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Earth's magnetic field

Electricity and magnetism · level 6, lesson 4 of 6

A question to start

A compass needle's north end points to the north. But north poles repel north poles. So what kind of pole is in the Arctic?

The idea

Earth's field is shaped roughly like that of a giant bar magnet inside the planet, tilted a little from the spin axis.

  • A compass's north end is attracted to the pole in the Arctic. So that pole is, magnetically, a south pole. We still call it the magnetic north pole, because it is in the north.
  • The magnetic north pole is not at the geographic North Pole, and it wanders a little each year.
  • The field is not made by a solid magnet. It comes from currents in Earth's liquid iron outer core, kept going as the molten metal churns. This is called a dynamo.

The field reaches far out into space, where it turns aside most of the charged particles streaming from the Sun.

Earth's field Value
Strength at the surface 24 to 66 µT, weakest near the equator, strongest near the poles
Magnetic north pole (2020 model) 86.5° N, 164.0° E, in the Arctic Ocean
SN spin axismagnetic north pole:a south pole, magnetically

Worked examples

1. The compass. Opposite poles attract, so the needle's north end is pulled towards the magnetic south pole that sits in the Arctic.

2. Strength. 66 µT near the poles against 24 µT in the weakest places: about 2.75 times stronger.

3. Not a solid magnet. Iron loses its magnetism above about 770 °C, and Earth's core is far hotter. So the field must be made by moving charge in the liquid core.

Try it

  1. What kind of magnetic pole is near the geographic North Pole?
  2. What makes Earth's magnetic field?
  3. Why can it not be a giant solid magnet?
  4. How does Earth's field help life on the surface?
Answers
  1. A magnetic south pole.
  2. Currents in the churning liquid iron of the outer core: a dynamo.
  3. The core is far too hot for iron to stay magnetized.
  4. It turns aside most of the charged particles streaming from the Sun.

Common mistake

"The magnetic and geographic north poles are the same." The magnetic pole is hundreds of kilometres away and moves. Navigators correct for the difference.

Where it leads