Pick one before you read on. It is fine to be wrong.
By the end you can
Make an electromagnet with a nail, wire and a cell
Test how more turns of wire change its strength
A wire that carries a current has a magnetic field round it. The field of one wire is weak. Wind the wire into a coil and the fields of all the turns add up.
A coil with a current acts like a bar magnet. It has a N pole at one end and a S pole at the other. Turn the current off and the field goes.
A bar magnet, standing in for the coil. A coil with a current has a field like this one. Reverse the current and the N and S ends swap.
Put an iron nail inside the coil. The nail is the core. Iron becomes a magnet when it is in a field. The nail adds its own field to that of the coil. Together they are much stronger.
An electromagnet is a coil with a current in it, usually wound round an iron core. It is a magnet that you can switch on and off.
Draw it first
Draw an electromagnet and switch it on
1 Wind wire round an iron nail and join it to a cell and a switch. Leave the switch open. No current flows, so no clips stick.
2 Close the switch. A current flows in the coil and the nail becomes a magnet. With 10 turns it holds 2 clips.
3 Wind more turns. With 20 turns, the same cell holds 3 clips. The magnet is stronger.
When the switch is opened, the current stops. The iron nail loses nearly all of its magnetism, and the clips fall. A steel nail may keep a little magnetism.
One learner's test, 20 turns and one cell. The iron nail makes the coil much stronger.
Two things make an electromagnet stronger. One is more turns of wire. The other is a bigger current, which you get from more cells. The results below are one learner's. Yours will differ.
See it three ways
As a picture
In words
More turns, or more cells, lift more clips. Clip counts are rough, so do not expect them to grow exactly in step.
In symbols
5 turns: 1 clip. 20 turns: 3 clips. More turns, more clips.
The results for the next example. One cell each time.
Worked example
One learner's electromagnet picks up 1 clip with 5 turns and 3 clips with 20 turns. How many times as many turns is that, and how many times as many clips?
Read the table: 5 turns pick up 1 clip. 20 turns pick up 3 clips.
The turns grew 20 ÷ 5 = 4 times.
The clips grew 3 ÷ 1 = 3 times.
More turns picked up more clips. Clip counts are rough, so they do not grow exactly in step.
Answer: 4 times as many turns and 3 times as many clips
Electromagnets are used in scrapyard cranes, door locks and electric bells. They are useful because they can be switched off. A crane drops its load when the current stops.
Try it
Make an electromagnet
You need: a long iron nail, about 8 cm, about 1 metre of thin wire, with the ends bared by an adult, one or two torch cells of 1.5 V, tape, small paper clips or pins.
Be safe: The wire and the cell can get hot. Connect for only a few seconds, then let go. Let them cool before you touch them. Keep the nail tip and the pins away from your face.
Wind the wire in 10 neat turns round the nail. Leave a tail at each end. An adult bares the ends, with care, as the blade is sharp.
Tape one end of the wire to one end of the cell. Hold the other end on the other end of the cell for 3 seconds. Count the clips the nail tip lifts.
Lift the wire off the cell. Do the clips fall? Let the wire and the cell cool for a minute.
Now wind 20 turns and count again. Then try two cells in a line.
What you will see: With more turns the nail holds more clips. With two cells it holds more again. When you lift the wire off the cell, most clips fall. Our 20 turns and one cell held 3 clips. Your counts will differ.
If you cannot try it, this is what happens.
A wrong idea many people have
An electromagnet is a permanent magnet that stays on once it is made.
Why it feels right: A nail with a coil looks like any other magnet, and a bar magnet never switches off.
The test that breaks it: Lift some clips with the electromagnet. Lift the wire off the cell and the clips drop. Join it again and they stick again.
What is true: An electromagnet works only while a current flows. An iron core loses nearly all its magnetism when the current stops.
Key words
coil
wire wound round and round.
turns
the loops of wire in a coil.
core
the iron nail inside a coil.
electromagnet
a coil with a current in it, usually round an iron core.
Back to your guess
The answer is B: Yes. A current in a coil of wire makes a magnet, and it stops when the current stops.
A current in a coil of wire makes a magnetic field. Switch the current off and the field goes. That is an electromagnet.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
An iron nail has a coil round it joined to a cell. Mei opens the switch. What happens to the nail?
See the answer
It loses nearly all of its magnetism.
The magnetism comes from the current. Switch it off and an iron core loses nearly all its magnetism, so the clips fall.
If you chose A: That is how a permanent magnet works. An iron core in a coil loses nearly all its magnetism when the current stops. Go to S3.10.7.
If you chose B: The poles swap when you turn the current round, not when you stop it. Go to S3.10.7.
Kofi wants to make his electromagnet stronger. What should he do?
See the answer
Wind more turns of wire round the nail.
More turns add more fields together. More cells would also help, because they give a bigger current.
If you chose B: The iron core makes the field much stronger. Without it the coil is weak. Go to S3.10.7.
If you chose C: Plastic does not become a magnet. Iron does, and that is why the nail makes the field stronger. Go to S3.10.7.
In one learner's test, an electromagnet with 10 turns and one cell held 2 clips, as shown. She winds 20 turns and keeps the one cell. Using the table in this chapter, how many clips does it hold?
See the answer
3 clips
The table gives 20 turns and one cell as 3 clips. More turns help, but clip counts are rough, so they do not grow exactly in step.
If you chose A: More turns give a stronger magnet, so it holds more clips than before. Go to S3.10.7.
If you chose C: That doubles the clips exactly. Clip counts are rough, so read the table. It gives 3. Go to S3.10.7.
Why is an iron nail put inside the coil?
See the answer
The nail becomes a magnet in the field of the coil, so the whole thing is stronger.
Iron becomes a magnet when it is in a field. Its field adds to the coil's, so the electromagnet is much stronger.
If you chose A: The current flows in the wire. The nail does not carry it. The nail becomes a magnet and makes the field stronger. Go to S3.10.7.
If you chose B: It does more than that. The iron adds its own field to the coil's, which makes the electromagnet much stronger. Go to S3.10.7.
From an earlier chapter
Review: the point Q is to the right of the N end of a bar magnet. Which way does the black end of a compass point at Q?
See the answer
Away from the magnet.
Field lines leave the N pole. At Q the field points away from the magnet, so the black end does too.
If you chose B: Field lines leave the N pole. At Q the line points away from the magnet, and the needle points that way. Go to S3.10.6.
If you chose C: The needle lines up with the field. At Q the field points along the axis, not up the page. Go to S3.10.6.
Ana tests her electromagnet with a cell. What is the safe way to do it?
See the answer
Join it for a few seconds, count the clips, then let go.
The wire and the cell can get hot. A few seconds is enough to count the clips.
If you chose A: The wire and the cell get hot. Connect for only a few seconds, then let go. Go to S3.10.7.
If you chose C: Taped on, the current keeps flowing. The wire and the cell get hot and the cell goes flat. Join it only while you count. Go to S3.10.7.
Keep it
Copy this into your exercise book.
A current in a coil makes a magnet. An iron core makes it stronger. More turns or more cells make it stronger. Switch off and an iron core loses most of its magnetism.
Draw: An iron nail with a coil of wire. A cell and a switch. Paper clips under the tip.
