Why do moving liquid metals generate a magnetic field?

Question

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?

Answer 1

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).

Answer 2

This is a topic that people ask about again and again in slightly different ways.

• What is the source of Earth's magnetic field?
• Even if molten iron is ionized & spins . . . how does it make a mag field?
• How does Earth's interior dynamo work?
• Why geologist do not consider molten iron in the magma to be the source of Earth's magnetic field?

Synthesizing the answers, I think the following things are true:

1. You will not get a magnetic field from rotating conductors alone unless there is some existing initial (possibly weak) field to generate the currents. (But, realistically, there will generally be such a field available.)
2. The liquid metal allows rotational patterns that are not coaxial with the earth.
3. In a liquid conductor, small random pockets of charge imbalance can be magnified by causing a magnetic field that creates moves other pockets in a way that magnifies the field. This can't happen in a solid. (See the answer to the second bullet above.)

Perhaps the official policy is that all but the first of these questions should be closed as duplicates, but many of them have been around a long time, I hate to lose some good answers, and different questions are addressing different knowledge gaps.