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Q & A: microwaves and dry ice

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Most recent answer: 08/19/2016
Q:
radiant heating = dielectric heating. Is this correct? I heated dry ice in the microwave. I assumed it would sublimate more quickly but there was no change I've thought several cause : 1. Because the dry ice is a collection of nonpolar molecules consisting of CO2 it can't be heated by way of dielectric heating 2. Even though dielectric heating is impossible it should be heated by absorbing radiation of 700W microwave at least. Looks like electromagnetic wave isn't absorbed into nonpolar molecules 3. radiant heating is effective only above infrared therefore if power of microwave is over 700W eventually dry ice will be sublimated 4. radient heating = dielectric heating therefore the reason earth is warm : first of all polar molecules of earth is heated then the heat is conducted to nonpolar molecules
- youngsik (age 20)
Incheon seogu, Korea
A:

Interesting experiment. I think you're right that the reason the dry ice didn't sublimate quickly is that it absorbs very little of the microwave energy. There are ways that non-polar molecules can absorb electromagnetic radiation (that's why CO2 is a good absorber in parts of the infrared spectrum, causing greenhouse warming of the earth) but those particular resonant absorption modes seem not to be important at the frequency of a standard microwave oven. 

This site shows some resonant absorption spectra for CO2 and other molecules: http://co.uk/infra-red-sensors. Notice that there are many frequencies at which the absorption is very small.  In a solid, there can be absorption modes different from those in the individual gas molecules, but because the solid has fairly rigid regular order these still tend to be sharply defined absorption lines.

The broad-spectrum heating of dielectric molecules by low-frequency EM waves will be absent in CO2, just as you say. This site shows that broad absorption spectrum for liquid water:  https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#/media/File:Absorption_spectrum_of_liquid_water.png. At low frequencies, it's largely just that the electric fields rotate the dipoles back and forth, with the friction between molecules generating heat.

Radiant heating is caused by all sorts of absorption of EM waves by materials, regardless of the specific type of absorption.

Mike W.

 


(published on 08/19/2016)

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