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Unit 9: Magnetism and electromagnets

The Earth as a giant magnet — HIA International - Grade 7 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 5 3 quizzes

📚 HIA International - Grade 7 Hub / Topical Revision
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Lesson summary

The Earth as a giant magnet — HIA INTERNATIONAL notes: Explain that the Earth has a magnetic field like a bar magnet, caused by its core, and how a compass uses it.Lodestone — a naturally occurring magnet (magnetite) that is not made by people.Invention of the compass — a small n…

  • Field lines
  • Magnetic poles
  • Geographic poles
  • Compass needle
  • Magnetic north
  • Invention of the compass — a small needle of lodestone floating on water always turned to point in the same direction

Part 1 of 6Early compasses

Lodestone
  • Lodestone a naturally occurring magnet (magnetite) that is not made by people.
Invention of the compass
  • Invention of the compass a small needle of lodestone floating on water always turned to point in the same direction.
Use
  • Use a compass lets people navigate on oceans and deserts and always shows which direction they are facing.
Today
  • Today ships and aeroplanes still use magnetic compasses, but satellite navigation does not use the Earth’s magnetic field.
Animals
  • Animals some animals, such as birds, use the Earth’s magnetic field to navigate over long distances.

Part 2 of 6The Earth’s magnetic field

Shape
  • Shape the Earth’s magnetic field is similar to the field of a bar magnet.
Field lines
  • Field lines they point towards the Earth’s north pole, because field lines point from north to south.
Magnetic poles
  • Magnetic poles the north pole of the Earth is actually a magnetic south pole.
  • A compass needle points to magnetic north, which is close to the geographic north pole.
  • Magnetic south is the magnetic pole close to the geographic south pole.
Geographic poles
  • Geographic poles the points where the spin axis of the Earth passes through the surface.
Compass needle
  • Compass needle the north pole of a compass points north because it is attracted to the magnetic south pole of the Earth.

Part 3 of 6Magnetic north moves

Position
  • Position magnetic north is not exactly at the geographic north pole and is currently in the Arctic Ocean north of Alaska.
Movement
  • Movement it moves very slowly, at about 60 km per year.
Reversals
  • Reversals the field has reversed in the past, the last time about 500 000 years ago.
Effect
  • Effect airport runway numbers are based on the direction from magnetic north, so they sometimes have to change.

Part 4 of 6Origin of the field

Core
  • Core the Earth’s core is the origin of the magnetic field.
Metals
  • Metals the core is mostly iron and also contains nickel, both magnetic metals.
Heat
  • Heat rocks deep inside lose any magnetism at high temperature, so the field cannot come from magnetic rocks.
Movement
  • Movement movement in the liquid outer core explains why the poles move slowly and have reversed.
Detail
  • Detail scientists have still to discover the exact reason.

Part 5 of 6Aurora

Cause
  • Cause particles from the Sun arrive in the stronger parts of the field near the poles.
Effect
  • Effect they produce natural lights in the night sky near the north and south poles.

Part 6 of 6Showing the Earth’s field with a magnet

1
Hang a bar magnet from a string in a paper support so that it hangs horizontally.
2
Keep other magnets and large magnetic objects away.
3
Let it come to rest and record the direction it points.
4
Move the apparatus to another part of the room and check that it points the same way.
5
Stroke a needle with one pole of a magnet, always in the same direction, and float it on a cork disc in water to make a compass.
Showing the Earth’s field with a magnet
The end of the magnet that points north is its north pole.
Such home-made compasses are not practical for navigation on a journey.

Key terms

TermMeaning
LodestoneA naturally occurring magnetic rock containing magnetite.
CompassA tool with a magnetised needle that points to magnetic north.
Magnetic northThe magnetic pole close to the geographic north pole.
Geographic poleA point where the Earth’s spin axis passes through the surface.
CoreThe centre of the Earth, mostly iron with some nickel, which produces the magnetic field.
Naturally occurringNot made by people.

Avoid these mistakes

1
Magnetic north is a magnetic north pole — it is a magnetic south pole, because it attracts the north pole of a compass.
2
Magnetic north stays fixed — it moves slowly, about 60 km per year.
3
The Earth’s field comes from magnetic rocks in the crust — it comes from the iron and nickel core.
4
Satellite navigation uses the Earth’s magnetic field — it does not.

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