Magnets and Magnetic Fields Notes

AQA GCSE Physics: Revision notes

Key facts

  • Magnetic materials are iron, steel, cobalt and nickel.
  • Like poles repel; unlike poles attract: a non-contact force.
  • Field direction at a point is the direction of the force on a north pole there; the field is strongest at the poles.
  • Permanent magnets have their own field; induced magnets are only magnetic in another field and always attract.
  • A compass points north–south because the Earth’s core is magnetic.

Magnetic fields

A magnetic field is the region around a magnet where it exerts a force on another magnet or on a magnetic material.

A magnetic field is strongest at the poles and weaker further away. Its direction at a point is the direction of the force a north pole would feel there.

Only some materials respond. Wood, plastic and copper are not affected.

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Field lines of a bar magnet (illustrative): they run from N to S and are closest together at the poles, where the field is strongest.

Magnetic materials

  • Iron
  • Steel
  • Cobalt
  • Nickel

Not magnetic

  • Wood
  • Plastic
  • Copper

Which material is attracted to a magnet?

Attraction and repulsion

Like poles repel and unlike poles attract, without the magnets touching.

Every magnet has two poles: north (N) and south (S). The forces act at a distance, so magnetism is a non-contact force like gravity and the electrostatic force.

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Unlike poles (S facing N) attract: the forces pull the magnets together. Reverse one magnet and they repel.

Like poles

Poles facing:
N and N, or S and S
Force:
Repel

Unlike poles

Poles facing:
N and S
Force:
Attract

Two north poles are brought together. What happens?

Permanent and induced magnets

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

An iron nail near a bar magnet becomes an induced magnet and is attracted to it. Move the nail away and it quickly stops being magnetic.

This is why a paperclip held by a magnet can pick up a second paperclip, but drops it when pulled away.

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An iron nail near a magnet becomes an induced magnet: its near end becomes the opposite pole, so it is attracted.

Permanent magnet

Own magnetic field:
Always
Force with the inducing magnet:
Attracts or repels
When removed from the field:
Stays magnetic

Induced magnet

Own magnetic field:
Only while in another magnet’s field
Force with the inducing magnet:
Always attracts, never repels
When removed from the field:
Loses its magnetism quickly

An iron nail is attracted to a magnet and is then pulled away. What happens to its magnetism?

Plotting a field pattern

Use a small plotting compass to mark the field direction at point after point, then join the marks.

The pattern shows field lines running from the N pole, curving round, into the S pole. Lines are closer together near the poles, where the field is strongest.

Closer lines mean a stronger field, a quick way to compare two regions on a diagram.

  1. 1

    Place the compass

    Near the magnet, and mark the direction the needle points.

  2. 2

    Move the compass

    To the position the needle pointed to, and mark the new direction.

  3. 3

    Repeat

    Join the marks to build up a field line.

  4. 4

    Start elsewhere

    Repeat from different points around the magnet.

Plotting a field line with a compass.

Field lines are closest together near the poles. What does this show?

The Earth as a magnet

A compass settles north–south because the Earth behaves as if it has a giant bar magnet inside it.

The Earth has a magnetic field extending into space. The consistent behaviour of a compass anywhere on the surface is evidence that the Earth’s core must be magnetic.

  1. 1

    Compass settles

    A compass needle always points roughly north–south.

  2. 2

    Earth has a field

    It behaves as if it has a giant bar magnet inside.

  3. 3

    Magnetic core

    This is evidence that the Earth’s core is magnetic.

Evidence that the Earth’s core is magnetic.

What does a compass pointing north–south show?

Try an exam question

Describe how a plotting compass can be used to show the magnetic field pattern around a bar magnet.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.