A First Course in Physics · Ch. IV, §§ 81–91 · Chapter 4: Pressure in Air
10. The weight of air and the barometer
Goal: Your student can explain why air presses on everything, describe Torricelli's and Pascal's experiments, and explain how a barometer works.
You'll need
- The book, open to these pages
- A drinking glass, water, and a piece of stiff paper or an index card (do this over the sink)
- A drinking straw and a glass of water
- A suction cup
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Air has weight; a can crushed by air pressure; why water rises in pumped-out tubes and the 32-foot limit of pumps; Torricelli's mercury tube (1643); the size of atmospheric pressure; Pascal's mountain experiment; the barometer and weather; Guericke's water barometer; the aneroid barometer. Nine problems follow, with a portrait of Otto von Guericke.
The lesson, step by step
Hook question
SayWhen you drink through a straw, are you pulling the drink up, or is something pushing it up?
Read together
Read §§ 81–91. Torricelli's conclusion:
the rise of liquids in exhausted tubes is due to an outside pressure exerted by the atmosphere on the surface of the liquid, and not to any mysterious sucking power created by the vacuum.
Watch only: mercury and the crushed can
Watch only: don't try this at home. Torricelli's tube (§ 84) uses a long column of mercury, which is poisonous; the crushed-can experiment in § 82 means boiling water in a can and sealing it while hot. Describe what happened: the mercury fell to about 76 cm (30 in.) and stayed, and the can was crushed flat as the steam inside condensed. Look up a video of a teacher doing the "can crush" and "Torricelli barometer" demonstrations instead.
Try it: air holds up water
Over the sink, fill a glass to the brim with water, lay the card on top, hold it on, and turn the glass over. Let go of the card: the water stays up (question 7). The air pushes up on the card harder than the water's weight pushes down.
Try it: straws and suction cups
Drink through a straw, then pinch a small hole in the straw near the top and try again. Press a suction cup on a smooth surface and pull. In each case, ask: where is the air pushing?
Explain the idea
Air presses about 1 kg on every square centimeter (about 15 lb per square inch). That pressure holds up 76 cm of mercury, or about 34 ft of water. As you climb a mountain there is less air above you, so the barometer falls (Pascal and Perrier, § 87). A falling barometer often means a storm is coming.
Today's weather reports give air pressure in hectopascals or millibars: about 1013 at sea level.
Say it back
Your student explains why no suction pump can lift water more than about 34 ft.
Problems
Work problems 1–9 on book page 64. Problem 9 (the Magdeburg hemispheres) is a favorite.
Afterwards
Check the air pressure in a weather app each day for a week and note the weather. Does falling pressure come before rain?
Tip
Do the paper-and-glass trick over a sink the first few times.
Answers to the book's problems
1. About 1152 kg (891 cu. m × 1.293 kg). 2. About 82.77 cm (1 m of water adds 100 ÷ 13.6 = 7.35 cm of mercury). 3. Pressing out the air leaves almost no air under the leather, so the outside air presses it down on the stone. 6. 2 in. × 13.6 = 27.2 in. 7. The air pressure upward on the paper is greater than the weight of water; the paper keeps the water surface flat so air cannot bubble in. 8. The outside air presses them together and nothing inside pushes back. 9. About 5700 lb. (π × 11² = 380 sq. in. × 15 lb.).
Worked out for this site from the scan. If a number in your copy differs, trust the book.