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In AC field, polar molecule are trying to align with external field. At high freguencies, changes of field are so quict, that molecules cannot align with it. This leads to increasing kinetic energy of molecules, therefore heating.

My question is this:

Is increase of kinetic energy caused 1) by transforming some portion of energy from EM field (= dipole molecules absorbs EM energy, which is then converted into kinetic energy), or 2) by simple presence of external field? (= the source of kinetic energy is not external field - ie. no energy absorbed, therefore no dielectric loss and so no energy transformed from external field)?

Could you please clarify the exact pathway by which is this happening?

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I'd guess that you've seen the Wikipedia article on dielectric heating. If so, I think the article is potentially confusing when it says:

Temperature is the average kinetic energy (energy of motion) of the atoms or molecules in a material

This is true, but in solids the energy is in lattice vibrations. It is kinetic energy in the sense that the vibrations cause atoms and molecules to move to and fro, but I think it's easier to just think of the lattice having vibrational energy.

Viewed this way, the heating process is conceptually very simple. As the polar molecules rotate they deform the lattice around them, and those deformations cause the lattice to vibrate i.e. energy gets transferred to the vibrations of the lattice and therefore increases the temperature.

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