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This schoolphysics webpage discusses some information regarding the heating of water using microwaves, and according to that page a common misconception that

"Microwave ovens operate at a frequency of 2.45 GHz (2.45x10^9 Hz) and this is NOT the resonant frequency of a water molecule"

But in any event the microwave energy is absorbed by the water molecules, and I assume the molecules at that point do vibrate in their various modes at their higher resonate frequencies.

But what I'm not sure of is if the polar nature of the water molecule plays any role in how the microwave energy is captured by the water molecule. I'm thinking that the microwaves being electromagnetic waves must have some influence on the water molecules that are somewhat electrically polarized by their shape.

Does the polar nature have any influence?

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    $\begingroup$ Not a duplicate, but very relevant: physics.stackexchange.com/a/136078/26969 $\endgroup$
    – Floris
    Commented Apr 3, 2017 at 20:56
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    $\begingroup$ @Floris Thanks, that was useful and actually the material you quoted mentions action of the water dipole so it appears it does serve a function in moving the water molecules, their phase relative to the EMR. So by electromagnetic forces. But is that the sole mode of energy transfer, or do the molecules absorb energy from the EMR by other means not related to their polar characteristics? Oils are well known as a non-polar liquid. Don't recall if microwaves heat oil. $\endgroup$
    – docscience
    Commented Apr 3, 2017 at 22:59
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    $\begingroup$ @Floris here goodhousekeeping.com/food-recipes/news/a18064/… mentions "Oils such as olive oil do not heat well in microwaves because their molecules lack the polarity found in water" $\endgroup$
    – docscience
    Commented Apr 3, 2017 at 23:02

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The water molecules undergo forced oscillations, but this is not resonance because the frequency of the microwaves (the forcing frequency) does not coincide with any natural frequency of the molecules. The molecules are forced to oscillate because they are polar and the oscillating electric field from the microwaves sets them into a 'libratory' oscillation, which is rotation in one direction, then the other direction, then the first direction and so on, rather like the 'balance wheel' of an old-fashioned clockwork watch, though I imagine the amplitude is much smaller. The motion is damped due to interactions with surrounding molecules, and the energy is randomised.

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  • $\begingroup$ You use the term 'libratory' oscillation, but dictionary.com defines libratory as 'oscillatory'. So oscillatory oscillation? Do physicists define libratory in another sense? $\endgroup$
    – docscience
    Commented Apr 3, 2017 at 23:08
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    $\begingroup$ I tried to explain what a libratory oscillation was in my answer above. Look up the Wiki entry under 'Libration (molecules)' and you'll find an animation. $\endgroup$
    – Philip Wood
    Commented Apr 5, 2017 at 8:46
  • $\begingroup$ Thanks. So the surrounding medium, intermolecular forces appear to provide a restoring force, and electromagnetic force, induced by the passing microwaves induce an exciting force onto the molecules dipole. I can see that this wouldn't necessarily require the microwave to have the same frequency as the natural frequency of the molecule, restoring force system. I wonder though if the libration frequency is a subharmonic of the microwave. If so you still might consider it a resonance. $\endgroup$
    – docscience
    Commented Apr 5, 2017 at 14:05
  • $\begingroup$ a quick search brought up this in chemical reviews physics.ncsu.edu/clarke/papers/… indeed molecular 'rotors' may librate at a subharmonic of the exciting wave. $\endgroup$
    – docscience
    Commented Apr 5, 2017 at 14:09

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