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.
An electric current is a movement of these free electrons in a certain direction
by the application of a voltage.
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.
The Amp can be defined in the following way: