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I’ve been reading about how microwave ovens heat up your food and I keep finding conflicting answers (some say it vibrates, others say it spins). On the one hand, I’ve also read that heat is the molecules kinetic energy making them collide and bounce off other molecules and, if that is the case, I can’t see how spinning the molecule would cause the molecule to go shooting off in a certain direction since spinning doesn’t move anything until there is something to push off of. On the other hand, it doesn’t make sense for an oscillating magnetic wave to push any real direction since the negative side of the wave would just pull it right back and cancel out any kinetic energy the molecule received, making little to no progress. What am I missing?

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rotating molecules will certainly cause heating (as also vibrating molecules). the molecules are not in isolation, but are embedded in the matter which is the rest of the food. this "embedding" is done by some forces, which in case of water, a highly polarized molecule, is depending on the orientation of the molecule, as the positive part of foodstuff molecules will be closer to the negative part of the water molecules (the oxygen atom) and the negative part of the foodstuff molecules will be closer to the positive part of the water molecules (the hydrogen atoms). as a side note, this is how water solutions work. back to the microwave: when water molecules move, either spinning or vibrating, the distances between their positive and negative parts and the respective negative and positive parts of the foodstuff molecules change, therefore they exert force on the foodstuff molecules, that "feel the urge" to rearrange themselves such that their orientation and distance will adjust the new orientation (or "stretching") of the water molecules. this movement propagates through the foodstuff matter and it heats up the food

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  • $\begingroup$ Does that mean that microwaves not only make a polar molecule spin, but also (in the case of a water molecule) will make the hydrogen atoms push away from the oxygen atom only for them to snap back and create vibrations $\endgroup$ May 13, 2022 at 15:05
  • $\begingroup$ If so, does that mean that a single hydrogen ion, being a single proton, in a vacuum will spin in a microwave but not vibrate? $\endgroup$ May 13, 2022 at 15:07
  • $\begingroup$ from here: wtamu.edu/~cbaird/sq/2014/10/15/…. (which seems credible) it seems that home appliances cook in a wide frequency, so they will both cause oscillation and vibrations, though the efficacy of both of these movements may change (you need to hit the resonance of vibrations in order to effectively transfer heat in such a way) $\endgroup$
    – user275556
    May 13, 2022 at 15:11
  • $\begingroup$ the proton has a net charge, so under an electric field it will move with it, and if the electric field oscillate it will follow its oscillations. in addition, the proton is not a point particle and has an internal structure, composed of two up-quarks (electric charge +2/3) and one down quark (electric charge -1/3), so it will also spin as the different internal components respond in opposite manner to the electric field $\endgroup$
    – user275556
    May 13, 2022 at 15:14
  • $\begingroup$ "rotating molecules will certainly cause heating" Microwave ovens do not cook by heating. The rotational kinetic energy obtained by the water molecule is randomized by means of collisions with other molecules increasing their translational KE and thus the temperature of the food. Microwaves also do. not produce vibrational KE. That requires higher energy (infrared) than that of the microwave . See my answer to the following post physics.stackexchange.com/questions/602336/… $\endgroup$
    – Bob D
    May 13, 2022 at 18:49

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