Pick one before you read on. It is fine to be wrong.
By the end you can
Charge a comb by rubbing it and use it to lift paper bits
Explain that like charges repel and unlike charges attract
Pull a woolly jumper off in the dark and you may hear a crackle. Rub a plastic comb and it can pick up paper. This is static electricity. The charge stays in one place. It does not flow round a loop.
Everything is made of tiny parts. Some carry a plus charge and some carry a minus charge. Most of the time they balance, so a thing has no charge overall.
Rubbing two materials together can move electrons from one to the other. The material that gains electrons has more minus charges. We say it is negative. The material that loses electrons has more plus charges. We say it is positive.
Draw it first
Draw a comb being charged
1 Draw a comb and wool. Each has equal plus and minus: 3 and 3 on the comb, 4 and 4 on the wool.
2 Rub them. 2 electrons move from the wool to the comb. The comb now has 5 minus marks and the wool has 2. They attract.
3 Hold the comb near bits of paper. The comb pulls on the paper. The paper has equal plus and minus marks, but they move a little.
There is a rule for the pulls and pushes between charges. Charges of the same sign repel, which means they push apart. Charges of different signs attract, which means they pull together.
Two negative balloons repel. A negative and a positive balloon attract.
See it three ways
As a picture
In words
Charges of the same sign push each other away. Charges of different signs pull each other together.
In symbols
+ and + repel. − and − repel. + and − attract.
Why does a charged comb lift paper that has no charge? The comb is negative. It pulls the plus charges in the paper a little nearer. It pushes the minus charges a little away.
The paper has no charge overall. Its plus marks move to the side nearest the comb.
The plus charges are now nearer to the comb than the minus ones. A pull from nearby is stronger than a push from farther away. So the paper is pulled to the comb.
After rubbing, the comb has 2 more minus marks than plus. The wool has 2 more plus marks than minus.
Worked example
A comb has 3 plus and 3 minus charges. Rubbing moves 2 electrons onto it. How many more minus than plus charges does the comb have now?
Before rubbing: 3 plus and 3 minus. They balance.
Rubbing adds 2 electrons. Each is a minus charge.
Minus charges now: 3 + 2 = 5.
The plus charges are still 3.
5 − 3 = 2, so the comb has 2 more minus than plus. It is negative.
Answer: 2 more minus than plus, so the comb is negative
Static electricity works best on a dry day. In damp air the charge leaks away into the moisture, so a comb loses its charge fast.
Try it
Lift paper with a comb
You need: a plastic comb, or a plastic pen or ruler, dry hair or a woollen cloth, tiny bits of paper, each the size of a fingernail, a dry day.
Tear the paper into tiny bits and spread them on a table.
Hold the comb above the bits without rubbing it. Does anything move?
Rub the comb on dry hair or wool, 20 quick strokes in one direction.
Hold the comb just above the paper, not touching. Count the bits that jump up.
What you will see: The comb that was not rubbed lifts nothing. The rubbed comb pulls up several bits. Some bits may jump off again. They take the comb charge and are pushed away. Your counts will differ from ours.
If you cannot try it, this is what happens.
A wrong idea many people have
The comb picks up paper because the comb becomes a magnet.
Why it feels right: A magnet also picks things up without touching them, so the two seem alike.
The test that breaks it: A magnet does not move bits of kitchen foil. A rubbed comb pulls them. The two kinds of pull are different.
What is true: The comb has an electric charge. It pulls the charges in the paper. A magnet works in a different way.
Key words
static electricity
charge that stays in one place and does not flow.
negative
having more minus charges than plus charges.
positive
having more plus charges than minus charges.
repel
to push apart.
attract
to pull together.
Back to your guess
The answer is C: Rubbing gives the comb an electric charge, and the charge pulls the paper.
Rubbing moves electrons onto the comb, so it has a charge. The charge pulls on the paper. Paper is not magnetic, and the comb is not sticky.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
A plastic comb is rubbed on wool. What happens?
See the answer
Electrons move from the wool to the comb.
Rubbing moves electrons. The comb gains them and becomes negative. The wool loses them and becomes positive.
If you chose B: Rubbing does not make charge. It moves electrons from one material to the other. Go to S3.10.5.
If you chose C: It is the other way round. The comb gains the electrons and becomes negative. Go to S3.10.5.
Two balloons have charges as shown. Do they attract or repel?
See the answer
They repel, because both are positive.
Each balloon has more plus marks than minus marks, so both are positive. Like charges repel.
If you chose A: Charged things do not always attract. Only unlike charges attract. Go to S3.10.5.
If you chose C: Any two charged things push or pull each other. Both of these have a charge. Go to S3.10.5.
Why does a charged comb pick up paper that has no charge?
See the answer
The comb pulls the opposite charges in the paper nearer, so the paper moves to it.
The charges in the paper shift. The unlike charges are nearer the comb, and a near pull beats a far push.
If you chose A: Paper is not magnetic. A magnet cannot lift paper. Go to S3.10.6.
If you chose B: Like charges repel. A paper bit that takes the comb's charge jumps off. Go to S3.10.5.
On which day does a rubbed comb lift paper best?
See the answer
A dry day, because dry air lets the charge stay on the comb.
In damp air the charge leaks away into the moisture. In dry air it stays on the comb.
If you chose B: Damp air lets the charge leak away. The comb loses its charge fast. Go to S3.10.5.
If you chose C: The weather matters. Moisture in the air takes charge away from the comb. Go to S3.10.5.
Two other balloons have charges as shown. Do they attract or repel?
See the answer
They attract, because one is positive and one is negative.
The left balloon has more plus marks, so it is positive. The right balloon has more minus marks, so it is negative. Unlike charges attract.
If you chose A: Charged things only repel when their charges are alike. One of these is positive and the other is negative. Go to S3.10.5.
If you chose C: The marks cancel only inside one thing that has equal plus and minus. Each balloon here has more of one sign, so each has a charge. Go to S3.10.5.
From an earlier chapter
Review: a 9 V supply is joined to a resistor of 15 Ω. What is the current?
See the answer
0.6 A
Current = voltage ÷ resistance = 9 ÷ 15 = 0.6 A.
If you chose A: You multiplied the voltage by the resistance. Divide instead: current = voltage ÷ resistance. Go to S3.10.4.
If you chose B: You added the voltage and the resistance. Divide the voltage by the resistance. Go to S3.10.4.
A comb has 7 minus charges and 3 plus charges. How many more minus than plus charges does it have?
See the answer
4
7 − 3 = 4. The comb is negative.
Keep it
Copy this into your exercise book.
Rubbing moves electrons. The one that gains them is negative. Like charges repel and unlike charges attract. A charged comb pulls paper. Dry days work best.
Draw: A comb with more minus marks near bits of paper, with an arrow of attract. Write + and − on both.
