A First Course in Physics · Ch. XIII, §§ 322–333 · Chapter 13: Static Electricity
37. Static electricity: charges, conductors and induction
Goal: Your student can explain the two kinds of charge and their law, tell conductors from insulators, charge an electroscope by induction, and describe the electron theory.
You'll need
- The book, open to these pages
- Two balloons, a wool sweater or your hair
- A plastic comb and tiny bits of tissue paper
- A homemade electroscope: a glass jar, a cardboard lid, a bent paper clip pushed through it with two thin strips of aluminum foil hanging from the hook inside
- Cellophane tape
- A notebook and pencil
What's in the book
Electrification by rubbing, from the Greeks' amber to Gilbert; two kinds of charge, positive (glass rubbed with silk) and negative (ebonite rubbed with fur); like charges repel, unlike attract; the unit of charge; conductors and insulators with the gold-leaf electroscope; induction; the two-fluid theory and J. J. Thomson's electron theory; charging by induction; equal plus and minus charges. Ten questions follow.
The lesson, step by step
Hook question
SayAfter you rub a balloon on your hair, it sticks to the wall. What is holding it there?
Read together
Read §§ 322–333. The law:
Electrical charges of like kind repel each other; those of unlike kind attract each other.
Try it: like and unlike
Rub both balloons on the sweater and hang them side by side on threads: they push apart. Rub a comb through dry hair and pick up bits of tissue. Stick two strips of tape to a table, rip them off together and hold them near each other (they repel); then stick one on top of the other, rip apart, and they attract.
Try it: an electroscope
Bring a charged balloon near the paper-clip top of the electroscope without touching it: the foil leaves spread (induction, § 327). Take it away: they fall. Now charge it by induction (§ 332): hold the balloon near, touch the clip with a finger, remove the finger, then remove the balloon. The leaves stay apart.
Explain the idea
Rubbing does not make electricity; it separates plus and minus charges that were there all along (§ 333). Conductors (metals, salt water, our bodies) let charges move; insulators (rubber, glass, plastic, dry silk) don't.
What we know now
The electron theory in § 330 won. J. J. Thomson discovered the electron in 1897, and in 1911 Rutherford showed the atom's positive charge sits in a tiny nucleus with electrons around it. The book also notes that the direction we call current flow (§ 341) is opposite to the way electrons actually move; that convention is still used today. The SI unit of charge is the coulomb.
Say it back
Your student explains how the electroscope can be charged by induction, step by step.
Questions
Work questions 1–10 on book pages 246–247. Your homemade electroscope can test several of them.
Afterwards
Notice static this week: a shock from a doorknob, clothes clinging out of the dryer, hair standing up under a hat.
Tip
Static experiments work best on a dry day; humid air carries the charge away.
Answers to the book's problems
1. If the divergence increases, the paper is charged + (like the electroscope); if it decreases, the paper is −. 2. Charge the electroscope + by induction from the rubbed glass rod, then bring the unknown near: if the leaves spread farther it is +; if they fall it is − (or uncharged). 3. Each figure takes a charge from the charged plate, is repelled to the other plate, gives up its charge there, and is attracted back. 4. Bring the charged sphere near two touching spheres, separate them while it is still near, and then take it away. 5. The negative is bound by the attraction of the positive rod. 6. Otherwise the electricity that went to earth through the finger flows back and the electroscope is left uncharged. 7. In the hand the brass's charge runs off through the body; held in rubber it is insulated. 8. It is attracted by induction, touches the rod and takes a charge of the same sign, and is then repelled. 9. An uncharged body is also attracted (by induction), but only a charged one can repel. 10. For example: charges exist on any body and can move through conductors, while magnetic poles exist only in iron and steel and stay fixed; a magnet always has both poles, but a body can have a single kind of charge.
Worked out for this site from the scan. If a number in your copy differs, trust the book.