Physics of Microwaves: Find Out Why Glass Gets Hot

[RestlessMind]

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?

 

[Drakkith]

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.

 

[RestlessMind]

Cool, thanks for the info.

 

[microwaveguru]

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.

 

[LostConjugate]

and out of the woodwork a Microwave Guru shows up. What are the chances.

 

[Drakkith]

and out of the woodwork a Microwave Guru shows up. What are the chances.

One in a google.

 

[microwaveguru]

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.