A First Course in Physics · Ch. XI, §§ 299–304 · Chapter 11: The Transference of Heat
34. Radiation, and heating and ventilating a house
Goal: Your student can explain radiation as the third way heat travels and how it differs from conduction and convection, and explain how stoves, furnaces and hot-water systems heat and ventilate a house.
You'll need
- The book, open to these pages
- A sunny window or a lamp, and a piece of cardboard to block it
- Two identical jars of water, one wrapped in black paper and one in white or foil, and a thermometer
- A candle, a tall glass jar and a saucer of water (with an adult)
- A notebook and pencil
What's in the book
Heat from a fire or the sun that is neither conducted nor convected; radiation travels at the speed of light, in straight lines, and through air without heating it; the principle of ventilation (a candle under a lamp chimney); fresh air needed in houses; hot-air, hot-water and indirect heating. Eight questions follow.
The lesson, step by step
Hook question
SayThe sun is 93 million miles away across empty space. How does its heat reach us?
Read together
Read §§ 299–304. The three differences in § 300: radiation is as fast as light, it goes in straight lines, and it can pass through something without heating it.
Try it: block the radiation
Sit in the sun (or near a lamp) and feel the warmth on your face. Hold up the cardboard: the warmth stops at once, even though the air is the same. That is radiation traveling in straight lines.
Try it: black and white jars
Set the black-wrapped and white-wrapped jars of water in the sun for an hour and take their temperatures. Dark surfaces take in more radiation. (Chapter XXII returns to this.)
Try it: candle and chimney (with an adult)
Stand a lit candle in a saucer of water and lower the tall jar over it so the rim sits in the water: the flame dims and dies as the oxygen is used up (§ 301). Lift the rim slightly and it brightens. Safety: an adult lights the candle and handles the jar; keep hair and sleeves back.
Heating a house
Trace the arrows in Figs. 211 and 213. Hot air or hot water rises from the furnace, gives up its heat in the rooms, and the cooler air or water sinks back. Ask: where does fresh air come in?
What we know now: today radiation from the sun and fire is understood as electromagnetic waves (infrared light), which Chapter XXII introduces.
Say it back
Your student names the three ways heat travels and gives a household example of each.
Questions
Work questions 1–8 on book page 226.
Afterwards
Find where the heat in your home comes from (furnace, radiators, heat pump, stove) and trace how it reaches each room.
Answers to the book's problems
1. Sawdust stops the air from circulating, so there is no convection. 2. Hot water rises out of the top of the boiler and the cooled water returns to the bottom. 3. Radiation. 4. Direct radiation: chiefly radiation and convection from the radiators in the rooms; indirect: convection of air warmed by the coils. 5. Breath warmer than the hands warms them; fanning in summer speeds evaporation from the skin. 6. Thick glass heats unevenly, since glass conducts poorly; the inside expands before the outside and it cracks. 7. Into the cold room: warm air flows out at the top and cold air in at the bottom. 8. Packed tightly it holds less trapped air, and air is the poorest conductor.
Worked out for this site from the scan. If a number in your copy differs, trust the book.