A First Course in Physics · Ch. IV, §§ 99–112 · Chapter 4: Pressure in Air
12. Siphons, pumps, divers and balloons
Goal: Your student can explain how a siphon, a lift pump and a force pump work, and make a Cartesian diver that sinks and rises on command.
You'll need
- The book, open to these pages
- A 1–2 meter length of clear plastic tubing and two buckets or bowls
- A plastic bottle with cap, a glass medicine dropper or a ketchup packet (for the Cartesian diver)
- A bicycle pump
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
The siphon and the intermittent siphon; air pumps and compression pumps; the lift pump and force pump; the Cartesian diver; balloons and parachutes; diving bells and diving suits; the air brake; the bellows; the gas meter. Fourteen problems follow.
The lesson, step by step
Hook question
SayHow could you empty a fish tank without tipping it over or using a pump?
Read together
Read §§ 99–112. Your student can skim the machine descriptions (§§ 110–112) and study the diagrams of the pumps (Figs. 78–79) closely.
Try it: a siphon
Fill the tube completely with water by holding it under water in a full bucket, cap both ends with thumbs, and put one end in the full bucket and the other in an empty bucket below it. Let go: water flows uphill over the edge and down. Raise the lower bucket above the upper one and watch the flow reverse (§ 99). Don't start the siphon by sucking with your mouth.
Try it: Cartesian diver
Fill a plastic bottle to the brim. Put in a dropper with just enough water in it to barely float (or a ketchup packet). Cap the bottle. Squeeze: the diver sinks. Let go: it rises. Ask your student to explain using three ideas: Pascal's law, Boyle's law and Archimedes' principle (§ 106).
Explain the pumps
A lift pump does not "suck" water up; it removes air so the atmosphere pushes the water up the pipe. That is why it can never lift water more than about 34 ft (§ 104). A force pump pushes water up from below. Have your student trace the valves in Fig. 79 stroke by stroke.
Balloons and divers
Read § 107: a balloon rises if it weighs less than the air it pushes aside. Ask: why is a balloon only partly filled at the start? Read § 109 and discuss why divers' ears hurt going down.
Party balloons and blimps today use helium, which cannot burn; hydrogen catches fire easily.
Say it back
Your student explains how a siphon works, using the picture in Fig. 72.
Problems
Work a selection of problems 1–14 on book pages 80–81: try 2, 4, 5, 8, 9, 12, 13 and 14.
Afterwards
Use the siphon to change the water in a fish tank or water the garden from a rain barrel.
Tip
The Cartesian diver works best with the bottle filled right to the top.
Answers to the book's problems
2. 1.5 atmospheres × 1033.6 g × 50 sq. cm, about 77,500 g (77.5 kg). 4. One half; 1/8 left after 3 strokes; 1/1024 after 10. 5. (3/4)⁵ ≈ 0.237 of the air is left; the barometer reads about 18 cm. 6. No; each stroke removes only a fraction of what is left. 8. 10 atmospheres. 9. About 1114 kg (1,200,000 g of air minus 85,700 g of hydrogen). 10. Each balloon carried about 2312 lb. (20,000 ÷ 64 = 312 lb. of letters, plus 2000 lb.); with the book's figure of 12 cu. ft. of air to the pound, coal gas lifts 1/30 lb. per cu. ft., so about 69,000 cu. ft. 11. When it weighs as much as the air it displaces. 12. One half (1033 cm of water is one atmosphere). 13. 40 cu. ft. (at 34 ft. the pressure is 2 atmospheres). 14. Density 0.0024; the barometer reads 152 cm.
Worked out for this site from the scan. If a number in your copy differs, trust the book.