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:
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.
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.
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.
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.