الأسبوع 24 · المحور 4، الباب 1 «الربط بين القوى» (ب): الأقطاب والقوة عن بعد وتصميم بوابة · 3 دروس
2
Magnets from a distance
Discover P3 Term 2: Axis 4, Chapter 1
GoalShow that magnets work from a distance and through paper, test how many sheets are too many, and use a compass.
New words
distance · how far one thing is from anothermagnetic field · the space around a magnet where its force can be feltcompass · a tool with a small magnet needle that shows where north issheet · one flat piece of paper
Explanation
A magnet does not have to touch something to pull it. A paper clip jumps towards a magnet that is a little way off. We say that a magnet works from a distance. The space around a magnet where its force can be felt is called its magnetic field. We cannot see the field, but we can see what it does. A magnet also works through thin things, such as paper, cloth and plastic. To test this, we slide sheets of paper into the gap above a paper clip under the magnet, adding one sheet each time. Every sheet puts the magnet a little farther away, so its pull gets weaker. When there are too many sheets, the paper clip no longer moves. A compass has a small magnet needle that can turn freely. The Earth has a magnetic field too, and the needle lines up with it, so one end points north. If we move a compass around a bar magnet, the needle turns to follow the magnet’s field. We keep other magnets away from a compass, or the needle gets confused. We use magnets every day: to stick notes on a metal board, to keep a fridge door shut, to join toy blocks, to close a bag, and to sort iron from rubbish at a recycling place.
Worked examples
What I do
What the clip does
Hold the magnet just above the clip
it jumps up to the magnet
Slide the magnet under a thin card with the clip on top
it slides along on the card
Two ways to move a paper clip without touching it with the magnet.
Sheets between
Clip lifted?
4
yes
5
yes
6
yes
7
no
Mina’s sheet test. Her magnet lifted the clip through 6 sheets but not through 7. So it works through at most 6 sheets. A stronger magnet might work through more.
Comparing magnets. Karim tested three magnets. A worked through 4 sheets, B through 6 and C through 9. C worked through 9 − 4 = 5 more sheets than A.
Class activity
How many sheets? With a grown-up. Put a paper clip on the table. Hold a magnet above it with one sheet of paper between them, then two, then three, and so on, until the clip no longer jumps up. First predict the number. Write the result, and compare with a friend. Did you get the same number? Why might two magnets give different numbers? Then draw a field line. Trace a bar magnet on a big sheet of paper. Put a small compass next to one pole and mark a dot where the needle’s tip points. Slide the compass along until the tail of the needle sits on that dot, and mark the tip again. Keep going, then join the dots. Start from other places and draw more lines. Keep magnets away from phones, cards and screens.
Practice
1
Can a magnet pull a paper clip without touching it? Explain.
Yes. A magnet works from a distance, because its force reaches out into its magnetic field.
2
What happens when more and more sheets of paper are put between the magnet and the clip?
The magnet is farther away, so it pulls less, and at last the clip no longer moves.
3
Tamer’s magnet lifted the clip through 3 sheets and 5 sheets, but not through 8. What is the greatest number of sheets we know it works through?
Sheets
Clip lifted?
3
yes
5
yes
8
no
5 sheets. He did not test 6 or 7, so we do not know about them.
4
Name two ways magnets are used at home or at school.
For example: a note on a metal board, a fridge door, toy blocks, a cupboard catch.
