Physics of Microwaves: Find Out Why Glass Gets Hot

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Discussion Overview

The discussion centers around the heating of glass in microwave ovens, particularly questioning why glass can become hot despite claims that materials like air and glass do not absorb microwaves effectively. Participants explore the mechanisms of microwave heating, the properties of different types of glass, and the implications for various materials used in microwaves.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that their school physics indicated air molecules do not affect glass in microwaves, yet their experiment showed the glass became hot.
  • Another participant suggests that glass may still be weakly affected by microwaves or that impurities in the glass could be responsible for heating.
  • A different viewpoint argues that the idea of glass being microwave transparent is not entirely accurate, as certain types of glass can absorb microwave energy and convert it to heat.
  • This participant elaborates that while borosilicate glass is generally transparent to microwaves, some energy can still be absorbed, leading to heating.
  • They also discuss how unglazed ceramics can absorb moisture and heat up, while improperly formulated glazes can cause rapid heating and potential hazards.
  • Another participant introduces an example of paper towels, explaining how a single sheet may not heat up, but a stack can due to insulation and the release of flammable volatiles.
  • A participant humorously acknowledges the presence of a knowledgeable contributor, referring to them as a "Microwave Guru."
  • A later reply from this "Guru" identifies themselves as a microwave scientist with extensive experience, expressing their own confusion about the heating mechanisms.

Areas of Agreement / Disagreement

Participants express differing views on the heating properties of glass and other materials in microwaves, indicating that multiple competing perspectives remain without a clear consensus on the mechanisms involved.

Contextual Notes

There are unresolved assumptions regarding the definitions of microwave transparency and the specific conditions under which different materials heat up. The discussion also highlights the variability in material properties that may affect heating outcomes.

Who May Find This Useful

This discussion may be of interest to individuals exploring the physics of microwaves, material science, and those curious about practical applications in cooking and heating methods.

RestlessMind
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My physics in school says: "Air molecules are too small to be excited [by microwaves], and [they] also have no effect on glass, plastic, or ceramic."

So I thought hey, let's test that! I'm glad I did. See, I put a empty glass measuring cup from the pantry into the microwave and put it on high for 1 minute. When I took it out, it was significantly hot.

So now I'm confused. My first thought was that it must be some kind of moisture conducting heat to the glass, but it was very dry, and it's winter where I live and 75 degrees Fahrenheit in the house.

What gives?
 
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Microwaves heat things up by transferring energy to molecules through Dielectric Heating. It is likely that the glass is still weakly effected by the microwaves, or that it has impurities that are.

Look up a Microwave Oven and Dielectric Heating on wikipedia for more.
 
Cool, thanks for the info.
 
The idea that things such as glass and ceramic dishes are microwave transparent and will not heat, it is not necessarily true. Microwaves will heat anything ultimately, because microwaves are a form of energy, not a form energy, and is the ability of the material in the microwave oven, such as your glass measuring cup, to capture the microwave energy and convert it into heat, that is important. Certain types of glass, such as soda lime glass, are rather absorptive and will heat quite well. On the other hand, borosilicate glass, which is probably what your measuring cup is made out of, is rather transparent. But the point is, that if there is nothing else in the microwave oven, some of the energy will, in fact, be absorbed by that measuring cup and it can get hot. In my laboratory we have melted Pyrex glass and about 1600°, as well as converted sand into glass. Both are thought to be microwave transparent, but again, there was nothing else in the oven and eventually it got hot. Ceramic dishes can be quite different. If the ceramic is unglazed, it can absorb water during washing, and then get hot when microwaved. If it is glazed, and the glaze is not properly formulated, the glaze can get incredibly hot very fast. This has caused numerous injuries.

An interesting situation occurs with paper, which is also quite microwave transparent: let's use a paper towel is an example. If you put one sheet of paper towel and microwave it, it will not get hot (although it could be heated if the glass turntable gets hot, and that's very common). Here any energy that is absorbed by the paper is dissipated into the air because of the extremely low thermal mass of the paper. However, if you put a stack of maybe 12 sheets of paper towel in a microwave oven, not only will it get hot, but is likely to catch fire. Part of the reason for that is that the paper stack insulates the interior of the stack which is then allowed to get hot. Add to that the evolution of volatiles from the paper, which are flammable and the result is a fire.
 
and out of the woodwork a Microwave Guru shows up. What are the chances.
 
LostConjugate said:
and out of the woodwork a Microwave Guru shows up. What are the chances.

One in a google.
 
Hi Folks - thanks for the comments - actually, I've answered several questions in the past, but those must have been "under the radar" - no pun intended. I'm a microwave scientist and have been working with microwaves, microwave ovens, etc. for over 50 years. I still don't really understand how they heat.
 

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