A First Course in Physics · Ch. XIV, §§ 375–386 · Chapter 14: Electricity in Motion
43. Batteries: how cells work and how to connect them
Goal: Your student can explain how a simple cell makes a current, what polarization is, how the Daniell, Leclanché and dry cells differ, and when to connect cells in series or in parallel.
You'll need
- The book, open to these pages
- Three or four lemons, the same number of galvanized (zinc-coated) nails and clean copper coins or strips of copper, and wires with alligator clips
- A red LED (a light-emitting diode) or a cheap digital clock, or a multimeter
- A used AA battery to look at (not to open)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
What happens in a zinc-copper-acid cell; local action and amalgamation with mercury; the ion theory of the cell; polarization and its causes; the bichromate, Daniell, Leclanché and dry cells; series and parallel connections of cells and when to use each. Ten problems follow.
The lesson, step by step
Hook question
SayA battery has no moving parts. Where does its electricity come from?
Read together
Read §§ 375–386. The rule for connecting cells:
Hence series connection should always be used when the external resistance is large in comparison with the internal resistance of a single cell.
Try it: a lemon battery
Push a galvanized nail and a copper coin into a lemon, not touching. That is a cell: zinc, copper and an acid (§ 377). One lemon gives roughly one volt but very little current. Join three or four lemons in series (the copper of one to the zinc of the next) and connect the ends to an LED (longer leg to the copper end) or a small digital clock. It glows faintly. A multimeter will show the voltage add up.
Safety: don't eat the lemons afterward.
Explain the idea
Zinc dissolves into the acid as positive ions, leaving the zinc plate negative; hydrogen comes out at the copper, making it positive (§ 377). Hydrogen bubbles on the copper weaken the cell (polarization, §§ 378–379). Better cells are designed to stop that (the Daniell cell) or recover (the Leclanché and dry cells).
Watch only: acids and mercury
Watch only: don't try this at home. The book's cells use sulphuric acid, bichromate, copper sulphate and mercury for amalgamating the zinc, all of which are hazardous. Never open a battery: the chemicals inside can burn skin.
What we know now
Most AA batteries today are alkaline cells, a later improvement on the book's dry cell (§ 383). Rechargeable batteries (lithium-ion in phones, lead-acid in cars) are storage batteries, which the next chapter introduces.
Say it back
Your student explains when to use series and when to use parallel, with the water pictures in Figs. 296 and 297.
Problems
Work problems 1–10 on book pages 290–291.
Afterwards
Look at how batteries are put into a flashlight or remote control: end to end (series) or side by side? Why?
Answers to the book's problems
1. Its E.M.F. stays nearly constant for a long time, with little polarization. 2. It needs no attention and does not waste away on open circuit, and doorbells need only short bursts of current. 3. In series: about 0.065 ampere (43.2 volts ÷ (500 + 160) ohms); one cell: about 0.0021 ampere; 40 in parallel: about 0.0022 ampere. 4. 40 in parallel: 5.4 amperes; 40 in series: about 0.27 ampere; one cell: about 0.26 ampere. 5. Its E.M.F. is high and its internal resistance low, so it gives a large current, but it soon weakens. 6. When the outside resistance is large compared with the cell's internal resistance. 7. About 2.16 ohms (taking its E.M.F. as 1.08 volts). 8. So that it draws too little current to lower the P.D. it measures. 9. So that putting it in the circuit does not reduce the current it measures. 10. Yes: the same current C flows through each wire, so the P.D. across each, C × r, is proportional to its resistance r.
Worked out for this site from the scan. If a number in your copy differs, trust the book.