The Space Around a Wire

What this book is about

The chapter title gives the game away, and almost nobody notices. It is not called "electricity" and "magnetism". It is called *the magnetic effect of an electric current* — and then, as if it were a separate matter, *magnetism*. Two halves, joined by an "and", taught a fortnight apart, examined in the same paper.

They are not two subjects. They are one, and this book is arranged to make that unmissable: every magnet you have ever held, including the Earth, is moving charge and nothing else. Once that is genuinely believed rather than merely recited, the second half of the chapter stops being a new topic and becomes an application of the first.

The one idea to carry through: there is no such thing as magnetism without moving charge. A current makes a field; a field pushes anything that moves through it; and a lump of iron is magnetic because inside every one of its atoms there is a charge going round.

If you are short of time, the pages that matter most are page 25 (the same field, by two completely different roads), page 33 (why a magnetic force does no work — the idea most often got wrong) and page 57 (a current loop and a bar magnet).

What is inside

• A current bends a compass: what a field is, how big a tesla is, and the right-hand grip rule
• The field of a current — Biot–Savart one piece at a time, the straight wire, the loop,

Ampère's shortcut, the solenoid and the electromagnet

• A field and a moving charge: the sideways force, why it does no work, circles and

helices, the cyclotron and the Hall effect

• A field and a current: F = BIL, Fleming's left hand, two wires and the definition of the

ampere, torque on a coil, the motor, the galvanometer, and building an ammeter

• Where magnetism comes from — the pole that is not there, a current loop as a magnet,

inside the atom, domains, the Curie point and hysteresis

• The Earth as a magnet, where its field comes from, the magnetosphere and reversals
• Two experiments with a torch cell, everything on one sheet, eighteen exercises with

answers, a Bangla–English glossary and a quiz