by johnnydeep » Tue Aug 01, 2023 12:47 pm
Chris Peterson wrote: ↑Mon Jul 31, 2023 9:10 pm
johnnydeep wrote: ↑Mon Jul 31, 2023 8:55 pm
Chris Peterson wrote: ↑Mon Jul 31, 2023 8:07 pm
But there is more to thermal transfer than the number of atoms.
Only away from the material, not into it. And all external directions are nominally equivalent in the simple case, so there's really only a single metric that defines the radiative gain/loss. So it's best understood as a one-dimensional process. (And yes, we could make a complex analysis where photons are emitted into the material, resulting in some messy combination of effects. But I think we're just considering the basic, first-order process here. Heat transfer inside the material is primarily conductive, and is weak in this insulating material. Heat transfer at the surface is primarily radiative, and is what is most important in considering the rate that the surface temperature changes with radiative environment.)
Ok. I'll have to add thermal properties of materials to optics as things I don't understand well. Which is a list that already had electromagnetism and relativity on it for quite a long time! (My latest confusion is how come a molten iron core generates a magnetic field due to rotation (like in the core of the Earth), but a rotating solid iron ball or ring does not? Electrons are "flowing" in both cases are they not?)
Well, the full answer requires a gruesome dive into magnetohydrodynamics, but the key difference is that in order to generate a magnetic field in a planet or star you need convection, and that obviously requires a fluid conductor, not a solid one. Rotation is also required, but is not enough by itself.
Ok. And of course Wikipedia has more about this process happening in the Earth. Pretty complex stuff!
https://en.wikipedia.org/wiki/Earth%27s_magnetic_field#Physical_origin wrote:
Earth's core and the geodynamo
The Earth's magnetic field is believed to be generated by electric currents in the conductive iron alloys of its core,
created by convection currents due to heat escaping from the core.
The Earth and most of the planets in the Solar System, as well as the Sun and other stars, all generate magnetic fields through the motion of electrically conducting fluids.[51] The Earth's field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer core.[52] The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F).[53] The heat is generated by potential energy released by heavier materials sinking toward the core (planetary differentiation, the iron catastrophe) as well as decay of radioactive elements in the interior. The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.[54]
The mechanism by which the Earth generates a magnetic field is known as a dynamo.[51] The magnetic field is generated by a feedback loop: current loops generate magnetic fields (Ampère's circuital law); a changing magnetic field generates an electric field (Faraday's law); and the electric and magnetic fields exert a force on the charges that are flowing in currents (the Lorentz force).[55]
...
The motion of the fluid is sustained by convection, motion driven by buoyancy. The temperature increases towards the center of the Earth, and the higher temperature of the fluid lower down makes it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is plated to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This is called compositional convection.
A Coriolis effect, caused by the overall planetary rotation, tends to organize the flow into rolls aligned along the north–south polar axis.[54][56]
A dynamo can amplify a magnetic field, but it needs a "seed" field to get it started.[56] For the Earth, this could have been an external magnetic field. Early in its history the Sun went through a T-Tauri phase in which the solar wind would have had a magnetic field orders of magnitude larger than the present solar wind.[57] However, much of the field may have been screened out by the Earth's mantle. An alternative source is currents in the core-mantle boundary driven by chemical reactions or variations in thermal or electric conductivity. Such effects may still provide a small bias that are part of the boundary conditions for the geodynamo.[58]
The average magnetic field in the Earth's outer core was calculated to be 25 gauss, 50 times stronger than the field at the surface.
[quote="Chris Peterson" post_id=332645 time=1690837801 user_id=117706]
[quote=johnnydeep post_id=332643 time=1690836931 user_id=132061]
[quote="Chris Peterson" post_id=332642 time=1690834079 user_id=117706]
But there is more to thermal transfer than the number of atoms.
Only away from the material, not into it. And all external directions are nominally equivalent in the simple case, so there's really only a single metric that defines the radiative gain/loss. So it's best understood as a one-dimensional process. (And yes, we could make a complex analysis where photons are emitted into the material, resulting in some messy combination of effects. But I think we're just considering the basic, first-order process here. Heat transfer inside the material is primarily conductive, and is weak in this insulating material. Heat transfer at the surface is primarily radiative, and is what is most important in considering the rate that the surface temperature changes with radiative environment.)
[/quote]
Ok. I'll have to add thermal properties of materials to optics as things I don't understand well. Which is a list that already had electromagnetism and relativity on it for quite a long time! (My latest confusion is how come a molten iron core generates a magnetic field due to rotation (like in the core of the Earth), but a rotating solid iron ball or ring does not? Electrons are "flowing" in both cases are they not?)
[/quote]
Well, the full answer requires a gruesome dive into magnetohydrodynamics, but the key difference is that in order to generate a magnetic field in a planet or star you need convection, and that obviously requires a fluid conductor, not a solid one. Rotation is also required, but is not enough by itself.
[/quote]
Ok. And of course Wikipedia has more about this process happening in the Earth. Pretty complex stuff!
[quote=https://en.wikipedia.org/wiki/Earth%27s_magnetic_field#Physical_origin]
[size=150][b]Earth's core and the geodynamo[/b][/size]
The Earth's magnetic field is believed to be generated by electric currents in the conductive iron alloys of its core, [b][color=#0000FF]created by convection currents due to heat escaping from the core[/color][/b].
The Earth and most of the planets in the Solar System, as well as the Sun and other stars, all generate magnetic fields through the motion of electrically conducting fluids.[51] The Earth's field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer core.[52] The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F).[53] The heat is generated by potential energy released by heavier materials sinking toward the core (planetary differentiation, the iron catastrophe) as well as decay of radioactive elements in the interior. The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.[54]
The mechanism by which the Earth generates a magnetic field is known as a dynamo.[51] The magnetic field is generated by a feedback loop: current loops generate magnetic fields (Ampère's circuital law); a changing magnetic field generates an electric field (Faraday's law); and the electric and magnetic fields exert a force on the charges that are flowing in currents (the Lorentz force).[55]
...
[b][color=#0000FF]The motion of the fluid is sustained by convection, motion driven by buoyancy.[/color][/b] The temperature increases towards the center of the Earth, and the higher temperature of the fluid lower down makes it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is plated to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This is called compositional convection. [b][color=#0000FF]A Coriolis effect, caused by the overall planetary rotation, tends to organize the flow into rolls aligned along the north–south polar axis.[/color][/b][54][56]
A dynamo can amplify a magnetic field, but it needs a "seed" field to get it started.[56] For the Earth, this could have been an external magnetic field. Early in its history the Sun went through a T-Tauri phase in which the solar wind would have had a magnetic field orders of magnitude larger than the present solar wind.[57] However, much of the field may have been screened out by the Earth's mantle. An alternative source is currents in the core-mantle boundary driven by chemical reactions or variations in thermal or electric conductivity. Such effects may still provide a small bias that are part of the boundary conditions for the geodynamo.[58]
The average magnetic field in the Earth's outer core was calculated to be 25 gauss, 50 times stronger than the field at the surface.
[/quote]
[img2]https://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Dynamo_Theory_-_Outer_core_convection_and_magnetic_field_generation.svg/1024px-Dynamo_Theory_-_Outer_core_convection_and_magnetic_field_generation.svg.png[/img2]