Magnets, magnetic fields and electromagnetism
A magnet affects magnetic materials and other magnets without touching them. An electric current makes a magnetic field too, and coiling the wire turns that into an electromagnet you can switch on and off.
Part 1 of 3: Learn it
In short
- Two north poles (or two south poles) push apart; a north and a south pull together.
- Field lines go from the north pole to the south pole; closer lines mean a stronger field.
- A current in a coil (solenoid) makes a field like a bar magnet's; an iron core makes it much stronger.
Where this is in your specification
Spec points: AQA 4.7.1.1, 4.7.1.2 and 4.7.2.1 (Combined Trilogy 6.7.1 and 6.7.2.1), Edexcel Topic 12, OCR P4.1
| Board | Topic: Magnetism and electromagnetism |
|---|---|
| AQA 8463 | 4.7 (8463); 6.7 (8464) |
| Edexcel 1PH0 | Topics 12 and 13 |
| OCR J249 | P4 and P3.4 |
| Eduqas C420 | 8.1 to 8.4 |
| Cambridge IGCSE 0625 | 4.1, 4.5 |
Poles and fields
The field around a magnet is the region where another magnet or a magnetic material feels a force. It is strongest at the poles. The direction of a field line is the way the north pole of a small compass would point, which is from north to south outside the magnet.
A compass needle lines up with the Earth's magnetic field when no magnet is nearby, which is evidence that the Earth's core behaves like a magnet.
Permanent and induced magnets
- A permanent magnet makes its own magnetic field all the time.
- An induced magnet becomes a magnet only when it is in a magnetic field, and loses most or all of it when taken away.
- Induced magnetism always causes attraction, which is why a magnet picks up any steel paper clip whichever pole you use.
- Iron, steel, cobalt and nickel are magnetic materials.
Electromagnetism
A current in a straight wire makes a circular magnetic field around it. Reversing the current reverses the field direction; a bigger current or a point closer to the wire gives a stronger field. The right-hand grip rule: thumb along the current, fingers curl the way the field goes.
Coiling the wire into a solenoid makes the fields add up. Inside, the field is strong and uniform; outside, it has the same shape as a bar magnet's.
Making a stronger electromagnet
- Increase the current.
- Use more turns of wire in the coil.
- Wind the coil round a soft iron core.
Electromagnets are used where a magnet must be switched off, such as cranes in scrapyards, door locks and relays.
Name three magnetic materials.
Show the answer
Any three of: iron, steel, cobalt, nickel.
Part 2 of 3: See it worked
Worked examples
Example 1
A steel bar is attracted to both ends of a magnet. Is the bar a permanent magnet or an induced magnet? Explain.
- A permanent magnet would be repelled by one end of the magnet
- Being attracted to both ends shows its magnetism is induced by the field
Answer: It is an induced magnet: it is attracted to both poles and never repelled.
Example 2
Describe how to test which end of a solenoid is the north pole.
- Switch the current on
- Bring a plotting compass near one end
- The compass needle's south pole points towards the solenoid's north pole
Answer: Use a plotting compass: the end that the compass's south pole points to is the north pole.
Common mistakes
- Drawing field lines from S to N, or letting them cross.
- Saying all metals are magnetic. Copper and aluminium are not.
- Saying a current makes a straight field along the wire. It is circular around it.
- Thinking an induced magnet can repel. It is always attracted.
Give three ways to make an electromagnet stronger.
Show the answer
More current, more turns on the coil, and an iron core.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Name three magnetic materials.
Any three of: iron, steel, cobalt, nickel.
Give three ways to make an electromagnet stronger.
More current, more turns on the coil, and an iron core.
What happens to the field around a wire if the current is reversed?
The field reverses direction.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Ces fiches sont en anglais car elles suivent les programmes d'examen britanniques.
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