A First Course in Physics · Ch. XV, §§ 404–411 · Chapter 15: Effects of Electrical Currents
46. Heating effects: watts, Joule's law and electric lights
Goal: Your student can use volts × amperes = watts, calculate the heat made by a current (Joule's law), and compare the electric lights of 1906 with today's.
You'll need
- The book, open to these pages
- A few light bulbs or their boxes showing watts (incandescent, compact fluorescent or LED)
- An appliance label or the electric bill (to find watts and kilowatt hours)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
A thin wire glows and melts with a current; the energy spent is volts × amperes watts; calories = volts × amperes × seconds × 0.24; Joule's law; carbon-filament incandescent lamps and the watt hour; the arc light; the Nernst lamp; the Cooper-Hewitt mercury lamp. Five problems follow.
The lesson, step by step
Hook question
SayWhy does a toaster wire glow red, while the cord that feeds it stays cool?
Read together
Read §§ 404–411. The rule:
volts x amperes = watts.
Explain the idea
Power in watts = volts × amperes (§ 405). Heat made in a wire is proportional to the resistance, the time and the square of the current (Joule's law, § 406): H = C²R × t × 0.24 calories. The thin, high-resistance wire gets hot; the thick, low-resistance cord does not.
You pay for energy by the kilowatt hour: 1000 watts for one hour (the book uses the watt hour, § 407).
Try it: read the labels
Find the watts on several bulbs and appliances. Work out how many watt hours each uses in 10 hours, as the book does for a 55-watt lamp (550 watt hours). Find the price per kilowatt hour on an electric bill and the cost of running each one.
What we know now
The carbon-filament lamp of 1906 used about 3.4 watts per candle power (§ 407). Tungsten filaments soon replaced carbon, and today an LED bulb gives the same light as an old 60-watt bulb using only about 8–10 watts. Mercury lamps (§ 411) are being phased out because mercury is poisonous.
Watch only: melting wire and arcs
Watch only: don't try this at home. Melting a wire across a cell (§ 404) and the arc light (§ 408) involve very hot metal, intense light that can harm the eyes, and fire. Never try to make sparks or heat wires from a wall outlet.
Say it back
Your student explains why a fuse (a short, thin wire) melts first if too much current flows.
Problems
Work problems 1–5 on book page 311. Use calories = volts × amperes × seconds × 0.24.
Afterwards
Make a list of the five appliances in your home with the highest wattage. Which run longest each day?
Answers to the book's problems
1. About 15.8 calories per second (110 × 0.6 × 0.24). 2. 5760 calories (10² × 4 × 60 × 0.24). 3. About 9300 calories (current 220/750 ampere; 220²/750 × 600 × 0.24). 4. 10 watts. 5. About 75 cells (746 ÷ 10).
Worked out for this site from the scan. If a number in your copy differs, trust the book.