Stir a vat of molten iron, get a magnetic field?

[cephron]

Hey all,

I'm a complete layman at all things magnetic, and lately I've been trying to understand how the Earth's geodynamo works.

From my understanding of Wikipedia and this site (http://es.ucsc.edu/~glatz/geodynamo.html), the fundamental thing that actually generates magnetic field is:--- electric current --- generated by shear in the molten iron in the outer core --- as coriolis force affects its convective motion.

So, to clarify this for myself, I have a simple thought experiment which does away with most of the complicated stuff.

The setup: Suppose we have a cylindrical vat full of molten iron. The ambient temperature is the same as that of the molten iron, so it is in thermal equilibrium and there is no convective motion in the molten iron. The molten iron is "at rest".
[Sanity check: we would observe no magnetic field right now, correct?]

Experiment 1: We lower a rotating rod (with some sort of blade or paddle on it) into the center of the vat, stirring the molten iron. There should now be shear in the molten iron, caused by the motion of the stirring rod relative to the vat. Therefore, current would be induced.
[Result 1: we would observe a magnetic field, right?]

Expermient 2: In this experiment, we set the vat itself rotating and observe as the molten iron eventually matches speed. No stirring rod is involved.
[Result 2: we would observe a magnetic field at first, but it would diminish and finally vanish as the molten iron matched the rotation of the vat. Is this what would happen?]

Any input, corrections and/or further thought are most welcome. Thanks!

 

[eljose79]

Hello there,

Thank you for your interest in understanding the Earth's geodynamo. You have a good understanding of the basic principles involved, but I would like to clarify a few points and provide some additional information.

Firstly, your thought experiment is a good way to conceptualize the generation of magnetic fields in the Earth's outer core. However, it is important to note that the Earth's outer core is not just a simple cylindrical vat of molten iron. It is a highly complex and dynamic system that involves not only the convective motion of the molten iron, but also the presence of other elements such as nickel and sulfur, as well as the influence of the solid inner core.

To answer your first question, yes, in your setup of a cylindrical vat of molten iron at rest, we would not observe a magnetic field. This is because there is no convective motion or shear occurring, which is necessary for the generation of electric currents.

In your first experiment, where you stir the molten iron with a rotating rod, you are correct in saying that this would induce a magnetic field. This is due to the shear caused by the motion of the stirring rod, which generates electric currents in the molten iron.

In your second experiment, where you set the vat itself rotating, you would indeed observe a magnetic field at first. However, as the molten iron matches the rotation of the vat, the magnetic field would diminish and eventually vanish. This is because the convective motion and shear in the molten iron would no longer be present.

I hope this helps to clarify your understanding of the Earth's geodynamo. It is a fascinating and complex phenomenon that is still not fully understood by scientists. If you have any further questions, please don't hesitate to ask.