The Earth’s Magnetic Field

The Earth’s Magnetic Field

In the picture below, you can see a representation of the Earth’s magnetic field which is very similar to the magnetic field of a giant bar magnet like the one on the right of the picture. The Earth has two magnetic poles, a north and a south pole just like a bar magnet.

In addition to the magnetic poles the Earth also has two geographic poles. The two geographic poles are the points on the Earth’s surface where the line of the Earth’s axis of rotation meets the surface. To visualise this you could take any round fruit (lemon, orange etc.) and stick a pencil through the middle so it comes out the other side. Turn the pencil, the pencil is the axis of rotation and the geographic poles are where the pencil enters and exits the fruit. We call the geographic north pole true north.

The Earth’s magnetic field has been measured very precisely and scientists have found that the magnetic poles do not correspond exactly to the geographic poles.

So the Earth has two north poles and two south poles: geographic poles and magnetic poles.

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Did You Know?

The direction of the Earth’s magnetic field flips direction about once every \(\text{200 000}\) years! You can picture this as a bar magnet whose north and south pole periodically switch sides. The reason for this is still not fully understood.

The Earth’s magnetic field is thought to be caused by flowing liquid metals in the outer core of the planet which causes electric currents and a magnetic field. From the picture you can see that the direction of magnetic north and true north are not identical. The geographic north pole is about \(\text{11.5}\)\(\text{°}\) away from the direction of the magnetic north pole (which is where a compass will point). However, the magnetic poles shift slightly all the time.

Another interesting thing to note is that if we think of the Earth as a big bar magnet, and we know that magnetic field lines always point from north to south, then the compass tells us that what we call the magnetic north pole is actually the south pole of the bar magnet!

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This is a lesson from the tutorial, Magnetism and Faraday's Law and you are encouraged to log in or register, so that you can track your progress.

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