Is the definition of conduction rigorous?

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Oort
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Is the definition of conduction rigorous?

Postby Oort » Sat Dec 05, 2009 11:04 pm UTC

Wikipedia defines it as: "In heat transfer, conduction (or heat conduction) is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient." Radiation is transfer of heat through photons. But at atomic scales, interactions between particles are mediated by photons anyway, right? Should conduction be considered a case of close-range radiation?

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Re: Is the definition of conduction rigorous?

Postby You, sir, name? » Sat Dec 05, 2009 11:45 pm UTC

Well, the photons involved in conductive heat transfer are virtual, and at least in my book, it would feel very strange to call that radiation.
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Re: Is the definition of conduction rigorous?

Postby Nande_Ebisu? » Sat Dec 05, 2009 11:50 pm UTC

Eh, if its proportional to T^4 its radiative, if it proportional to the second derivative w.r.t. position its conductive, i figure if you're in engineering it doesn't really matter too much beyond that. Also, I always assumed that conduction had some sort of collision aspect to it, or some sort of brownian motion analog since it can behave similarly to mass diffusion.
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Re: Is the definition of conduction rigorous?

Postby PM 2Ring » Sun Dec 06, 2009 10:39 am UTC

Why worry about photons in heat conduction? They're not as important as electrons and phonons, at least in solids.
Wikipedia wrote:In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann-Franz law, as freely moving valence electrons transfer not only electric current but also heat energy. However, the general correlation between electrical and thermal conductance does not hold for other materials, due to the increased importance of phonon carriers for heat in non-metals. As shown in the table below, highly electrically conductive silver is less thermally conductive than diamond, which is an electrical insulator.

Thermal conductivity depends on many properties of a material, notably its structure and temperature. For instance, pure crystalline substances exhibit very different thermal conductivities along different crystal axes, due to differences in phonon coupling along a given crystal axis. Sapphire is a notable example of variable thermal conductivity based on orientation and temperature, with 35 W/(m·K) along the c-axis and 32 W/(m·K) along the a-axis.

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Re: Is the definition of conduction rigorous?

Postby You, sir, name? » Sun Dec 06, 2009 1:43 pm UTC

PM 2Ring wrote:Why worry about photons in heat conduction? They're not as important as electrons and phonons, at least in solids.


Keep in mind that phonons are quasiparticles. What makes phonons possible is electromagnetic interaction in the crystal lattice, which is mediated by (mostly) virtual photons. You get some degree of real photons as well, from the fact that vibrating ions are oscillating charges = antennas. Though that probably just adds up to blackbody radiation.
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Re: Is the definition of conduction rigorous?

Postby PM 2Ring » Sun Dec 06, 2009 3:07 pm UTC

You, sir, name? wrote:Keep in mind that phonons are quasiparticles. What makes phonons possible is electromagnetic interaction in the crystal lattice, which is mediated by (mostly) virtual photons. You get some degree of real photons as well, from the fact that vibrating ions are oscillating charges = antennas. Though that probably just adds up to blackbody radiation.

True, but my point is that a lot of heat is being transferred by essentially mechanical processes. Eg heat diffusing through a metal as energetic electrons jostle their neighbours. Or the vibrational energy in the bonds between atoms in a crystal lattice moving in a similar fashion. IIRC, the energy in the metal electron gas, or the non-metal phonon gas is substantial, and more significant than the energy in the black body photon gas. But I am happy to be corrected on this matter. :)

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Re: Is the definition of conduction rigorous?

Postby JWalker » Sun Dec 06, 2009 5:22 pm UTC

You, sir, name? wrote:Well, the photons involved in conductive heat transfer are virtual, and at least in my book, it would feel very strange to call that radiation.


Photons involved in radiative heat transfer are virtual as well...

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Re: Is the definition of conduction rigorous?

Postby defaultusername » Sun Dec 06, 2009 11:16 pm UTC

JWalker wrote:Photons involved in radiative heat transfer are virtual as well...
No, because there you can detect the actual photons, while in conductive heat transfer they are a theoretical model.
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Re: Is the definition of conduction rigorous?

Postby JWalker » Mon Dec 07, 2009 1:04 am UTC

defaultusername wrote:
JWalker wrote:Photons involved in radiative heat transfer are virtual as well...
No, because there you can detect the actual photons, while in conductive heat transfer they are a theoretical model.


All observed photons are virtual photons. A real photon is a limiting case of a photon that never interacts with anything. By detecting the actual photons you have forced them to appear as an internal line on some Feynman diagram exactly in the same way they appear in the conductive heat transfer model.


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