Molten iron can not be magnetized

[pallidin]

Hi everyone.

It is my(perhaps wrong) understanding that molten iron can not be magnetized nor attracted to a magnet. Is this true?

If so, how can the Earth's magnetic field be attributed to molten iron at its core?

This confuses me, likely due to my ignorance.
Thanks...

 

[fzero]

Since the Earth is rotating, electric currents are produced in the outer core, where the molten iron is pushed by surface effects with the solid crust. These currents create a magnetic field. This field further polarizes the molten iron through magnetic polarization and creates currents deeper into the fluid core. This creates what's called a dynamo in the molten core: the complex system maintains a magnetic field over long time scales due to convective motions of a conducting fluid.

 

[pallidin]

Since the Earth is rotating, electric currents are produced in the outer core, where the molten iron is pushed by surface effects with the solid crust. These currents create a magnetic field. This field further polarizes the molten iron through magnetic polarization and creates currents deeper into the fluid core. This creates what's called a dynamo in the molten core: the complex system maintains a magnetic field over long time scales due to convective motions of a conducting fluid.

OK, hold on here.
How are electric fields produced by a rotating earth?
Please give specifics, because it makes absolutely no scientific sense to me.

Is there ANY object, compound or not, that I can rotate in free space and create electrical fields? Again, molten iron I would suppose is excluded due to its inability to hold or maintain a magnetic field.

Please clarify.

 

[Dickfore]

Yes, every ferromagnetic material (like Fe) becomes paramagnetic, i.e. it cannot attain spontaneous magnetization, above a certain critical temperature, called Curie temperature. For Fe, the Curie temperature is 770 ^oC, far lower than its melting point of 1535 ^oC.

 

[fzero]

OK, hold on here.
How are electric fields produced by a rotating earth?
Please give specifics, because it makes absolutely no scientific sense to me.

Imagine the interface between the molten core and the solid crust. The crust is rotating because of the overall rotation of the Earth. The layer of fluid along the crust will be dragged along with the crust due to friction. This is what I meant by surface effect. This generates fluid flows in the core that are generally in the same direction as the Earth's rotation. This propagation of flows from the edge to the middle is also called convection.

Since the fluid is comprised of charged particles, there is an overall current that we could compute by summing up the products of the particle charges and their velocities. These currents generate magnetic, not electric fields.

Is there ANY object, compound or not, that I can rotate in free space and create electrical fields? Again, molten iron I would suppose is excluded due to its inability to hold or maintain a magnetic field.

Please clarify.

In analogy with the description of the core of the rotating Earth, we could take a jar of salt water and put it on a spinning turntable. Since the ions in the water are charged, friction at the surface of the jar generates electric currents. So there will be some small magnetic field produced. The strength depends on the concentration of salt, the size of the jar and the rotation frequency. There's also cancellations between the fields generated by the positive and negative ions. Therefore the field will more than likely be too [STRIKE]strong[/STRIKE] weak to generate a dynamo that would enhance the strength of the field, as for the Earth.

There's also a small time-dependent electric field being produced because the charges are accelerating. This field is mainly associated to electromagnetic radiation from the accelerating particles.

 

[Drakkith]

From the wikipedia on Magnetohydrodynamics:

The idea of MHD is that magnetic fields can induce currents in a moving conductive fluid, which create forces on the fluid, and also change the magnetic field itself.

Looks to me like a small magnetic field from something induces a current in the liquid metal in the outer core, which then causes its own magnetic field.

 

[pallidin]

OK, so. if I'm understanding this correctly, the phenomenon of the Earth's magnetic field is more so with respect to a "friction" event of differing layers of our Earth's core?
That is, a massive, though low-level, static elctricity production resulting in a low, but massive magnetic field?
In other words, our molten core, though not inherently producing this fiield, yet produces the potentiality by "rubbing" against an upper core?

 

[Dickfore]

No, you did not understand it correctly or it was not correctly explained.

 

[JohnGano]

A ferromagnetic material may be paramagnetic above the Curie temperature, but it will still align with the application of a strong enough external magnetic field. Right? This thread brought this question to my mind...

 

[pallidin]

No, you did not understand it correctly or it was not correctly explained.

Please offer your opinion.

 

[fzero]

OK, so. if I'm understanding this correctly, the phenomenon of the Earth's magnetic field is more so with respect to a "friction" event of differing layers of our Earth's core?
That is, a massive, though low-level, static elctricity production resulting in a low, but massive magnetic field?
In other words, our molten core, though not inherently producing this fiield, yet produces the potentiality by "rubbing" against an upper core?

The frictional effect is only one part of the driving force behind the magnetic field. The other part is the magnetization of the entire core, which generates an even larger contribution to the total magnetic field. The convective flow is what prevents the core from slowing down and losing its magnetic field.

 

[Dickfore]

A ferromagnetic material may be paramagnetic above the Curie temperature, but it will still align with the application of a strong enough external magnetic field. Right? This thread brought this question to my mind...

Of course, but this is not spontaneous magnetization.

 

[fzero]

A ferromagnetic material may be paramagnetic above the Curie temperature, but it will still align with the application of a strong enough external magnetic field. Right? This thread brought this question to my mind...

Yes, paramagnetic materials can be magnetically polarized.

 

[Dickfore]

Please offer your opinion.

Please find a quote where someone mentions static electricity in the above posts as I am lazy to scan.

 

[pallidin]

Yes, paramagnetic materials can be magnetically polarized.

Under high heat conditions?

 

[Dickfore]

Under high heat conditions?

Temperature only diminishes the magnitude of the susceptibility of the material due to the possibility of random flipping of the magnetic moment of the constituting particles in an opposite direction of the applied magnetic field, but it can never make it negative, i.e. cause the majority of them to be aligned in the opposite direction. Of course, this only holds for paramagnetic materials. There are diamagnetic materials as well.

 

[fzero]

Under high heat conditions?

The polarization will be proportional to the external field. The temperature dependence is probably complicated, but intuitively the polarizability should get smaller as as the temperature increases, because it's harder to line up the dipole moments. But given a strong enough external field there should always be some polarization.

Note that the induced field in the dynamo is a bit different from the standard treatment of polarizability of a dielectric because there are convective currents whose contribution to the induced magnetic field is much larger than any effect due to lining up magnetic dipole moments.

 

[pallidin]

OK, so let's look at a few things:

The Earth's moon has no magnetic field.
Mars has no magnetic field.

In both cases there is no molten core.

Just my thoughts: A previous post has suggested that a molten core, and it's differential to the sliding of upper cores is key to this.
Specific to the attribute of creating a low-level, but wide-spread electrical moment; resulting in a very low though pervasive magnetic field by virtue of sliding event.

 

[Drakkith]

Let me ask this. Is there any need for the outer core to be polarizeable if the induced current through it is the mechanism that creates the magnetic field of the earth?

 

[mikeph]

I don't quite understand how the current is generated. My current visualisation is a soup of both ions and electrons. Surely if the friction from the mantle acted equally on these, there would be no current? (Each with opposite charge moving in the same direction).

What causes one species with a certain sign of charge to move relative to the other?

 

[Dickfore]

Surely if the friction from the mantle acted equally on these, there would be no current? (Each with opposite charge moving in the same direction).

Well, it doesn't act the same on both of them. They have different masses and different mean free path lengths.

 

[mikeph]

Ah, cheers, that's what I didn't realize.

 

[davenn]

Hi everyone.

It is my(perhaps wrong) understanding that molten iron can not be magnetized nor attracted to a magnet. Is this true?
If so, how can the Earth's magnetic field be attributed to molten iron at its core?
This confuses me, likely due to my ignorance.
Thanks...

Hey Pallidin
your confusion comes from a wrong assumption... the Earth's core isn't totally molten ! :)

The inner core is solid, the outer core is molten and its the interaction between the 2 of them that generates the magnetic field... have a look at this brief article by the Geological Survey of Canada as one example...
http://gsc.nrcan.gc.ca/geomag/field/field_e.php

@MikeyW --- the mantle is not part of the generation system

google will bring up dozens of references to the generation of the Earth's magnetic field :)

cheers
Dave

 

[pallidin]

Nice. Thanks dave.