KS3 · Physics

Electromagnets

Flick a switch and an iron nail grabs a clump of paper clips. Flick it off and they fall.

Physics · Magnetism

Where there's a current, there's a magnetic field

coil of wire

Magnetic field — Current in a straight wire. current out. This is one straight wire seen end-on, with the current coming out of the screen towards you. The field makes circles around the wire. The arrows show which way a compass needle would point if you put it there. The lines are thin and spread out: the field around a single wire is weak. (The coil on the right has no current in it yet, so it has no field.). static

Switch between the two

This is one straight wire seen end-on, with the current coming out of the screen towards you. The field makes circles around the wire. The arrows show which way a compass needle would point if you put it there. The lines are thin and spread out: the field around a single wire is weak. (The coil on the right has no current in it yet, so it has no field.)

Magnetic field lines around a current-carrying straight wire, and around a current-carrying coil. Arrows show the way a compass needle would point; closer lines mean a stronger field.

Watch out: These field lines only exist while there is a current. With no current in the wire, there is no magnetic field at all — nothing for a compass needle to feel.

Predict, then check

Think back to charged balloons. Does 'electric' mean 'magnetic'?

You rub a balloon on your jumper so it's charged. It can now pick up tiny bits of paper. You hold it right next to a compass. What does the compass needle do?

Making an electromagnet

  1. Start with an iron coreTake an iron rod or a big iron nail. This is what's going to become your magnet.
  2. Wind insulated wire round itWrap insulated wire round and round the core to make a coil. Each loop of wire is called one turn.
  3. Connect the coil to a cellJoin the two ends of the wire to a cell, with a switch in the circuit. Close the switch and a current flows round and round the turns.
  4. It's a magnetWhile the current flows, you have an electromagnet. Dip its end into a pile of paper clips and it picks them up.
  5. Switch offOpen the switch and the current stops. The magnetic field disappears and the paper clips drop. It's a magnet only while the current flows.

Physics · Electromagnets

Stronger, weaker, or hardly a magnet at all?

Each card is one change to a working electromagnet. Where does each one belong?

Still to sort

Stronger (0)

It picks up more paper clips.

Weaker (0)

Still a magnet, but it picks up fewer paper clips.

Hardly a magnet at all (0)

No strong magnetic field.

Where the line is: 'Weaker' still works as a magnet. Here the current never flows round the turns, so the coil can't make a strong field.

7 of 7 still to sort.

Picture a working electromagnet: a coil of insulated wire joined to a cell. Each card is one change, made on its own.

Working scientifically

From paper clips to a pattern

A class tests how the number of turns affects an electromagnet's strength by counting how many paper clips it picks up. These are made-up practice numbers, not real results. With 20 turns, one group's four repeat counts were 12, 11, 4 and 13 paper clips. What should they record, and how should they plot the class's results?

  1. Look along the repeats: 12, 11, 4, 13. Three of them sit close together.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Send a current through a wire and it makes a magnetic field. Coil it round iron and you've built a magnet with an off switch.

What you need to know

  • A current in a wire makes a magnetic field around the wire, and the field deflects a compass needle. With no current, there's no magnetic field.
  • Magnetic and electrostatic forces are different types of force: a charged balloon doesn't move a compass needle, but a current does.
  • Magnetic field lines show a field. Their arrows show the way a compass needle would point, and lines closer together mean a stronger field.
  • The field around a single straight wire is weak. Coiling the wire makes the loops' fields add up to a stronger field, the same shape as a bar magnet's.
  • An electromagnet is a coil of insulated wire around an iron core. It's a magnet while a current flows and stops being one when the current stops.
  • More turns, a larger current or an iron core make an electromagnet stronger. A higher-resistance coil lets less current through, so the field is weaker.

The big picture

When an electric current flows through a wire, it makes a magnetic field around the wire, and you can spot it because it moves a compass needle. The field around a single straight wire is weak, but coil the wire up and the fields from all the loops add together into a stronger field shaped just like a bar magnet's. Wrap that coil of insulated wire around an iron core and you have an electromagnet: a magnet you can switch on and off with the current. More turns, a bigger current and an iron core all make it stronger.

Key points

1Current on, magnetic field on. Current off, field gone.
2A compass needle responds to a current, not to a charged balloon.
3Closer field lines mean a stronger field.
4Coil the wire and the loops' fields add up into a bar-magnet shape.
5Insulated wire keeps the current flowing round the turns, not through the core.
6Testing strength: repeat the paper-clip counts, leave out anomalies, plot the means.

Worked example

Problem

Ali and Beth each wind 30 turns of insulated wire round an identical iron nail and connect it to the same kind of cell. Ali's coil has a higher resistance than Beth's. Whose electromagnet is stronger, and why?

⚠ Watch out

Thinking an electromagnet stays magnetic like a bar magnet. It doesn't: stop the current and the magnetic field disappears, so the paper clips drop.

🧠

Memory hook

No flow, no field. Switch it off and the paper clips drop.

✓

Check yourself

Your friend says, 'An electromagnet is just a bar magnet with some wire wrapped round it.' Give one way they're right and one way they're wrong.

Flashcards

(14)
What is there around a wire when a current flows through it?
A magnetic field. When the current stops, the field disappears.
How can you show there's a magnetic field around a current-carrying wire?
Put a compass near it. The field deflects the compass needle.
Does a charged balloon make a compass needle move?
No. Magnetic and electrostatic forces are different types of force, so the needle isn't affected by the charge. A current does move it.
What do the arrows on magnetic field lines show?
The way a compass needle would point at that spot.
On a field-line diagram, what do lines drawn closer together mean?
A stronger magnetic field there.
Why is a coil's field stronger than the field around the same wire left straight?
A single wire's field is weak. In a coil, the fields around all the loops add together.
What shape is the magnetic field around a current-carrying coil?
The same shape as the field around a bar magnet.
What is an electromagnet made of?
A coil of insulated wire wrapped around a core such as an iron nail, with a current passing through the coil.
How do you turn an electromagnet off?
Stop the current. The field disappears and anything it was holding, like paper clips, drops.
Name three changes that make an electromagnet stronger.
More turns on the coil, a larger current, and an iron core inside the coil.
Why must an electromagnet's wire be insulated?
So the current flows round the turns of the coil. Bare wire lets it flow straight through the core, giving no strong field.
What does increasing the resistance of the coil do to an electromagnet?
It reduces the current, so the magnetic field gets weaker.
When you work out the mean of repeat paper-clip counts, what do you do with an anomalous result?
Leave it out. Take the mean of the readings that fit together.
In the paper-clip investigation, what goes on each axis of the graph?
Number of turns on the x-axis, mean number of paper clips on the y-axis.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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