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3

CHAPTER

Electrical systems

electron
3.1 Electrical and electronic principles proton

3.1.1 Electrical fundamentals
To understand electricity properly we must start by fi nding out what it really
is. This means we must think very small. The molecule is the smallest part
of matter that can be recognized as that particular matter. Subdivision of the
molecule results in atoms. The atom is a basic unit of matter and consists
of a central nucleus made up of protons and neutrons. Around this nucleus
electrons are in orbit, like planets around the sun ( Fig. 3.1 ). The neutron is a
very small part of the nucleus. It has an equal positive and negative charge. neutron
nucleus
It is therefore neutral and has no polarity. The proton is another small part of
the nucleus. It is positively charged. As the neutron is neutral and the proton is
Figure 3.1 Basic representation of an atom
positively charged, this means the nucleus of the atom is positively charged.
The electron is an even smaller part of the atom, and is negatively charged.
It is held in orbit around the nucleus by the attraction of a positively charged
proton. When atoms are in a balanced state the number of electrons orbiting
the nucleus equals the number of protons. The atoms of some materials have
electrons which are easily detached from the parent atom and join an adjacent

atom. In so doing they move an electron (like polarities repel) from this atom to
a third atom and so on through the material. These are called free electrons.
Materials are called conductors if the electrons can move easily. However, in
some materials it is diffi cult to move the electrons. These materials are called
insulators ( Fig. 3.2 ).
If an electrical pressure (voltage) is applied to a conductor, a directional
Figure 3.2 Insulated conductors
movement of electrons will take place. There are two conditions for electrons
to fl ow: a pressure source, e.g. from a battery or generator, and a complete
conducting path for the electrons to move, e.g. wires.
An electron fl ow is termed an electric current. The battery positive terminal is
Defi nition
connected, through a switch and lamp, to the battery negative terminal. With
the switch open, the chemical energy of the battery will remove electrons from Electron
the positive terminal to the negative terminal via the battery. This leaves the A subatomic particle that carries a
negative electric charge. It has a
positive terminal with fewer electrons and the negative terminal with a surplus
mass that is approximately 1/1836
of electrons. An electrical pressure exists between the battery terminals. With
that of the proton (not very big then!).
the switch closed, the surplus electrons on the negative terminal will fl ow
through the lamp back to the electron-defi cient positive terminal. The lamp
will therefore light until the battery runs down.

Automobile Mechanical and Electrical Systems.
© 2011 Tom Denton. Published by Elsevier Ltd. All rights reserved.