A First Course in Physics · Ch. VII, §§ 177–190 · Chapter 7: Thermometry; Expansion Coefficients
19. Thermometers and temperature scales
Goal: Your student can explain how a thermometer is made and marked, convert between Celsius (Centigrade) and Fahrenheit, and explain absolute zero and the absolute (Kelvin) scale.
You'll need
- The book, open to these pages
- An alcohol or digital thermometer
- A glass of ice and water, stirred
- Three bowls: warm, room-temperature and cold water
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Why our sense of hot and cold is unreliable; Galileo's air thermometer; making and marking a Centigrade mercury thermometer at melting ice and steam; the Fahrenheit scale; converting between them; the range of mercury and alcohol thermometers; the standard hydrogen thermometer; Charles's law; absolute zero and the absolute scale; low temperatures; the maximum and minimum thermometer. Eleven problems follow, with a portrait of Lord Kelvin.
The lesson, step by step
Hook question
SayPut one hand in warm water and one in cold for a minute, then both in the room-temperature bowl. Does the middle bowl feel warm or cold?
It feels different to each hand. That is why the book says we need thermometers (§ 178).
Read together
Read §§ 177–190. The meaning of absolute zero:
This temperature is called the absolute zero, because no lower temperature can possibly exist
Try it: check the zero point
Stir a glass of crushed ice and water for a couple of minutes and put the thermometer in. It should read 0° C (32° F), just as in Fig. 131. If it doesn't, your thermometer reads that much off; note the error.
Explain the scales
On the Centigrade (Celsius) scale ice melts at 0° and water boils at 100°. On the Fahrenheit scale they are 32° and 212°: 180 degrees apart instead of 100. So:
F = C × 9/5 + 32, and C = (F − 32) × 5/9.
Practice: 15° C = 59° F; 100° F ≈ 37.8° C; −40° is the same on both scales.
Absolute zero
Gas pressure falls by 1/273 of its 0° value for every degree of cooling. At −273° C the molecules would stop pushing at all (§ 188). Absolute temperature T = t + 273.
What we know now: absolute zero is −273.15° C, and the absolute scale is now called the Kelvin scale (K), after Lord Kelvin, whose portrait is in this chapter. Scientists have cooled atoms to within a billionth of a degree of absolute zero, but it can never quite be reached.
Watch only: mercury
Watch only: the book's thermometers and gas thermometer use mercury, which is poisonous. Use an alcohol or digital thermometer at home, and if an old silver mercury thermometer breaks, don't touch the beads; ask an adult to follow local cleanup advice.
Say it back
Your student explains how the 0° and 100° marks are put on a new thermometer (§ 180).
Problems
Work questions 1–11 on book page 137.
Afterwards
Record the outdoor temperature at the same time each day this week in both °C and °F.
Answers to the book's problems
1. 59° F. 2. About 37.8° C. 3. About −17.8° C; −40° C. 4. −292° F. 5. −436° F. 6. About −459° F (−273° C). 7. A long thin bulb has more surface, so it takes up the body's temperature quickly. 8. The glass bulb warms and expands first, before the mercury does. 9. The narrower the bore, the farther apart the marks. 10. The temperature at which gas molecules would stop moving and a gas would exert no pressure: −273° C. 11. Pumping away the vapor makes the liquid air evaporate faster, and evaporation cools it.
Worked out for this site from the scan. If a number in your copy differs, trust the book.