Historical Introduction

We usually associate electricity with the 20th Century, during which it revolutionized the lives of countless millions of ordinary people, in much the same manner as steam power revolutionized lives in the 19th Century. It is, therefore, somewhat surprising to learn that people have known about electricity for many thousands of years. In about 1000 BC, the ancient Greeks started to navigate the Black Sea, and opened up trade routes, via the river Dnieper, to the Baltic region. Amongst the many trade items that the Greeks obtained from the Baltic was a substance which they called ``electron'', but which we nowadays call amber. Amber is fossilized pine resin, and was used by the Greeks, much as it is used today, as a gem stone. However, in about 600 BC, the ancient Greek philosopher Thales of Miletus discovered that amber possesses a rather peculiar property: i.e., when it is rubbed with fur, it develops the ability to attract light objects, such as feathers. For many centuries, this strange phenomenon was thought to be a unique property of amber.

In Elizabethan times, the English physician William Gilbert coined the word ``electric'' (from the Greek word for amber) to describe the above mentioned effect. It was later found that many materials become electric when rubbed with certain other materials. In 1733, the French chemist du Fay discovered that there are, in fact, two different types of electricity. When amber is rubbed with fur, it acquires so-called ``resinous electricity.'' On the other hand, when glass is rubbed with silk, it acquires so-called ``vitreous electricity.'' Electricity repels electricity of the same kind, but attracts electricity of the opposite kind. At the time, it was thought that electricity was created by friction.

Scientists in the 18th Century eventually developed the concept of electric charge in order to account for a large body of observations made in countless electrical experiments. There are two types of charge: positive (which is the same as vitreous), and negative (which is the same as resinous). Like charges repel one another, whilst opposite charges attract. When two bodies are rubbed together, charge can be transfered from one to the other, but the total charge remains constant. Thus, when amber is rubbed with fur, there is transfer of charge such that the amber acquires a negative charge, and the fur an equal positive charge. Likewise, when glass is rubbed with silk, the glass acquires a positive charge, and the silk an equal negative charge. The idea that electrical charge is a conserved quantity is attributed to the American scientist Benjamin Franklin (who is also to blame for the unfortunate sign convention in electricity). The law of charge conservation can be written:

In any closed system, the total electric charge remains constant.

Of course, when summing charge, positive charges are represented as positive numbers, and negative charges as negative numbers.

In the 20th Century, scientists discovered that the atoms out of which ordinary matter is composed consist of two components: a relatively massive, positively charged nucleus, surrounded by a cloud of relatively light, negatively charged particles called electrons. Electrons and atomic nuclii carry fixed electrical charges, and are essentially indestructible (provided that we neglect nuclear reactions). Under normal circumstances, only the electrons are mobile. Thus, when amber is rubbed with fur, electrons are transferred from the fur to the amber, giving the amber an excess of electrons, and, hence, a negative charge, and the fur a deficit of electrons, and, hence, a positive charge. Substances normally contain neither an excess nor a deficit of electrons, and are, therefore, electrically neutral.

The SI unit of electric charge is the coulomb (C). The charge on an electron is $-1.602\times 10^{-19}$ C.