Introduction

Orientation

This short opening section sets up the chapter rather than supplying examinable results. It records that Hans Christian Oersted first discovered that a magnetic field is produced by an electric current in a wire, and that the later work of Gauss, Henry, Faraday and Maxwell showed the electric and magnetic fields to be closely linked partners — the basis of motors, generators, communication systems and computers.

Where electrostatics dealt with charges at rest and the forces between them, this chapter deals with charges in motion: the fields they create and the forces they feel.

The two facts carried forward

• Right hand thumb rule: grab the current carrying wire with the right hand, thumb pointing along the current; the curled fingers give the direction in which the magnetic field circles the wire.
• Two hanging wires connected to a cell repel each other when their currents are in opposite directions and attract when the currents are in the same direction. The wires stay electrically neutral overall, so the force cannot be electrostatic — it is magnetic in origin. This experiment is the thread the whole chapter follows, and it is settled quantitatively in Section 10.11.