The Origin of Electricity

18.1. The Origin of Electricity

An intrinsic property of protons and electrons, which make up all matter, is electric charge. Only two types of electric charge have been discovered, positive and negative; a proton has a positive charge, and an electron has a negative charge.

The SI unit for measuring the magnitude of electric charge is the coulomb (C). Each proton carries a charge +e, and each electron carries a charge -e, where

e = 1.60 × 10^-19 C

This charge e is the smallest amount of free charge that is possible. Charges larger than this are integer multiples of e, that is, q = Ne where N is the number of electrons (or protons) present.

Example 1

How many electrons are contained in 3.20 × 10^-6 coulombs of charge?

Since the total charge is q = Ne we can write

Because any electric charge, q, occurs as integer multiples of the elementary charge e, electric charge is said to be quantized.

Since the charges on the electron and proton have identical magnitudes but opposite signs, the algebraic sum of the two charges is zero. Since most materials have equal numbers of protons and electrons, most matter is electrically neutral. However, it is possible to transfer electric charge from one object to another. Usually electrons are transferred, and the body that gains electrons acquires an excess of negative charge. The body that loses these electrons then acquires an excess of positive charge. In this sense, charge can be "separated". However, during any process, the net electric charge of an entire isolated system remains constant (is conserved). This is referred to as the law of conservation of electric charge.

Charge separation is important in the operation of electrical equipment. For example, batteries, alternators in cars, and electric power generators all depend on the separation of electric charges for their operation.

It is easy to demonstrate that two electrically charged objects exert a force on one another. If the two objects contain charge of the same sign, the force will be one of repulsion, i.e., the objects push one another apart. If the objects contain charge of opposite sign, the force will be one of attraction and the objects will pull each other closer together. It is a fundamental characteristic of electric charges that like charges repel and unlike charges attract each other. The magnitude of the force exerted by one point charge on another was experimentally determined by the French physicist Charles-Augustin Coulomb. His result, which has become known as Coulomb's law, is stated in the next section.