How can I see the invisible push and pull around a magnet?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Map a magnet's field with a compass or bits of steel wool
Explain that the Earth has a magnetic field
A magnet can pull a pin across a table without touching it. The push or pull reaches out into the space round the magnet. That space is the magnetic field of the magnet.
You cannot see a field. But a small magnet, such as a compass needle, feels it. The needle turns until it lines up with the field.
We draw a field with field lines. They run from the north pole of the magnet round to the south pole. An arrow on each line shows the way a north pole would be pushed.
Draw it first
Draw the field of a bar magnet
1 Draw a bar magnet. Write N on one end and S on the other.
2 Draw lines that leave the N end, curve round and come into the S end. Put an arrow on each line, pointing from N to S.
3 Put a compass on a line. The black end of the needle points the way the arrow points.
The lines are closest together near the poles. That is where the field is strongest. Far from the magnet the lines are far apart, and the field is weak.
The lines crowd together near the poles, where the field is strongest.
See it three ways
As a picture
In words
A compass needle lines up with the field. Its black end points the way the field line points. Near the N pole it points away from the magnet. Near the S pole it points towards it.
In symbols
Outside a magnet, field lines run from N to S.
Two magnets make a joined field. If a N pole faces a S pole, the lines join across the gap. The magnets pull together. If two N poles face each other, the lines push apart. The magnets repel.
N facing S: the lines join across the gap and the magnets attract.N facing N: the lines push apart. At the neutral point the field of one magnet cancels the other.
The Earth has a magnetic field too. It is as if a huge bar magnet were inside the Earth. A compass needle lines up with this field. The black end points roughly to the geographic north.
The Earth acts like a huge magnet. Almost everywhere, a compass needle points roughly north.
The end of the Earth's magnet near the geographic north is a magnetic S pole. It pulls the black north end of a compass towards it. This works in most places. Very near the magnetic poles, compasses are not reliable.
Try it
Map a magnet's field
You need: a bar magnet, a plain sheet of paper, tiny bits of steel wool, cut small by an adult with scissors, a compass, if you have one.
Be safe: Steel wool is sharp. Ask an adult to cut it. Keep it away from your eyes and mouth, and wash your hands after.
Lay the magnet flat on the table. Put the paper over it.
Sprinkle the bits of steel wool over the paper. Tap the paper gently.
Look at the pattern. Which way do the lines curve? Where are they thickest?
With a compass, move it slowly round the magnet. Watch the needle turn.
What you will see: The steel wool lines up in curves from one end of the magnet to the other. It is thickest near the ends of the magnet, where the field is strongest. The compass needle turns as you move it, always along the lines. Your pattern will look like this, but not exactly.
If you cannot try it, this is what happens.
A wrong idea many people have
A magnet acts only at its two ends.
Why it feels right: Pins stick to the ends of a magnet. The ends seem to be the only places that pull.
The test that breaks it: Put a compass above the middle of a bar magnet. The needle turns to line up with the magnet. The field is there, too.
What is true: The field is all round the magnet. It is strongest near the poles, but it is not zero anywhere near the magnet.
Key words
magnetic field
the space round a magnet where it pushes or pulls.
field lines
lines that show a field. Outside a magnet they run from N to S.
compass
a small magnet on a pivot. It lines up with a field.
Back to your guess
The answer is A: A magnet has a field all round it, and a compass or steel wool can show it.
A magnet has a magnetic field all round it. The field is strongest near the poles, but it spreads out in every direction. A compass or bits of steel wool can show it.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Which way do field lines run outside a bar magnet?
See the answer
From the N pole round to the S pole.
Outside a magnet the field lines run from the N pole round to the S pole.
If you chose A: It is the other way. Outside the magnet the lines leave the N pole and come into the S pole. Go to S3.10.6.
If you chose C: The lines curve round. Those that leave the N pole come back into the S pole. Go to S3.10.6.
Where is the field of a bar magnet strongest?
See the answer
Near the poles, where the lines are closest together.
Field lines crowd together where the field is strong. They are closest near the poles.
If you chose A: Lines far apart mean a weak field. The field is strongest where the lines are closest. Go to S3.10.6.
If you chose B: The field gets weaker as you move away. The lines spread out, and the field falls. Go to S3.10.6.
A small compass is put at P, to the right of the magnet. The N pole is on the left. Which way does the black end point?
See the answer
Towards the S end of the magnet.
The black end points the way the field line points. At P the line points into the S pole, so the needle points towards the S end.
If you chose B: The black end is a north pole, and a north pole is pulled towards a S pole. Go to S3.10.6.
If you chose C: The needle lines up with the field. At P the field points along the axis, not up the page. Go to S3.10.6.
Why does a compass needle point roughly north in most places on the Earth?
See the answer
The Earth is like a huge magnet, and the needle lines up with its field.
The Earth acts like a huge magnet. A compass needle lines up with its field, so its black end points roughly north.
If you chose A: Ice is not magnetic. The needle lines up with the magnetic field of the whole Earth. Go to S3.10.6.
If you chose C: A compass points the same way at noon and at night, with no Sun in the sky. The Earth's own field turns the needle. Go to S3.10.6.
Kofi lays a bar magnet on a table with its N pole on the left. He puts a small compass to the left of the N pole, on the line of the magnet. Which way does the black end of the needle point?
See the answer
Away from the magnet, along the field line.
Field lines leave the N pole, and the black end points the way the line points. To the left of the N pole, the line runs away from the magnet.
If you chose A: A north pole is pushed away by a N pole. The needle follows the field line, and near the N pole the lines point away from the magnet. Go to S3.10.6.
If you chose B: Close to a magnet, its field is far stronger than the field of the Earth. The needle lines up with the field of the magnet, not with the Earth. Go to S3.10.6.
From an earlier chapter
Review: a comb and a piece of wool have the charges drawn here. Do they attract or repel?
See the answer
They attract, because one is negative and one is positive.
The comb has more minus marks, so it is negative. The wool has more plus marks, so it is positive. Unlike charges attract.
If you chose B: Charged things only repel when their charges are alike. Here one is negative and the other positive. Go to S3.10.5.
If you chose C: Any two charged things push or pull each other. Unlike charges attract. Go to S3.10.5.
Keep it
Copy this into your exercise book.
A magnet has a field all round it. Field lines run from N to S, closest near the poles. A compass lines up with them. The Earth has a field too.
Draw: A bar magnet marked N and S. Three curved lines with arrows from N to S. A compass on one line.
