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Magnetism and Matter
Introduction
The Bar Magnet
Practice: The Bar Magnet
Magnetism and Gauss's Law
Magnetisation and Magnetic Intensity
Magnetic Properties of Materials
Practice: Magnetisation and Magnetic Materials
Summary
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Magnetism and MatterIntroduction

Reading time: ~10 min

Aman's class is on a trek to a high meadow in Uttarakhand. Mist hides the trail markers, so the guide hands Aman a small compass: whichever way he turns it, the needle swings back and settles pointing north. That evening, Riya drops her fridge-magnet keychain on a rock and it snaps cleanly in two.

Riya picks up the two halves. What does she have now?

One half with only a north pole, the other with only a south pole
Two smaller magnets, each with its own north pole and south pole
Two pieces of plain iron, with no magnetism left
One magnet and one piece that is not a magnet

Each half is a complete magnet with a north pole and a south pole of its own, only a little weaker. In the last chapter you saw that moving charges, electric currents, produce magnetic fields: a discovery of the early nineteenth century, credited to Oersted, Ampère, Biot and Savart among others. Magnetism is everywhere, from distant galaxies to the tiniest atoms, and the earth's magnetism is older than the human race. In this chapter we look at magnetism as a subject in its own right.

The Monopole Hunt

Can you cut your way to a lone north pole? Tap anywhere on the magnet to cut it there, then cut the pieces again.

Here are the ideas about magnets that everyone knows:

  1. The earth behaves like a magnet, with its magnetic field pointing roughly from the geographic south to the geographic north.
  2. A bar magnet hung freely turns to point north–south. The end that points to the geographic north is its north pole; the end that points south is its south pole.
  3. Two north poles (or two south poles) brought close together repel each other; the north pole of one magnet and the south pole of another attract.
  4. A north or south pole cannot be isolated. Break a bar magnet in two and you get two similar, weaker magnets. Unlike electric charges, isolated north and south poles, called magnetic monopoles, do not exist.
  5. Magnets can be made out of iron and its alloys.

The word magnet comes from Magnesia, an island of Greece where deposits of magnetic ore were found as early as 600 BC. In this chapter we describe a bar magnet and how it behaves in an external field, Gauss's law for magnetism, and how materials are classified by their magnetic properties into diamagnetic, paramagnetic and ferromagnetic ones.

Sina