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

1. Mar 1, 2012

### 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!