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I thought microwaves causing water molecules to rotate, thus raising the temperature of my food but temperature isn't caused by rotation or vibration degree of freedom:

We also pointed out that temperature is a measure of the average kinetic energy due to translational motions of molecules. If vibrational or rotational motions are also active, these will also accept thermal energy and reduce the amount that goes into translational motions. Because the temperature depends only on the latter, the effect of the other kinds of motions will be to reduce the dependence of the internal energy on the temperature, thus raising the heat capacity of a substance.

https://chem.libretexts.org/Bookshelves/General_Chemistry/Chem1_(Lower)/14%3A_Thermochemistry/14.03%3A_Molecules_as_Energy_Carriers_and_Converters

Qmechanic
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iwab
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  • Did you look up the corresponding Wiki articles, in particular the one on dielectric heating linked in the principles section of the microwave oven article? – ACuriousMind Oct 04 '23 at 09:48
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    It doesn't really matter how you dump thermal energy into a complex interconnected system. All the internal interactions in the system will distribute the energy into all the ways the system can have energy until steady state is reached. – g s Oct 04 '23 at 10:01
  • @gs so what you are saying is: Temperature indeed just depends on translational and if I dump energy that initially is stored in form of rotation into my system, that will fastly spread into translation (in a solid it's vibration, not the same vibration as the common vibrational mode but vibration of the whole molecule around a steady center)? – iwab Oct 04 '23 at 16:33
  • I'm not sure if that's an accurate definition of temperature except for an ideal gas, hence giving a comment and not an answer, but yes to the other part. – g s Oct 04 '23 at 16:41
  • @gs I always wondered, why would (hypothetical) pure rotation or vibration doesn't cause any temperature? I mean those are movements too, just not as "large" as translation... – iwab Oct 04 '23 at 17:02
  • Randomly distributed Pure rotation doesn't stay pure rotation in an interconnected system. Consider a room full of car tyres piled halfway to the ceiling. One of them is attached to a wheel and axle and powerful motor bolted to the wall so that it sits a little above the floor. We start the engine, adding energy to the systemvia the rotation of the attached wheel. How does the room look a few seconds later? – g s Oct 04 '23 at 18:02
  • @gs yes, I already got that. That's because I wrote "hypothetical", so exclude the possibility of translation for a moment, why wouldn't energy in form of rotation and/ or vibration cause any temperature (why why wouldn't it cause a warm feeling when touched), even if things move a little (molecules spinning or parts of the molecule wiggle due to vibration). – iwab Oct 04 '23 at 19:49
  • Why, for temperature, the molecule as a whole have to move like in translation? – iwab Oct 04 '23 at 19:50

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