How does Earth’s magnetic field come from its metallic molten outer core if metals are great charge conductors and thence tend to be electrically neutral?
In: Physics
The molten iron layer is very big (planet sized) and thus creates a very strong field by its rotation, so even tough there would be charge dissipating internally it can’t ‘dampen’ the field on that scale.
u/JohnnyJordaan’s answer is essentially correct, and more Eli5 friendly than I can manage when I try to fill some details. This is the best I can do:
We can’t think of how that metal would behave at the surface here, because the outer core (where Earth’s magnetic field is essentially generated) is molten and so it’s not so important that it is electrically neutral (as it would be, even fluidised there are enough localised electrons to balance out a charge in the big picture).
Like I say, that’s not important because that the charge thing creating aligned spin of electrons is what produces a ferromagnetic effect. Ferromagnets are [not possible above certain temperatures](https://en.wikipedia.org/wiki/Curie_temperature) and the outer core is waaaay above that temperature.
So what’s going on?
First, “neutral” is only a property when viewing the whole system at large. When looking at tiny scales, any particular temperature will have random fluctuations and produce micro-pockets of positive and negative charges in any conductor, whether it’s a copper wire or a giant ball of molten iron.
We know that a moving electric charge creates a magnetic field. So if you allow these microscopic pockets to move relative to each other, they can generate magnetic fields which will move oppositely-charged pockets in the opposite direction. In turn, those oppositely-charged pockets will produce magnetic fields that move still more charges around, each time reinforcing the initial motion.
At some point, this feedback effect will produce large numbers of positive charges moving in one direction and large numbers of negative charges moving in the opposite direction, giving you a large-scale current in a neutral conductor, all because you allowed charge fluctuations to move relative to each other. (This phenomenon is closely related to turbulence in non-electromagnetic fluid dynamics, which also gives large scale flow effects from microscopic fluctuations.)
A key part to this self-reinforcing current being setup like this is the ability for those tiniest of charge fluctuations to move past eachother. The outer core is molten metal, so it’s mechanically supported, but able to flow, just what’s necessary. Conversely, electrically conductive solids don’t let the microfluctautions within move relative to eachother, which is why you won’t generate a magnetic field by waving a piece of copper wire around.
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