Magnetism Notes

Edexcel GCSE Physics: Revision notes

Key facts

  • Like poles repel; unlike poles attract, without touching.
  • A permanent magnet has its own field; an induced magnet is only magnetic in an external field.
  • Magnetic materials: cobalt, steel, iron and nickel.
  • Field lines run from north to south outside a magnet; closer lines mean a stronger field.
  • A compass points in a consistent direction, so the Earth's core is magnetic.

Like poles

  • N and N, or S and S
  • Repel

Unlike poles

  • N and S
  • Attract

Magnetic poles

Like poles repel and unlike poles attract, through a non-contact force.

Every magnet has a north and a south magnetic pole. The force acts without touching, through the magnetic field.

repelA0A1A2A3A4A5SNB0B1B2B3B4B5NS
Like poles repel: the two north poles (shaded) face each other and push apart.

Like poles

  • N and N, or S and S
  • Repel

Unlike poles

  • N and S
  • Attract

What happens when the north poles of two magnets are brought together?

Permanent and induced magnets

A permanent magnet always has its own field; an induced magnet is magnetic only while in another magnet's field.

Materials used for magnets include cobalt, steel, iron and nickel. An induced magnet is always attracted to the magnet inducing it, whichever pole faces it.

attractA0A1A2A3A4A5SNB0B1B2B3B4B5SN
An induced magnet (right, soft iron) gets a south pole nearest the magnet's north pole, so the force is always attraction.

Permanent magnet

Own magnetic field:
Always present
Loses magnetism?:
No: stays magnetised
Example material:
Steel

Induced magnet

Own magnetic field:
Only in an external field
Loses magnetism?:
Yes: quickly, once the field is removed
Example material:
Iron

An iron nail is attracted to a magnet and falls off when the magnet is removed. What was it?

Magnetic fields

Field lines run from north to south outside the magnet and are closest together where the field is strongest.

A magnetic field line shows the direction a north pole would be pushed. A uniform field has evenly spaced, parallel lines: the same strength and direction everywhere.

A plotting compass needle lines up with the field, so it can map field lines.

Near the poles

  • Lines close together
  • Strong field

Far from the magnet

  • Lines spread out
  • Weak field

Uniform field

  • Evenly spaced parallel lines
  • Same strength everywhere
  1. 1

    Place

    Put the compass near the magnet and mark where the needle points.

  2. 2

    Move

    Move it along the direction it pointed.

  3. 3

    Repeat

    Mark many positions and join them to build a field line.

Mapping a field with a plotting compass

Where is the field around a bar magnet strongest?

The Earth's field

A compass points consistently, so the Earth has a magnetic field and acts like a giant bar magnet.

A compass needle lines up with the Earth's field and points roughly towards geographic north. Because it responds only to a magnetic field, this is evidence that the Earth's core is magnetic.

  1. 1

    Compass lines up

    Wherever it is on the surface.

  2. 2

    Consistent direction

    It responds only to a magnetic field.

  3. 3

    Earth has a field

    Like a giant bar magnet.

  4. 4

    Core is magnetic

    The source of the field.

From compass to magnetic core

What does the behaviour of a compass suggest about the Earth?

Try an exam question

Describe how to use a plotting compass to find the shape of the magnetic field around a bar magnet.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.