Date: Fall 2012
So I have been reading about the geodynamo hypothesis. I need some help trying to explain this to 11 year olds. I understand this idea to be that as the molten material of the outer core convects it also has electricity and these two factors combined create the geomagnetic field. Is that a reasonable re-statement? From where does this electricity appear? Is there any analogy that can be made with electromagnetism of an electric generator/motor? Does the differential in rotation speed between the inner and outer core have anything to do with it?
Take a bare copper wire and attach both ends to the appropriate connectors of a small light-emitting diode (forming a loop). Emphasize to the students that there is no power source attached to the loop. Then, run a strong magnet back and forth along the wire (without touching the wire) and the diode lights up. [Note: you might want to test this first before doing the demonstration as, depending on the strength of the magnet and the voltage needs of your diode, you may have to make several turnings of the wire.] This is demonstrated on this video:
http://www.youtube.com/watch?v=gfJG4M4wi1o&feature=related [although the part of the flashlight at the end is badly explained].
From here you can extend the principle to the Earth - that the motion of molten iron in the Earth's core produces currents that in turn produce a magnetic field.
Greg (Roberto Gregorius)
Your analogy is a very good one!
Just for orientation, from cool to hot: The crust is cool, asthenosphere is a hot gel like magma. The magma is hot and is circulating. The outermost core is molten iron-nickle(Fe-Ni) and the innermost core is Fe-Ni that is hot, but the gravity is so great it is solid. Heat circulates from hot to cool(the asthenosphere and crust) with a very distinct magma and outermost Fe-Ni outercore flow.
As the Fe-Ni, Metals and SiO compounds circulate, electrons(e-) are stripped off of atoms and begin to move setting up electric currents. They move through the outermost Fe-Ni core, setting up a magnetic field. It is a feedback loop: that e- motion sets up an electric field in the super hot Fe-Ni, which creates a magnetic field that is in motion which sets up e- current flow...
Both fields exert a Lorentz force that keeps the molten metals in motion, which generates a magnetic and electric field...
The combined motion of compounds, e-, the field generation and Lorentz forces is referred to as the geomagnetic dynamo.
I am not sure if there is any rotational differential effects between inner and outer core. In terms of the dynamo, I suspect it would not matter very much because the e- moving is the critical factor in geomagnetic force formation.
Hope this helps! Peter E. Hughes, Ph.D. Milford, NH
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