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PHYSICS · Stage 8 · Chapter 10: The Earth in Space (Galaxies and Asteroids)

Physics 10.3 Magnetic Earth — Earth’s Core Dynamo, Magnetosphere and Auroras

5 min read 5 parts Lesson 3 of 5

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Physics 10.3 Magnetic Earth — Earth's Core Dynamo, Magnetosphere and Auroras — Alhussan International Academy — Physics (Stages 7–9) notes: Explain what a compass does, describe the Earth's magnetic field and explain why the Earth has one.Compass = a small magnet on a pivot that …

  • Suspended magnet
  • Bar magnet
  • Magnetic poles
  • Outer core
  • Dynamo effect
  • Solar wind

Part 1 of 5The compass

  • Compass a small magnet on a pivot that shows which way is north.
  • Suspended magnetA magnet hung on cotton lines up in the same north-south direction each time.
  • ReasonThe Earth's magnetic field exerts a force on the needle, so it lines up with the field.
  • HistoryPeople have used compasses to navigate since ancient times, and some birds and insects use the field to navigate.

Part 2 of 5Shape of the Earth's field

  • Bar magnet — The Earth behaves as if it has a bar magnet inside it, with a field like a bar magnet's.
  • Magnetic poles — Compass needles point to the Earth's magnetic pole, not the geographic North Pole at the top of the axis of rotation.
    • The magnetic north pole is in Canada, about 400 km from the geographic North Pole.
    • It is moving at about 30 km a year from Canada towards Russia.
    • The magnetic pole near the Arctic is really a south pole of the field because it attracts the north end of a compass.
  • Dip — The field lines also go into the ground at an angle, so a magnet free to turn in three dimensions points diagonally downwards.
  • Dip size — It is small near the equator and increases towards the poles, where the field points straight down or up.

Part 3 of 5Why the Earth has a magnetic field

  • No bar magnet — There is no permanent magnet inside the Earth.
  • Outer core — It is made of liquid iron, with some nickel, that can move and conducts electricity.
    • Moving currents of iron in the core act like an electromagnet.
    • The movement is driven by heat from the inner core and the Earth's rotation.
  • Dynamo effect — This process of making a field from moving conducting liquid is called the dynamo effect.
  • Particles — The particles in a liquid can flow past each other, whereas in a solid they can only vibrate, so a liquid core is needed.
  • Reversals — The field has flipped in the past, with the poles changing places about 780 000 years ago, and it may happen again.

Part 4 of 5Magnetosphere and the solar wind

  • Solar wind = a stream of electrically charged particles emitted by the Sun in all directions.
    • It is not wind you can feel.
    • The particles can damage or kill cells.
  • Magnetosphere = the region around the Earth where its magnetic field deflects the solar wind, so most does not reach the surface.
  • Protection — Without the field, the solar wind would endanger living organisms.

Part 5 of 5Auroras

  • Aurora the dramatic lights in the sky called the northern and southern lights.
  • CauseSome solar wind particles are guided to the north and south poles, where they collide with atoms and molecules in the upper atmosphere.
  • LightThe collisions release energy as light.
  • RequirementsA planet with a magnetic field, charged particles from the Sun and an atmosphere.

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