Refer to
Figure 2). All metals have less than their full complement of electrons
in the outer shell, and those that are present are loosely bound to
their parent atom. They can easily be detached from the atom and
move about in the space between atoms. For this reason they are
called free electrons.
So if an electron leaves, remember it takes its negative charge with it,
leaving behind a positive ion. The interior of the metal under normal
conditions can now be visualized as a framework of positive ions in a
fixed regular pattern known as a crystal lattice, through which the free
electrons may move freely. At temperatures above absolute zero the
free electrons are in a constant state of motion that changes, with

temperature. The positive ions are also vibrating about their mean
position in the crystal lattice.
In spite of all this intense activity within the interior of the metal, there
is no overall movement of electrons, and the piece of metal as a
whole is electrically neutral since the total number of negative
charges is equal to the positive charges.
External Charge
If we now bring an external charge near that metal the electrons will
be forced into a flow either towards the charge or away from it
depending on the type of external charge. If the external charge was
a battery, we know it has 2 terminals, a positive and negative.
Therefore the electrons of our metal would be attracted to the positive
terminal and you have an instantaneous current flow. You would also
get a force of attraction of the positive ions towards the negative
terminal but as they are held in the crystal lattice they cannot move.
We will look at these 2 effects a little later.

Non metallic insulators
Non-metallic materials are normally materials that do not have many
free electrons in their outer shell and the attraction between the
electrons and their parent atom is very strong. It therefore follows
that current flow through these materials would be virtually
impossible, these materials we call insulators. Typical insulators are
rubber, ceramics, glass and PVC’s.

Semiconductors
There is another type of material that falls in between both conductor
and the insulator, we call it a semiconductor. It is this material that
has given rise to the electronic age of computers. The special
properties of the semiconductor are such that under normal
conditions it is an insulator and does not pass current, but under
certain conditions it can be made to pass current, and then can be
made to return to its normal non conducting state again without any
damage. This switching can be done hundreds of thousands of times
a second if required. Materials commonly used are silicon and
germanium. The force that is used to switch semiconductors is
voltage.

The structure of matter

All pure substances are made up from a relatively few basic
substances called elements, either separately or combined together
to form what is known as a compound. There are 92 such elements
occurring naturally.
The ultimate particle to which an element can be reduced to is called
the atom. However, many elements cannot exist in a stable form as
individual atoms, but only in groups of atoms. The smallest part of an
element or compound that normally exists in a free state is the
molecule. The molecule of an element may consist of one or more
atoms of that element. The molecule of a compound consists of 2 or
more atoms of different elements. For example, the molecule of the
element oxygen is made up of 2 oxygen atoms, whereas the
compound of water is made up of 2 hydrogen atoms and one oxygen
atom. In a pure compound each molecule contains the same number
of atoms of each element. It follows therefore that elements can only
combine to form compounds in certain fixed proportions.
Of course, not all substances are pure in the sense that every
molecule is identical. Many of the substances met in every day life
are simply mixtures of elements or compounds.
When the structure of matter is looked at below the atomic level, it is
found that the atoms of all elements are made up of 3 main
components, protons, electrons and neutrons. All except hydrogen,
which does not contain a neutron. The characteristics of a particular
element are determined by the number of each of these components
in its atom.

Every atom consists of a central nucleus around which one or more
electrons orbit. Electrons carry a negative electrical charge; the
protons situated in the nucleus have a positive charge. The third part
of atom is the neutron; this is also in the nucleus but has no charge.
Not a lot is known about the nature of these charges except that there
are 2 kinds and they are opposite to each other, and when we bring
them together, like charges repel and unlike charges attract. Under

normal conditions the number of electrons and protons in an atom are
equal, it is then said to be electrically neutral
The neutron has the same mass as the proton, so nearly all the
weight of an atom is contained in the nucleus. Electrons have a
much smaller mass and may be easily removed from their orbit
around the nucleus.
The electrons orbit the nucleus in layers we call shells, and there is a
limit to the number of electrons which can be accommodated in each
shell. Working outwards from the nucleus, the K shell can have a
maximum of 2 electrons, the L shell can have eight, M shell eighteen
and so on. For example copper has twenty-nine protons and twentynine
electrons orbiting. The layers or shells are made up as 2
electrons in the K shell 8 in the L shell, eighteen in the M shell leaving
one to orbit in the N shell. For the atoms to combine to form
elements they share the electrons