They keep saying that spinning molten iron generates Earth's magnetic field.
But, even if that iron is somewhat ionized, those two lost electrons are still nearby in the fluid. So the spin of the ++ and the spin of the — should cancel out any magnetic field they'd generate.
[Physics] Even if molten iron is ionized & spins . . . how does it make a mag field
electromagnetism
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Best Answer
The Earth's magnetic field is not generated by spin alignment, because you can't make a ferromagnet out of molten iron. The Earth's core is at a temperature of 5700 K, far higher than iron's Curie temperature (1043 K), so any domains you would try to form would be broken by thermodynamic fluctuations.
What actually generates the Earth's magnetic field is large-scale electric currents flowing in the molten iron. Convection in the core causes the molten iron to flow, creating powerful currents that generate the Earth's magnetic field in the same way that running a current through a loop of wire does.
As to why you can get large internal currents by moving a neutral conductor around, we need to understand two things. First, "neutral" is only a macroscopic property. At any finite temperature, thermodynamic fluctuations will produce microscopic pockets of positive and negative charges in any conductor, whether it's a copper wire or a ball of molten iron. 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 self-reinforcing dynamic 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, which also gives macroscopic flow from microscopic fluctuations.
The key to having this self-reinforcing current generation is allowing fluctuations to move relative to each other inside the conductor. In other words, you must have the ability to support internal mechanical flow inside your conductor. A copper wire, for example, does not have this property. As such, you won't get current from waving a copper wire around.