A First Course in Physics · Ch. VI, §§ 160–167 · Chapter 6: Molecular Forces
17. Molecular forces in liquids: surface tension and capillarity
Goal: Your student can explain why liquids pull into drops, why soap films shrink, why water rises in thin tubes and paper towels, and how a needle can float.
You'll need
- The book, open to these pages
- A bowl of water, a needle or paper clip, a small piece of tissue paper
- Dish soap and a pipe cleaner or wire loop with a loop of thread
- A paper towel strip and a glass of colored water
- Two clean glass plates or two clear plastic rulers (optional)
- A notebook and pencil
What's in the book
Water clings to a glass plate; why a free liquid forms a sphere (oil in alcohol-and-water); soap films shrinking; liquids rising (water) or falling (mercury) in capillary tubes, and why; capillarity in wicks, towels, sugar and blotting paper; a needle floating on water. Six questions follow.
The lesson, step by step
Hook question
SayHow can a water strider walk on a pond without sinking?
Read together
Read §§ 160–167. Why drops are round:
Now the geometrical figure which has the smallest area for a given volume is a sphere.
Try it: float a needle
Lay a small square of tissue on the water and gently set a needle or paper clip on it. The tissue soaks and sinks, leaving the needle floating. Look closely for the little dent in the surface around it (§ 167). Now touch the water with a soapy finger: the needle sinks.
Try it: soap films
Tie a loose loop of thread across a wire loop, dip it in soapy water, and pop the film inside the thread loop with a dry pencil point. The thread snaps into a perfect circle (§ 162, Fig. 110).
Try it: capillary rise
Dip the end of a paper-towel strip in colored water and watch the color climb. If you have two glass plates, hold them together with a matchstick between them at one edge and stand them in colored water: the water climbs highest where the gap is narrowest (§ 163).
Explain the idea
Water molecules pull on each other, so the surface acts like a stretched skin. Where water touches glass, the glass pulls it up the wall (adhesion stronger than cohesion), so the water rises in a narrow tube. Mercury does not wet glass, so it is pushed down (described in the book; we don't use mercury at home).
Say it back
Your student explains why water rises higher in a thinner tube.
Questions
Work questions 1–6 on book page 121.
Afterwards
Watch drips from a faucet, raindrops on a waxed car, and water climbing a sugar cube dipped in coffee or tea.
Answers to the book's problems
1. Blotting paper is full of tiny open channels (capillary tubes); glazed paper has its pores filled. 2. 320 cm (ten times as high in a tube one tenth as wide). 3. The melted grease rises into the fine pores of the blotting paper. 4. The water pulls itself into drops, the shape with the least surface. 5. When soft, the glass's surface pulls it into the shape with least surface, rounding the corners. 6. The pressure forces the particles so close together that cohesion holds them.
Worked out for this site from the scan. If a number in your copy differs, trust the book.