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Two poles and one rule, which metals a magnet pulls, and a pull that reaches across a gap.
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 two magnets pull together or push apart, sort objects into magnetic and not magnetic, and measure a magnet's strength by the mass it holds.
You have probably stuck a magnet to a refrigerator, or seen one pick up a paper clip. You may also have noticed that two magnets sometimes snap together and sometimes push each other away. This lesson explains both, with one simple rule.
You also know how to measure masses in grams. You will use that to measure how strong a magnet is, by finding the mass it can hold up.
| Term | What it means |
|---|---|
| Magnet | Something that pulls on iron and steel, and pulls or pushes other magnets. |
| Pole | One end of a magnet, where its pull is strongest. |
| North pole | One kind of pole, often marked N. |
| South pole | The other kind of pole, often marked S. |
| Attract | To pull toward. |
| Repel | To push away. |
| Magnetic | Pulled by a magnet: iron, steel and nickel are magnetic. |
Every magnet has two ends, called poles. One is the north pole and the other is the south pole. Often they are marked N and S, or painted two different colors.
When two magnets come close, the rule is:
A magnet also pulls on some metals: iron, steel and nickel. It does not pull on other metals like aluminum and copper, and it does not pull on wood, plastic, paper or glass.
And a magnet does not have to touch to pull. Its pull reaches across a gap, which is why a force from a magnet is called a non-contact force.
Another way: action
Hold two magnets with the same poles facing and push them together. Feel them push back. Now turn one around. Feel them snap together.
Another way: steps
Many people think a magnet pulls on any metal. Try it and you will find it does not.
| Object | Made of | Pulled? |
|---|---|---|
| paper clip | steel | yes |
| nail | iron | yes |
| soda can | aluminum | no |
| penny | copper and zinc | no |
| ring | gold | no |
Only a few metals are magnetic: iron, steel (which is mostly iron) and nickel. That is useful. At a recycling center, a big magnet lifts the steel cans out of a pile, and the aluminum cans stay behind, so the two can be recycled separately.
Most pushes and pulls happen when two things touch: a hand on a door, a foot on a ball. A magnet's pull is different. Hold a magnet a little way above a paper clip, and the clip jumps up to meet it before they touch.
The pull even reaches through things that are not magnetic. A magnet under a sheet of paper drags a paper clip around on top. A magnet on the outside of a glass of water pulls a steel paper clip inside the water. The paper, the glass and the water do not block it, because a magnet does not pull on them at all.
A magnet's pull does get weaker as the gap gets bigger. Close up, it is strong; far away, you cannot feel it. The next lesson measures exactly how the pull changes with distance.
One way to measure a magnet's strength is to count what it can hold up. Hang steel paper clips from it, one below the other, until one more would make the chain fall. Then weigh the clips.
Suppose a magnet holds $8$ clips and each clip has a mass of $2$ g. The magnet holds $8 \times 2 = 16$ g. A stronger magnet might hold $15$ clips, which is $30$ g. Comparing the grams tells you which magnet is stronger, and by how much.
This is a fair test only if everything else stays the same: the same kind of clips, hung the same way, touching the same pole. Change only the magnet, and the difference in grams belongs to the magnets.
The whole Earth acts like a giant magnet, with poles near the top and bottom of the planet. That is why a compass works.
A compass needle is a tiny magnet balanced on a pin so that it can turn freely. The Earth's magnetic pull turns the needle until it lines up, so one end always points north. People have used compasses to find their way for about a thousand years: sailors crossing oceans, explorers in forests, and hikers on mountain trails.
If you hold a magnet near a compass, the needle swings toward it instead, because the magnet is much closer than the Earth's poles. That is a quick way to find which end of an unmarked magnet is its north pole.
Before you answer a magnet question, run three checks.
The second check catches the most common mistake, saying a magnet pulls on all metal. The first catches the next: forgetting that two magnets can push as well as pull.
What happens if you break a bar magnet in the middle? You might expect to get one piece that is all north and one that is all south. You do not. Each piece becomes a complete, smaller magnet, with its own north pole and its own south pole.
Break one of those pieces again and the same thing happens. However small you make the pieces, every one has two poles. Nobody has ever found a magnet with only one pole, even though scientists have looked very hard.
This tells you something about how magnets are made inside. A magnet is made of enormous numbers of tiny magnetic bits, all lined up the same way, like a crowd of people all facing forward. Each tiny bit has a north end and a south end. When they all line up, their pulls add together and the whole piece acts as one magnet.
You can even make a weak magnet yourself. Stroke a steel needle the same way, many times, with one pole of a magnet. The strokes line up the tiny bits inside the needle, and afterward it can pick up a light paper clip on its own.
Some magnets work only when electricity flows. Wind a long wire in a coil around an iron nail and connect the ends to a battery. While the current flows, the nail becomes a magnet and picks up paper clips. Disconnect the battery and the clips drop.
This kind of magnet is called an electromagnet. It follows the same rules as any other magnet: it has a north and a south pole, it pulls on iron and steel but not on aluminum, and its pull reaches across a gap. The difference is that you can turn it on and off, and you can make it stronger by adding more turns of wire or using a bigger battery.
Electromagnets are all around you. There is one in every doorbell, every electric motor, and every speaker in a phone or headphones. The speaker's electromagnet pushes and pulls against a permanent magnet many times each second, and that shaking makes the sound you hear.
At a recycling center, cans of every kind arrive mixed together on a moving belt. Steel cans and aluminum cans are recycled in different ways, so they must be separated, and doing it by hand would take far too long.
Instead, the belt passes under a big magnet. The steel cans jump up and stick to it; the aluminum cans ride on underneath. A plant might sort $5{,}000$ kg of cans in an hour this way. If $2{,}000$ kg of them are steel, the magnet lifts out all $2{,}000$ kg and leaves $3{,}000$ kg of aluminum on the belt.
The whole machine works because only some metals are magnetic. A magnet that pulled every metal would be useless for sorting.
At a scrapyard, old cars and machines are broken up for their steel. A crane there does not use a hook. It uses a huge round electromagnet, a magnet that works only while electricity flows through it.
The crane operator lowers the magnet onto a pile of scrap and switches it on. Steel pieces weighing hundreds of kilograms leap up and cling to it, and the crane swings them over to a truck. Then the operator switches the magnet off, and the steel drops. A big scrapyard magnet can lift more than $1{,}000$ kg at once.
The same rules apply as for a paper clip on a desk: the magnet pulls only on iron and steel, and it pulls across a gap before it even touches the pile.
The most common idea about magnets is that they stick to anything metal. They do not. Aluminum foil, a copper penny and a gold ring are all metal, and a magnet ignores them. Only iron, steel and nickel are pulled.
A second mistake is thinking a magnet only pulls. Two magnets can also push each other away, when the same poles face each other. If you have ever tried to push two magnets together and felt them slide sideways, that was the push.
A third is thinking a magnet must touch to pull. It does not. Its pull reaches across a gap and through paper, plastic and water, getting weaker the further away it is.
A last mistake is thinking the bigger magnet is always the stronger one. Size is a poor guide: a small, powerful magnet can hold far more than a large, weak one. The only way to know is to test them fairly and compare the grams each one holds. A number you measured beats a guess from how big something looks, every time, and it is a number anyone else can check by testing the same magnet again themselves.
Name the facing poles.
$\text{north and south}$
The north end of one faces the south end of the other.
Decide same or different.
$\text{different}$
North is not the same as south.
Apply the rule.
$\text{different poles attract}$
They pull together.
Turn one magnet around.
$\text{now north faces north}$
The facing poles are the same.
Apply the rule again.
$\text{same poles repel}$
Now they push apart.
List the objects.
$\text{steel scissors, plastic ruler, aluminum sharpener}$
Test each one with a magnet.
Check the scissors.
$\text{steel: pulled}$
Steel is magnetic.
Check the ruler.
$\text{plastic: not pulled}$
A magnet does not pull on plastic.
Check the sharpener.
$\text{aluminum: not pulled}$
It is metal, but aluminum is not magnetic.
Write the sorted lists.
$\text{pulled: scissors} \quad \text{not pulled: ruler, sharpener}$
Two groups.
Check against the rule.
$\text{only iron, steel and nickel are pulled}$
The steel scissors are the only magnetic thing.
Test the first magnet.
$\text{holds } 6 \text{ clips}$
Hang clips until one more would fall.
Find its mass.
$6 \times 2 = 12\ \text{g}$
Each clip is 2 grams.
Test the second magnet.
$\text{holds } 11 \text{ clips}$
Same clips, same way of hanging them.
Find its mass.
$11 \times 2 = 22\ \text{g}$
More clips, more grams.
Compare the two.
$22 - 12 = 10\ \text{g}$
The second holds ten grams more.
Check the test was fair.
$\text{only the magnet changed}$
So the difference belongs to the magnets.
State the answer.
$\text{the second magnet is stronger}$
It held more mass.
Name the facing poles.
$\text{south and south}$
Both ends are south.
Decide same or different.
Apply the rule.
Two bar magnets lie on a table. Which pair of ends will pull together?
Complete the worked solution: a magnet holds a chain of $7$ paper clips, each $2$ g. A second magnet is stuck to the first, north to south, and now $3$ more clips hang on. What mass did one magnet hold, and what do both hold together?
Find what one magnet held.
$7 \times 2 =$ a g
That many clips of two grams each.
Explain why they stick together.
$\text{north facing south: pull together}$
Different poles pull, so the two magnets join into a stronger one.
Find what both hold.
$(7 + 3) \times 2 =$ b g
All the clips hanging now.
Match each object to whether a magnet pulls on it.
| pulled by a magnet | not pulled by a magnet | |
|---|---|---|
| a steel paper clip | ||
| an aluminum soda can | ||
| a copper penny | ||
| an iron nail |
A paper clip sits on top of a sheet of paper. A magnet is moved around underneath the paper, without touching the clip. What does the clip do?
For each pair of facing poles, say whether the magnets pull together or push apart.
| Facing poles | What they do |
|---|---|
| north facing north | |
| south facing north | |
| north facing south |
For each object, say whether a magnet pulls on it.
| Object | Pulled by a magnet? |
|---|---|
| an iron washer | |
| a gold ring | |
| a steel bottle cap |
A class tests how strong a magnet is. Hanging from it are a chain of $9$ steel paper clips, each with a mass of $2$ g, and a steel key of $6$ g. A plastic tray sits underneath, not touching. What total mass is the magnet holding up?
Answer: unit: g
On a hiking trip, a scout holds a compass. Its needle is a tiny magnet on a pin. Why does one end of the needle always swing to point north?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each pair of facing poles, say whether the magnets pull together or push apart.
| Facing poles | What they do |
|---|---|
| south facing south | |
| north facing north | |
| south facing north |
You can use the pole rule and the magnetic metals. Tell someone why a magnet picks up a steel paper clip but not an aluminum can.
18. Your turn: two magnets with south facing south. What happens?, step 2
$\text{the same}$
Two south poles.
18. Your turn: two magnets with south facing south. What happens?, step 3
$\text{same poles repel: they push apart}$
You can feel them push.