There’s an Enormous Bar Magnet in the Earth’s Center, Right?
Looking at the magnetic field of the Earth from a great distance, it seems that the source of this field is a huge and slightly tilted bar magnet which is about 450 kilometers (280 miles) displaced from the Earth’s center towards 140 degrees eastern longitude¹. And though all the compass needles appear to point north, the magnetic poles don’t coincide with the geographic poles. The inclination of the “magnet” in the Earth’s interior towards the rotational axis is currently about 11 degrees.
How, after all, comes this magnetic field into being in the first place? Is there really a gigantic bar magnet consisting of a red and a green side sitting in the center of the globe? (And are there maybe even a big “N” and “S” on its halves?)
Do we actually know anything about the origin of our global magnetic field? I mean, the deepest hole ever digged by humans is just some lousy kilometers deep – that’s nothing compared to the 6400 kilometers distance between us and the Earth’s center. Thus, it’s absolutely impossible that somebody had a look and the chance to directly see an enormous bar magnet.
Well, it’s quite impossible that there’s indeed a big bar magnet like the ones depicted in the physics schoolbooks – you probably have already expected this. However, can there still be a magnet of whatever shape which produces the observed magnetic field in the interior of the Earth?
Well… Looks like we cannot dig down and just go and see – we have to find other ways to identify the source of the global magnetic field.
For this purpose, it might be useful to recall some general properties of magnets.
Magentism is the result of the interaction between tiny (magnetic) dipole moments on atomic scales. Many substances contain loads of incredibly small (atoms which have) dipole moments. For the sake of simplicity, just imagine them as tiny little bar magnets, bound to the atomic particles of the material. It’s a common phenomenon in the entire universe that things wiggle and shake on the level of particles and molecules. The lower the temperature, the lesser the shaking – but the temperature is always above absolute zero (-273.15 °C), therefore there is always some shaking of the particles. The vibrations in some materials result in the tiny bar magnets (= magnetic dipole moments) being completely jumbled up. In this state, the little magnets aren’t able to align themsevles in order to point in a single common direction. You would need an external magnetic field to getting them aligned. Materials in which the dipole moments are randomly aligned without an external magnet field, but can be unjumbled by applying a magnetic field, are called “paramagnetic materials”. They aren’t really magnetic on their own, but they amplify a magnetic field if applied.
Of course, there are other materials as well, in which the thermal vibrations of the components aren’t strong enough to jumble the magnetic dipole moments completely. The minute bar magnets of these materials (more or less) align themselves with each other spontaneously. All such materials are called “ferromagnetic” – we usually mean ferromagnets when we talk of “magnets” in our day-to-day lifes.
If we raise the termperature of ferromagnets, their atomic building blocks begin to jitter more and more. At some point, the shaking is too intense to maintain the dipole moments’ well-arranged order. Then, the vibrations of the tiny “bar magnets” are so heavy that a magnetization on a grand scale isn’t possible anymore. We now have all these dipole moments randomly aligned in all spatial directions. Again, this state pefectly matches our definition of paramagnets.
The temperature at which ferromagnets suddenly become paramagnets is called the Curie point .
Meanwhile, I’d really like to refer those of you who prefer vivid visualizations (like I do) to the brilliant video “MAGNETS: How Do They Work?” by MinutePhysics and Veritasium! In it, Henry and Derek bautifully explain how the different kinds of magnetism arise. Furthermore, there’s an awesome interactive Periodic Table of Elements by Henry which shows how the magnetic properties of materials change depending on the temperature.
However, we still want to find out if there’s an enormous bar magnet in the Earth’s interior, right?!
At this point, we have collected enough knowledge to being able to anwer this question.
Bar magnets are ferromagnets. But we know (by different kinds of considerations) that all ferromagnetic materials in the inside of the Earth have a temperature above the Curie point. That’s why the global magnetic field cannot arise from any kind of solid magnet in the interior!
Even though we’ve never digged down to the Earth’s core, we can exclude this kind of origin of the magnetic field.
What, then, causes the terrestrial magnetic field?
As long ago as 1820, the Danish physicist and chemist Hans Christian Ørsted discovered the deviation of a compass needle when near a current-carrying conductor during one of his lectures. Moving charges (= currents) can obviously produce magnetic fields. (Forty-odd years later, James Clerk Maxwell was able to formulate the mathematical theory of electromagnetism, which can explain this phenomenon (and all the other electromagnetic occurrences) to a remarkable degree of accuracy.) So moving electrical charges creates a magnetic field. This doesn’t only apply to conductors, but also to the Earth’s interior!
Consequently, the terrestrial magnetic field originates in magma currents containing electrically charged particles in the fluid inside of the Earth. These circular currents run symmetrically with respect to the dipole axis. They exist because of different temperature gradients (“liquid matter ascends, cools down, and descends again”), because of dynamic effects which cause relative motions between the fluid and the solid parts of the Earth, and because of other reasons which I shall not discuss further here.
There are other reasons supporting the idea that the global magnetic field is produced by moving currents. Let’s take all the field’s small, local differences which can be measured and the slow changing of the global magnetic field as examples.
So without literally having to dig down to the Earth’s core, we discovered that there is no giant bar magnet, but instead circular currents of charged particles which cause the terrestrial magnetic field. In doing so, we haven’t even had to leave our desk. How convenient!
¹ Source: http://de.wikipedia.org/wiki/Erdmagnetfeld#Form_und_St.C3.A4rke
Posted on December 17, 2013, in Earth, Electromagnetism, Geology, physics, science and tagged Curie point, Earth, Earth's magnetic field, Electromagnetism, Ferromagnetism, global magnetic field, Magnetic field, Magnetism, Materials Science, Paramagnetism. Bookmark the permalink. 2 Comments.