The earth's magnetic field is generated due to the movement of liquid metals, but why not solid? Why is it that moving molten metals create a magnetic field, but when nothing of the sort happens when I move a metal rod?


You may really be surprised, but you do create both electric and magnetic fields when you move a metal rod.

But, here is a simple and intuitive example:

An electric generator. It is made of solid metals, but it has bearings and the rotor can rotate. Rotate it fast enough and you get both magnetic fields (inside) and electric fields (at the wires).

The Earth's interior works in the similar manner.

First, imagine that the core is solid (and conductive), but there is a thin layer of lubrication that allows it to rotate in respect to the rest of the Earth's mass. And, imagine some monstrous mechanism that rotates it. Bingo - we have a generator.

We do have the monstrous mechanism required, together with the lubrication, 2 in 1.

We are lucky to have both a liquid-ish outer core and a heat source inside. A convective heat exchange emerges.

The convection constantly mixes not only the masses of different temperature, they have different momentums as well. When a mass plumes upwards, coriolis forces make it lag in regard to the Earth mantle. When the same mass accelerates to the mantle angular velocity (and cools), it plunges down and the same coriolis forces make it advance in regard to the core.

This is why the inner core has somewhat higher angular velocity.

This "motor" is powered by the temperature difference between the internal core and the lower mantle.

(Yes, it is AC. The fields reverse once in a half million years approximately).

  • $\begingroup$ it looks interesting but don't you think if earth rotated the solid core can create magnetic field and I am also interested to know why the current reverses. Why about shaking an iron rode instead of rotating would it still work , if not ,Why? $\endgroup$
    – Mohd Saad
    Sep 20 '21 at 14:13
  • $\begingroup$ No, the solid core won't do. The essence of the electric generator is the parts moving one in respect to another. The convective plumes would create a (somewhat chaotic) magnetic fields even if the Earth didn't rotate. $\endgroup$
    – fraxinus
    Sep 20 '21 at 14:24
  • $\begingroup$ Why is it so that this happens? $\endgroup$
    – Mohd Saad
    Sep 20 '21 at 14:30
  • 1
    $\begingroup$ @fraxinus Nitpick: I think that if the liquid outer core (inner is solid) were a solid shell, but spinning relative to the rest of the Earth (maybe with 'thin" liquid layers on each boundary to reduce friction) you'd still get the AC field. $\endgroup$ Sep 20 '21 at 14:45
  • 1
    $\begingroup$ @CarlWitthoft I think I can improve the answer with your suggestion $\endgroup$
    – fraxinus
    Sep 20 '21 at 15:18

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