A First Course in Physics · Ch. XII, §§ 305–314 · Chapter 12: Magnetism
35. Magnets, poles and lines of force
Goal: Your student can state the law of magnetic poles, explain induced magnetism, and map a magnet's field with a compass and iron filings.
You'll need
- The book, open to these pages
- Two bar magnets (or strong fridge magnets) and a compass
- Iron filings in a sealed clear plastic bag or jar, or a few paper clips and small iron nails
- A sheet of paper or thin cardboard
- A sewing needle, a slice of cork and a bowl of water
- A notebook and pencil
What's in the book
Lodestones and the first compasses; poles of a magnet; like poles repel and unlike attract, with force falling off as the square of the distance; the unit magnet pole; magnetic materials; induced magnetism in soft iron; permeability and retentivity; lines of force traced by a floating magnetized needle; fields of force shown with iron filings; field strength as lines per square centimeter.
The lesson, step by step
Hook question
SayA compass needle swings to point north. What is pulling it?
Read together
Read §§ 305–314. The law of magnets:
Like poles repel each other, unlike poles attract.
Try it: poles and the law
Hang one bar magnet from a thread (or lay it on a floating dish) and let it settle north–south. Mark its north end. Bring the second magnet's ends close to each end in turn: like ends push away, unlike ends pull together.
Try it: a chain of nails
Hang a nail from one end of a magnet, then a second nail from the first, and a third (Fig. 219). Pull the magnet away carefully: the chain falls apart. The nails were magnets only while the magnet was near: induced magnetism (§ 310).
Try it: map the field
Lay the paper over a magnet and sprinkle iron filings from the sealed bag (or tap a bag of filings flat over it) to see the pattern of Fig. 223. Or move the compass around the magnet and draw an arrow where it points at each spot. Join the arrows into curved lines from N to S: lines of force.
Explain the idea
Soft iron becomes a magnet easily but loses it easily (high permeability); hard steel is hard to magnetize but keeps it (high retentivity) (§ 311). Field strength is shown by how closely the lines crowd together.
What we know now: the strongest modern magnets are made of special alloys such as neodymium-iron-boron, far stronger than the steel magnets of 1906. Today field strength is measured in teslas.
Try it: a floating needle
Stroke a sewing needle 30 times in one direction with one end of a magnet, push it through a thin slice of cork and float it in the bowl. It turns to point north–south: you have made a compass (and it moves along the lines of force when a magnet is held below the bowl, Fig. 222).
Say it back
Your student explains why a compass needle sets itself along a line of force.
Afterwards
Keep the homemade compass and check it against a real one each day.
Tip
Keep magnets away from phones, credit cards and computer screens, and never let small children swallow small magnets.