Electric charges

One aspect of electricity that we are familiar with in our everyday lives is that of static cling - when two objects, such as a piece of Saran wrap and a wool sweater, are rubbed together, they cling. One feature of this that we don't encounter too often is static ``repulsion'' - if each piece of Saran wrap is rubbed on the wool sweater, then the pieces of Saran wrap will repel when brought near each other. These phenomena are interpreted in terms of the objects acquiring an electric charge, which has the following features:

• There are two types of charge, which by convention are labelled positive and negative.
• Like charges repel, and unlike charges attract.
• All objects may have a charge equal to an integral number of a basic unit of charge.
• Charge is never created or destroyed.