Basic units

Charge and conduction

The basic element of charge is the electron, which is assumed to have a negative charge – by convention equal to 1.6x10-19 Coulombs1 (the reason for the strange value is that the Coulomb was defined without reference to the basic particles) and we often represent charge as Q. Normally, electrons are bound closely around atoms and molecules with equal numbers of positive charges and this means that there is charge neutrality in most materials. However, in certain materials, the electrons can be persuaded to break way from the atoms and move through the material. This leaves an atom in the material with a positive charge that is not balanced by an electron but the atom will capture other electrons moving in the material and, over time, the number of electrons leaving will balance the number of electrons flowing into a region of the material. The type of material influences the difficulty of persuading electrons to break free and move. In metals, this process is easy and metals conduct charge without much difficulty – such materials are called conductors. Certain materials conduct quite badly: it is relatively difficult to persuade the electrons to move – such materials are called semiconductors. Finally, there are certain materials in which it is extremely difficult to persuade electrons to break free – such materials are called insulators.


1Named after Charles Augustin de Coulomb, 1736-1806
(see:http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Coulomb.html )