# Mechanisms for heating of water by microwaves

1. Jul 24, 2011

### bcrowell

Staff Emeritus
There are four qualitatively different effects by which microwaves can heat matter:
1. dielectric heating
2. ionic conductivity
3. electronic conductivity
4. hysteresis
In most foods, 1 and 2 are of approximately equal strength. 3 can occur in soot particles formed when food burns. In pure water and ice, only 1 is significant. Effect 1 is not a resonant effect, contrary to what many people believe.

I think what I've said so far is noncontroversial. What I'd like to hear more about is the mechanisms of dielectric heating. WP http://en.wikipedia.org/wiki/Dielectric_heating describes it as a phenomenon in which molecules with a permanent dipole moment oscillate rotationally in response to the electric field. Since collisions are frequent, the molecule doesn't rotate freely. The collisions dump the energy out of the rotation and thermalize it almost as fast as the energy comes in. (Quantum-mechanically, I think this corresponds to the fact that discrete rotational band structure does not exist in solids of liquids.) This description seems to do a good job of explaining certain observations. It explains why water heats faster than ice in a microwave oven (because $W=\tau\Delta\theta$, and in a solid, you can't get much of a rotation angle). It explains why water, which has a big dipole moment, is heated more efficiently than lipids.

When I asserted the above picture on an online forum, I got the response that I was totally wrong, and that the correct answer is as follows:
-- http://physics.stackexchange.com/questions/12657/ice-in-a-microwave

Gee whiz, he sounds so impressive, but I can't find any support for this explanation anywhere on the web. He also seems to contradict himself by saying that there is orientational polarization but no rotation (how do you change orientation without rotation through some angle?). [Edited] I'm not sure if the hydrogen bonds he refers to are intermolecular or intramolecular. The latter seems unlikely, since then he'd be describing vibrational modes, which cause absorption in the infrared region.

Opinions? Information? Web searching seems to turn up mostly either popularizations, which may be presenting an over-simplified picture, or empirical engineering data, which doesn't necessarily imply any specific mechanism.

http://www.btinternet.com/~martin.chaplin/vibrat.html -- mostly about visible and infrared
http://www.btinternet.com/~martin.chaplin/microwave.html -- microwave

Last edited by a moderator: Apr 26, 2017