A First Course in Physics · Ch. XI, §§ 291–298 · Chapter 11: The Transference of Heat
33. Heat on the move: conduction and convection
Goal: Your student can tell good conductors from poor ones, explain why metal feels colder than wood at the same temperature, and explain convection currents, winds and sea breezes.
You'll need
- The book, open to these pages
- A metal spoon, a wooden spoon and a plastic spoon, and a mug of hot tap water
- A metal tray and a wooden board or carpet (to touch)
- Food coloring, a clear glass of cold water and a small jar of hot tap water
- A notebook and pencil
What's in the book
Metals conduct heat much better than wood or glass; a table of conductivities; water and air are poor conductors; conduction as molecule passing motion to molecule; why metal feels colder; air in fur, feathers and snow; the Davy safety lamp; convection currents in water; winds, land and sea breezes, and ocean currents. Six questions follow.
The lesson, step by step
Hook question
SayOn a cold morning, why does a metal doorknob feel colder than a wooden door, when both are the same temperature?
Read together
Read §§ 291–298. The book's explanation:
In general, the better a conductor the hotter it will feel to a hand colder than itself, and the colder to a hand hotter than itself.
Try it: spoon race
Stand the three spoons in the mug of hot tap water. After a minute, feel the handles. The metal one is warm; wood and plastic stay cool. Then touch the metal tray and the wooden board: same room temperature, different feel.
Try it: watch convection
Put a few drops of food coloring in the small jar of hot water, cover the top with your thumb (or plastic wrap), lower it gently into a big glass or bowl of cold water and uncover it. A colored plume rises to the top: hot water is lighter and rises (§ 297).
Explain the idea
Conduction passes heat from molecule to molecule (§ 293). Convection carries heat by moving whole currents of liquid or gas. Air is a very poor conductor, so things full of trapped air (fur, feathers, wool, fresh snow, foam cups) keep heat in (§ 295). Sea breezes come from the land heating faster than the water (§ 298).
Watch only: gauze, flames and burning ether
Watch only: don't try this at home. The Davy-lamp gauze over an open gas jet (§ 296) and the burning ether on water (§ 292) need a lab. Describe what happens: the flame will not pass through the wire gauze, because the gauze carries the heat away.
Say it back
Your student explains why a wool sweater keeps you warm even though it makes no heat itself (question 6).
Questions
Work questions 1–6 on book page 219.
Afterwards
Notice convection this week: shimmering air above a hot road, a radiator warming a room, smoke rising.
Answers to the book's problems
1. Flannel is a poor conductor: in summer it keeps the outside heat away; in winter it keeps the body's heat in. 2. The metal carries the heat away from the paper so fast that it cannot reach its burning point; wood does not. 3. The metal pan conducts heat into the hand much faster than the bread. 4. Iron draws heat from the moist skin so quickly that the moisture freezes; wood conducts too slowly. 5. Paper is a poor conductor and keeps the earth's heat around the plants. 6. No; it keeps the body's own heat from escaping.
Worked out for this site from the scan. If a number in your copy differs, trust the book.