When you turn on an electric light an electric current flows in the
wire. Do not think of it like water coming from a tap – the electricity current does not flow out from
the switch – it is already in the wire – connecting the lamp to the power supply via the switch
simply gives it the energy to flow.
This energy can come from a variety of sources –
kinetic as in a dynamo, a chemical reaction in a cell, light falling on a photoelectric cell, heating the
junction of two metals in a thermocouple, sound in a microphone or mechanical stress in a piezo-
electric crystal.
When an electric current flows electrical energy is converted to other
forms of energy such as heat, light, chemical, magnetic and so on.
We will now look
more closely at the nature of an electric current.
Consider a piece of metal wire - a very much
enlarged view of which is shown in Figure 1.
A piece of wire is made of millions of atoms and each one of these has its own cloud of
electrons. However in a metal there is a large number of electrons that are not held around
particular nuclei but are free to move at high speed and in a random way through the metal. These
are known are free electrons and in a metal there are always
large numbers of these.
It is when these free electrons are all made to move in a certain direction
by the application of a voltage across the metal that we have an electric current.
The
difference between a metal (a large and constant number of free electrons), a semiconductor (a
few free electrons, the number of which varies with temperature) and an insulator (which has no free
electrons) is shown in Figure 2.
Each
electron has only a very small amount of electric charge, and it is more convenient to use a larger
unit when measuring practical units of charge. This unit is the coulomb.
The
charge on one electron is -1.6x10-19 C.
Usually written as e. You would need about
5x1018 electrons to have a charge of one coulomb!
The electrical charge
passing any one point in a circuit in one second is called the electric current, and it is measured in
Amperes (A).
The Amp can be defined in the following way: