Introduction

So far, electric charges have been studied mainly at rest -- electrostatics. This chapter turns to charges in motion: an electric current is nothing but charge flowing steadily through a conductor, and understanding how and why it flows is the foundation for every practical circuit, from a torch bulb to a power grid. The chapter builds up in a deliberate order: first the microscopic picture of current in a metal, in terms of the drift of free electrons and their mobility (Sections 3.2-3.4); then Ohm's law and the macroscopic quantities resistance, resistivity and conductivity that describe how strongly a material resists this flow, together with how resistance depends on temperature (Sections 3.5-3.7); then how resistors combine when wired together (Section 3.8); then the EMF and internal resistance of a real cell, and how cells combine (Sections 3.9-3.10); then Kirchhoff's two general rules for solving any network too complicated for simple series-parallel reduction (Section 3.11); and finally three classic null-deflection measuring instruments built entirely from these ideas -- the Wheatstone bridge, the metre bridge, and the potentiometer (Sections 3.12-3.14).