A First Course in Physics · Ch. XII, §§ 315–321 · Chapter 12: Magnetism
36. Why magnets work, and the earth as a magnet
Goal: Your student can explain the molecular theory of magnetism (breaking, heating, jarring, saturation), and describe the earth's magnetism, declination and dip.
You'll need
- The book, open to these pages
- A steel sewing needle or a paper clip straightened out, a magnet and a compass
- Pliers or wire cutters (adult)
- A steel rod or long screwdriver and a hammer (for § 321)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
A tube of iron filings becomes a magnet when stroked; heating, jarring and breaking a magnet; the theory that molecules are tiny magnets lined up; saturation; Gilbert and the earth as a great magnet; declination and the map of lines of equal declination; dip of the compass needle; magnetizing a steel rod by tapping it in the earth's field. Ten questions follow, with a portrait of William Gilbert.
The lesson, step by step
Hook question
SayIf you break a magnet in half, do you get a north piece and a south piece?
Read together
Read §§ 315–321. Gilbert's idea, as the book gives it:
the earth itself is a great magnet with an S pole near the geographical north pole and an N pole near the geographical south pole.
Try it: break a magnet
Magnetize a straightened paper clip by stroking it with a magnet, check both ends with the compass, then have an adult snip it in half. Each half has both a north and a south end (Fig. 226).
Explain the theory
Each molecule acts as a tiny magnet. In unmagnetized iron they point every which way; magnetizing lines them up (Figs. 227–229). Jarring or heating shakes them out of line. When all are lined up the magnet is saturated.
Try it: the earth magnetizes a rod
Hold a steel rod pointing north and tilted steeply down (the dip in most of the United States is about 60–70°, as the book says) and tap the top end sharply with a hammer a dozen times. Test with the compass: the lower end now attracts the compass's south end (§ 321).
What we know now
The earth's magnetic poles wander. The north magnetic pole has moved hundreds of miles since Ross found it in 1831 and is now in the Arctic Ocean, heading toward Siberia. Its magnetism comes from currents in the earth's molten iron core. And the "tiny magnets" in iron are now known to be the spinning electrons of its atoms, lined up in small regions called domains.
Say it back
Your student explains why a compass does not point exactly north in most places (declination, § 319).
Questions
Work questions 1–10 on book page 237. For question 5, the force between poles is (strength × strength) ÷ distance², in dynes.
Afterwards
Look up your town's magnetic declination online and check it with the compass against a street that runs true north–south.
Answers to the book's problems
1. No; it only turns to point north–south, since the earth pulls its two poles equally in opposite directions. 2. Yes, a magnet's near pole attracts more strongly than the far pole repels; the earth is so far away that its pull on the two poles is equal. 3. So that it is balanced before it is magnetized and any dip afterward is due only to magnetism. 4. No; the earth's pull on its N and S poles is equal and opposite. 5. 6 × 1 ÷ 25 = 0.24 dyne; with a pole of strength 9, 2.16 dynes. 6. The magnet induces an opposite pole in the near end of the iron, which it then attracts. 7. An S pole. 8. Hang or float the magnet and bring the iron near it: the magnet moves toward the iron. 9. A compass needle would have no fixed direction; a dipping needle would point straight down. 10. The keeper becomes a magnet by induction and helps hold the molecules in line.
Worked out for this site from the scan. If a number in your copy differs, trust the book.