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Magnetism: 9.5 Origin of Earth's magnetic field and how a compass works

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9.5 Earth's magnetic field and compass

Swirling currents of heated and cooling molten (mainly) iron metal in the Earth's outer core create an electric current, which, due to the rotation of the Earth, create a magnetic field.

diagram of the Earth's magnetic field and the Earth's north and south poles The Earth's magnetic field

A compass works by the alignment of its needle with the Earth's magnetic field.

The geographic north pole is actually the south pole of the Earth's magnetic field - this is due to the convention established from electromagnetic science - it was an arbitrary choice, rather like the convention for the direction of flow of electricity - which is opposite to the flow of electrons!

Therefore the Earth's magnetic north pole is at the geographical south pole.

Therefore the magnetic south pole of the compass needle is attracted and points to the geographic north pole of the Earth.

The magnetic north is a few degrees away from the true geographic north pole based on the Earth's spin axis.

The fact that a magnetised-magnetic compass needle always aligns itself in a particular direction on the Earth's surface, is proof that the Earth has a permanent magnetic field.

However, it has been deduced from geological studies of ancient rocks that the Earth's magnetic poles flip every so often, perhaps as often as every 200 00 years now.

a compass for navigating using an OS map

The trusty compass to help you navigate the countryside with your OS map.

However, these days, many walkers are using GPS systems for navigation, but I still like a 'real map' and a 'real compass'!

INDEX for physics notes on magnetism

Keywords, phrases and learning objectives on magnetism

Be able to describe the origin of Earth's magnetic field and explain how a compass works.

Know the Earth's iron core creates a permanent magnetic field.


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