Magnets and Magnetic FieldsAQA GCSE Physics: Revision notes
Section 1
What is a magnetic field?
A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material. Magnetic materials include iron, steel, cobalt and nickel — most other materials (wood, plastic, copper) are not affected.
The field is strongest at the poles of a magnet and gets weaker further away. At any point, the direction of the field is defined as the direction of the force that would act on a north pole placed at that point.
If a question asks for field direction at a point, always answer in terms of the force on a (hypothetical) north pole there — never 'towards the magnet'.
Section 2
Attraction and repulsion between poles
Every magnet has two poles: north (N) and south (S).
- Like poles repel (N–N or S–S)
- Unlike poles attract (N–S)
These forces act without contact — magnetism is a non-contact force, like gravity or the electrostatic force.
Do not say magnets 'touch' to attract or repel — the force acts at a distance, which is exactly what makes it a non-contact force.
Section 3
Permanent magnets vs induced magnets
A permanent magnet produces its own magnetic field all the time — it does not need another magnet nearby.
An induced magnet is a material that only becomes magnetic when it is placed inside another magnetic field. Key facts about induced magnets:
- They always experience a force of attraction towards the magnet inducing them (never repulsion)
- They lose their magnetism quickly once removed from the field
This is why a paperclip picked up by a bar magnet can itself pick up a second paperclip, but drops it as soon as it is pulled away from the first magnet.
An iron nail near a bar magnet becomes an induced magnet and is attracted to it; move the nail away and it stops being magnetic almost immediately.
Section 4
Plotting a magnetic field pattern
The field pattern of a bar magnet can be plotted using a small plotting compass:
- Place the compass near the magnet and mark the direction the needle points (N pole of compass points along the field line)
- Move the compass to the position the needle pointed to and mark the new direction
- Repeat, joining the marks to build up a field line
- Repeat starting from different points around the magnet
The resulting pattern shows field lines running from the N pole, curving round, and into the S pole. Lines are closer together (denser) near the poles, showing the field is strongest there, and spread out where the field is weaker.
Closer-together field lines = stronger field. This is a quick way to compare field strength between two regions on a diagram or description.
Section 5
The Earth as a magnet
A compass needle always settles pointing roughly north–south. This only happens because the Earth itself behaves as if it has a giant bar magnet inside it, with a magnetic field extending out into space.
The consistent, predictable behaviour of a compass anywhere on Earth's surface is used as evidence that the Earth's core must be magnetic.
Must Know
- Like poles repel, unlike poles attract — magnetism is a non-contact force
- Magnetic materials: iron, steel, cobalt, nickel
- Field strength is greatest at the poles and decreases with distance; field direction at a point = direction of force on a north pole there
- Permanent magnets always produce their own field; induced magnets only become magnetic in a field, always attract, and lose magnetism quickly when removed
- A plotting compass is used to build up a magnet's field line pattern, from N to S
- A compass points north-south because the Earth's core is magnetic
That's the notes covered.
Carry on to the next subtopic.