Back to the on-screen lesson ·
Rubbing moves charge; different charges pull, the same charges push, and a charged balloon lifts paper.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will say whether charged things pull together or push apart, tell static electricity from magnetism, and predict how a balloon's pull grows with rubbing.
You have probably rubbed a balloon on your hair and stuck it to a wall, or felt a little shock touching a doorknob in winter. Both are static electricity.
From the magnet lessons you know a rule: different poles pull together, the same poles push apart, and the pull reaches across a gap. Static electricity follows a rule with exactly the same shape, even though it is a different force.
| Term | What it means |
|---|---|
| Static electricity | Electric charge that builds up on a thing, often by rubbing. |
| Charge | What rubbing gives a balloon: it can be positive or negative. |
| Positive and negative | The two kinds of charge. |
| Charged | Carrying more of one kind of charge than the other. |
| Uncharged | Carrying equal amounts of both, so it has no overall charge. |
| Spark | Charge jumping across a small gap, with a flash and a snap. |
Everything is made of tiny particles that carry electric charge, some positive and some negative. Usually a thing has equal amounts of each, so it is uncharged. Rubbing two things together moves some charge from one to the other. Rub a balloon on hair, and the balloon ends up negative and the hair positive.
The rule for charged things is:
A charged thing also pulls on small uncharged things, like bits of paper, tissue or a thin stream of water. And, like a magnet, it does not need to touch: the pull reaches across a gap.
Another way: action
Rub a balloon on your hair for ten seconds, then hold it a little above your head. Watch your hair rise to meet it without touching.
Another way: steps
The rule for charges has the same shape as the rule for magnets. That can make them look like the same thing. They are not.
| Magnets | Static electricity | |
|---|---|---|
| two kinds | north and south poles | positive and negative charges |
| rule | different pull, same push | different pull, same push |
| pulls on | iron, steel, nickel | small light things of any material |
| comes from | the material itself | rubbing |
Here is a test that tells them apart. A magnet picks up a steel paper clip but not a bit of paper. A charged balloon picks up a bit of paper but hardly moves a steel clip. Two different forces, with rules that happen to look alike.
Every material is made of tiny particles. Some of those particles carry negative charge and can be moved from one material to another. When you rub a balloon on hair, some negative charge is scraped off the hair and onto the balloon.
Now the balloon has extra negative charge, so it is negative. The hair has lost some, so it is positive. The two have different charges and pull together, which is why your hair lifts toward the balloon.
Nothing new was made. The charge that ended up on the balloon came from the hair. The total amount of charge is the same before and after; it has only moved.
A bit of paper has no overall charge. So why does a negative balloon pull it?
When the balloon comes near, it pushes the negative charge inside the paper a tiny distance away and pulls the positive charge a tiny distance closer. The near side of the paper becomes slightly positive and the far side slightly negative. The near side is closer to the balloon, so its pull wins, and the paper jumps up.
This only works for small, light things. A heavy book is uncharged too, but the pull is far too weak to lift it. That is why a rubbed balloon picks up confetti and hair but not your shoes.
The more you rub, the more charge moves, and the stronger the pull. A class can measure this by counting how many bits of paper a balloon picks up after different numbers of rubs.
| Rubs | Bits picked up |
|---|---|
| 5 | 3 |
| 10 | 6 |
| 15 | 9 |
| 20 | 12 |
Each extra five rubs adds three more bits. That steady pattern lets the class predict: after $30$ rubs, which is six lots of five, the balloon should pick up $6 \times 3 = 18$ bits. Then they can test the prediction. In real life the pattern stops growing eventually, because a balloon can only hold so much charge.
Static shocks happen far more in winter. Dry winter air lets charge build up on you as you walk across a carpet or pull off a sweater. In damp summer air, a thin film of water on everything lets the charge leak away almost as fast as it builds.
When a charged hand comes close to a metal doorknob, the charge leaps across the last tiny gap as a spark. You feel a small snap and may even see a flash in the dark. Lightning is the same thing on a gigantic scale: charge built up in a storm cloud jumping to the ground.
Run three checks.
The third check catches the most common mix-up in this whole unit.
Charge moves easily through some materials and hardly at all through others. Materials that let charge move easily are called conductors. Metals are good conductors, which is why a spark jumps to a metal doorknob and why the wires in your home are made of copper.
Materials that hold charge in place are called insulators. Rubber, plastic, glass, wool and dry air are insulators. That is why a rubber balloon can hold its charge long enough to stick to a wall: the charge cannot flow away through the rubber.
It also explains a puzzle. You cannot easily charge a metal spoon by rubbing it while you hold it, because any charge you give it flows straight through the metal and into your hand. Rub a plastic ruler instead and the charge stays put, ready to pick up bits of paper. Choosing an insulator is the secret to every static experiment.
Once you know what to look for, you find static electricity everywhere.
In every case, something was rubbed, charge moved from one thing to another, and the charged thing then pulled on something light across a small gap. Once you can name those three parts, you can explain any static surprise you meet.
Inside a photocopier or laser printer, static electricity does the drawing. A drum is given a charge, and a light shines the picture of the page onto it. Where light hits, the charge leaks away; where the page was dark, the charge stays.
Then a fine black powder called toner is dusted over the drum. The toner is charged so that it is pulled only to the charged places, making a copy of the page's dark parts. The drum rolls the toner onto paper, and heat melts it in place. A busy office copier can make $50$ pages a minute this way, which is almost one page every second.
Every step is the rule from this lesson: charged things pull on each other, and where there is no charge, nothing is pulled.
Inside a thunderstorm, ice crystals and hailstones rub against each other as the wind tosses them. The rubbing moves charge, just like a balloon on hair. The bottom of the cloud becomes strongly negative, and the ground below becomes positive.
When the charge is big enough, it leaps across the gap as a giant spark: lightning. A lightning bolt can be $5$ km long and is hotter than the surface of the Sun. The National Weather Service counts about $20$ million lightning strikes to the ground in the United States each year.
Tall buildings have lightning rods: metal rods on the roof, joined by a thick wire to the ground. They give the charge an easy path, just as a doorknob does for the charge on your hand, so it goes safely into the ground instead of through the building.
Because the rules look alike, many people think a rubbed balloon has become a magnet. It has not. A magnet pulls steel and ignores paper; a charged balloon pulls paper and all but ignores steel. Test it and see.
A second mistake is thinking rubbing makes electricity out of nothing. It only moves charge that was already there, from one thing to the other. What one gains, the other loses.
A third is thinking charged things only pull. Two things with the same charge push apart, just as two north poles do.
A last mistake is thinking static electricity is dangerous in the same way as the electricity from a wall socket. The little shock from a doorknob is startling but harmless, because only a tiny amount of charge jumps. Wall sockets are different and must never be played with. Lightning, the giant version of a static spark, is dangerous too: go indoors when you hear thunder. The size of the spark, not the kind of force, is what makes the difference between a tickle and a real danger to people.
Find what was rubbed.
$\text{a balloon on hair}$
Rubbing moves charge between them.
Name the charges.
$\text{balloon negative, hair positive}$
Charge moved from the hair to the balloon.
Decide same or different.
$\text{different}$
One positive, one negative.
Apply the rule.
$\text{different charges attract}$
They pull together.
Say what you see.
$\text{hair rises toward the balloon}$
Across the gap, without touching.
Find what was rubbed.
$\text{both balloons on the same hair}$
Rubbed the same way.
Name the charges.
$\text{both negative}$
Each took charge from the hair.
Decide same or different.
$\text{the same}$
Both negative.
Apply the rule.
$\text{same charges repel}$
They push apart.
Say what you see.
$\text{they hang apart, not straight down}$
The push holds them away from each other.
Check it is not magnetism.
$\text{balloons are rubber, not steel}$
So the force is static electricity.
Read the first two readings.
$5 \text{ rubs: } 3, \quad 10 \text{ rubs: } 6$
Bits of paper picked up.
Find the step.
$6 - 3 = 3 \text{ bits per } 5 \text{ rubs}$
How much each extra five rubs adds.
Check the next reading.
$15 \text{ rubs: } 6 + 3 = 9$
It matches the table.
Choose a new number of rubs.
$30 \text{ rubs}$
Beyond the table.
Count the lots of five.
$30 \div 5 = 6$
Six steps.
Multiply by the step.
$6 \times 3 = 18 \text{ bits}$
The prediction.
Plan the test.
$\text{rub 30 times and count}$
A prediction is worth checking.
Find what was rubbed.
$\text{the comb on the sweater}$
They were rubbed together.
Name the charges.
Apply the rule.
Two balloons are each rubbed on the same person's hair, then hung on strings side by side. What do they do?
Complete the worked solution: a balloon picks up $6$ bits after ten rubs and $9$ after fifteen. Find the step, then the counts after twenty and twenty-five rubs.
Find the step for five rubs.
$9 - 6 =$ a bits
How much each extra five rubs adds.
Add it for twenty rubs.
$9 + \text{step} =$ b bits
One more step.
Add it for twenty-five rubs.
$\text{that} + \text{step} =$ c bits
The pattern keeps going the same way.
Match each observation to the force causing it.
| static electricity | magnetism | |
|---|---|---|
| a rubbed balloon lifts bits of paper | ||
| a bar magnet lifts a steel paper clip | ||
| your hair stands up after you pull off a wool hat | ||
| a compass needle swings toward an iron nail |
A balloon is rubbed on a wool scarf and held just above some tiny bits of paper on a table. What happens?
For each pair, say whether they pull together or push apart.
| Pair | What they do |
|---|---|
| two combs both rubbed on the sweater | |
| a rubbed comb and the sweater it was rubbed on | |
| a rubbed comb and a bit of tissue |
A balloon picks up $4$ bits of paper after $5$ rubs and $8$ after $10$ rubs. The pattern goes up by the same step each time. Fill in the counts for $15$ and $20$ rubs.
| bits of paper picked up | |
|---|---|
| 15 rubs | |
| 20 rubs |
A class rubs a balloon on a wool scarf and counts how many tiny bits of paper it picks up. After $5$ rubs it picks up $3$ bits, after $10$ rubs $6$, and after $15$ rubs $9$. If the pattern keeps going, how many bits will it pick up after $30$ rubs?
Answer: bits
On a dry winter day, a student shuffles across a carpet in socks and then touches a metal doorknob. There is a tiny spark and a snap. What happened?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each pair, say whether they pull together or push apart.
| Pair | What they do |
|---|---|
| two balloons both rubbed on hair | |
| a rubbed balloon and a bit of paper | |
| a balloon and the hair it was rubbed on |
You can use the charge rule and tell it apart from magnetism. Tell someone why a rubbed balloon lifts paper but a magnet does not.
19. Your turn: a comb is rubbed on a sweater. Do the comb and the sweater pull or push?, step 2
$\text{different}$
Charge moved from one to the other.
19. Your turn: a comb is rubbed on a sweater. Do the comb and the sweater pull or push?, step 3
$\text{different charges attract: they pull}$
The sweater's fibers cling to the comb.