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 Originally Posted by OngBonga
I figured it was obviously due to molten iron moving around.
The earth contains a large amount of molten iron. It is rotating on its axis, while drifitng through space at breakneck velocity. Other local moving objects such as the Moon, Jupiter, et al will cause further fluctuations in the electric currents moving through the core.
A large magnetic field seems inevitable, considering these aspects.
But why, after 3.5 billion years, is the Iron still moving around?
(insert Nobel Prize for correct answer)
The thing is that molten Iron is sticky stuff. It's viscous. It drags on the surrounding material unless that material is moving with the same velocity. The fact that this shear friction has not completely locked any molten currents into a single rate of rotation (i.e. 1 revolution per day) is mind-boggling with any known models of the Earth's core.
However, the electric current can flow even if the Iron isn't flowing, so there's that. BUT there's a similar effect called Eddy Currents that are produced by any current in a conductor. Basically, if you have a current in a conductor, it will induce other currents nearby to the main current. Kinda like if there's a strong ocean current, the surrounding waters may actually flow in the opposite direction nearby because of the generation of vortices.
So even if it's just the electric current flowing through the Iron, given 3.5 billion years of Energy being dissipated by Eddy Currents and Joule heating (stuff gets hot when you pass a current through it)... given all that it's hard to figure why there's any current that is left undissipated. Many younger planets and moons are completely tidally locked and have no magnetic fields.
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The sun, moon, Jupiter, etc. are pretty much neutral of charge and have next to no bearing on the electromagnetic fields on and quite near Earth. That said, Jupiter's and Saturn's magnetic fields are pretty immense, IIRC, and could maybe be measured on or near Earth with sensitive equipment.
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