Skip to content

Physics · Ch 5 — Magnetism and Matter

Introduction

5.1

Introduction

The Universal Nature of Magnetism

Magnetism is not a rare phenomenon — it is everywhere. From the largest galaxies to the smallest atoms, magnetic fields are present. Even the Earth itself is a giant magnet, and its magnetism existed long before humans evolved. The word magnet comes from Magnesia, a region in Greece where magnetic rocks (lodestone) were found around 600 BC.

Historical Context

In the previous chapter, you learned that moving charges (electric currents) produce magnetic fields. This key discovery was made in the early 1800s by scientists like Oersted, Ampere, Biot, and Savart. Now, we study magnetism as a subject in its own right.

Five Key Ideas About Magnetism

The chapter begins by recalling five fundamental observations:

  1. Earth as a Magnet: The Earth behaves like a giant magnet. Its magnetic field points approximately from the geographic south to the geographic north.
  2. Bar Magnet Orientation: A freely suspended bar magnet always aligns itself in the north-south direction. The end pointing towards the geographic north is called the north pole, and the end pointing towards the geographic south is called the south pole.
  3. Force Between Poles:
    • Like poles (north-north or south-south) repel each other.
    • Unlike poles (north-south) attract each other.
  4. No Magnetic Monopoles: Unlike electric charges, you cannot isolate a single magnetic pole (north or south). If you break a bar magnet into two halves, you do not get a separate north pole and a separate south pole. Instead, you get two smaller bar magnets, each with its own north and south pole. This means magnetic monopoles do not exist.
  5. Magnetic Materials: Magnets can be made from iron and its alloys.

What This Section Covers

This introductory section sets the stage for the rest of the chapter, which will cover:

  • The detailed behaviour of a bar magnet in an external magnetic field.
  • Gauss's law of magnetism.
  • Classification of materials based on their magnetic properties: paramagnetism, diamagnetism, and ferromagnetism.
← Previous chapterCh 4 — Moving Charges and MagnetismNext · 5.2 →The Bar Magnet