Magnetism and Matter

Lesson

Part 1 — the bar magnet seen as a tiny current loop: its field lines, its field at different points and its behaviour in a uniform field.

In this lesson you will

Describe the field lines of a bar magnet and compare them with electric field lines
Explain why a bar magnet behaves like an equivalent solenoid
Describe the field of a magnetic dipole on its axis and equatorial line, and the torque on it in a uniform field (qualitatively)

Concept

Properties of a bar magnet

A freely suspended bar magnet settles roughly north–south. Its two ends are its north (N) and south (S) poles.

Like poles repel; unlike poles attract.
Breaking a magnet gives two smaller magnets, each with N and S: isolated poles (monopoles) are not found.
The magnetic moment $\vec m$ points from S to N inside the magnet (unit A m²).

Table of bar-magnet properties: directive, poles, poles in pairs and breaking into complete magnets
Property	Observation
Directive	Hangs roughly north–south
Poles	Attraction is strongest near the ends
Pairs	N and S always come together
Breaking	Each piece is a complete magnet

Diagram

Traced numerically from the equivalent solenoid (Biot–Savart over its circular turns).

Magnetic field lines form continuous closed loops: outside the magnet they run from N to S, and inside from S to N.