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Introduction

We have seen that the particle model of matter helps us understand the properties of gases and the phenomena of phase changes. To do that, we had to look at matter at a molecular and atomic level.

If we look even closer, we find that atoms are made of nuclei surrounded by electrons. Nuclei and electrons share a property called electric charge.

Nuclei carry positive charge, and electrons have a negative charge. The unit of electric charge is the coulomb, symbol C; the charge on an electron is −1.6 × 10−19 C, and its mass is 9.1 × 10−31 kg.

Like energy, charge is conserved; in any process, the total charge before is the same as the total charge after.

In a metal, most of the electrons are attached to particular nuclei; these electrons can only move around their respective nuclei. But some electrons are only loosely attached to nuclei and are free to move throughout the volume of the metal; these are called free electrons. Because nuclei are much heavier than electrons, they cannot move—they are fixed in a lattice of positive charges. This chapter deals with what happens when free electrons are forced to move in the same direction in an electric circuit.