A First Course in Physics · Ch. IX, §§ 238–248 · Chapter 9: Work and Heat Energy
27. Heat from work: Joule and the conservation of energy
Goal: Your student can explain where wasted work goes, define the calorie, describe Joule's paddle-wheel experiment, and state the law of conservation of energy.
You'll need
- The book, open to these pages
- Hands to rub together
- A bicycle pump
- A thermometer and a jar of sand with a lid (shake it hard for several minutes)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Rubbing, drilling and hammering make things hot; Rumford's view that heat is molecular motion; the calorie; Joule's paddle-wheel experiment and others (about 427 gram meters of work per calorie); heating by compressing a gas and cooling by expansion; the conservation of energy; perpetual motion; chains of energy changes; the sun as the source of nearly all our energy. Six problems follow, with a portrait of Joule.
The lesson, step by step
Hook question
SayRub your hands together hard for ten seconds. Where did the warmth come from?
Read together
Read §§ 238–248. The great law:
Energy may be transformed, but it can never be created or destroyed.
Try it: shake the sand
Take the temperature of a jar of dry sand. Put on the lid and shake it hard for five minutes (take turns). Measure again: it is a little warmer. Your work went into faster-moving molecules, just as Joule's falling weights warmed the water (§ 240).
Pump up a bicycle tire and feel the pump barrel get warm (§ 242).
Explain the idea
A calorie is the heat that warms 1 gram of water 1° C. Joule found that the same amount of work, about 427 gram meters (about 4.19 joules), always makes one calorie, whether by friction, collision or squeezing a gas. So heat is a form of energy, and energy is never lost: wasted work becomes heat (§ 245). That is why no perpetual-motion machine can work (§ 246).
Watch only: the fire syringe
Watch only: don't try this at home. The fire syringe in § 242 uses carbon bisulphide, a poisonous and very flammable liquid. Describe what happens: squeezing air very suddenly heats it enough to light the vapor with a flash. Look up a video of a teacher's "fire syringe" or "fire piston" demonstration instead.
What we know now
The food "Calorie" on labels is 1000 of the book's calories (a kilocalorie). In SI units we measure heat in joules: 1 calorie = 4.184 joules. Today we also know a source of energy not traced to the sun: the energy stored in atomic nuclei, which powers the sun itself.
Say it back
Your student traces the energy in a waterfall from the sun to the electric streetcar, as the book does in §§ 247–248.
Problems
Work problems 1–6 on book page 182. Use 427 gram meters of work per calorie.
Afterwards
Spot energy changes for a day: food to motion, electricity to light and heat, a ball rolling to a stop.
Answers to the book's problems
1. About 46,800 calories (20,000,000 gram meters ÷ 427). 2. Their great kinetic energy becomes heat when they strike. 3. About 0.11° C (each gram falls 48.8 m: 48.8 ÷ 427). 4. About 281,000 calories. 5. 2,500,000 g. cm; about 2,450,000,000 ergs; about 58.5 calories. 6. Friction with the air turns their kinetic energy into heat until they glow.
Worked out for this site from the scan. If a number in your copy differs, trust the book.