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Name: Will
Status: educator
Grade: 6-8
Location: IL
Country: USA
Date: N/A 


Question:
Glass seems to have properties of both a conductor and an insulator. In class, we have a glass pot that we can use to boil water. The heat must get through the glass to the water (also as in a beaker) but the heat does not conduct to the handle (or to the top rim of a beaker). So is glass a conductor or an insulator or both? How can I explain our observations? Thanks for any information that you can provide.



Replies:
Hi Will,

In fact, glass is a poor conductor of heat (that is, a fairly good thermal insulator). The reason that the heat is transferred to the water in a beaker, is simply that it does not have far to go (only though the glass that is a couple of millimeters thick), and also, there is a lot of area for heat transfer (the entire area of the bottom of the beaker.

The reason that the heat does not appear to conduct up the side of the beaker, is that it has a long way to go, and very little cross-sectional area to travel though. If the beaker walls were much thicker, then more heat would travel up the sides.

Regards,

Bob Wilson


Will,

Classifying objects into categories such as "conductor" or "insulator" is just a shorthand way to describe differences in properties. Perhaps a more strict terminology would be a "good conductor" (conductor) or a "poor conductor" (insulator). All objects conduct heat, but some do it faster than others. For example, glass is a poor conductor of heat compared with metals, but glass still can conduct heat. In the case of your glass pot, the glass absorbs heat and conducts it into the water, but it does so more slowly than metals. The reason the handle stays cool is because the glass cannot move heat quickly into the handle, and air is pulling heat from the handle just as fast as the glass can supply it (so the temperature of the handle does not rise).

Hope this helps,
Burr Zimmerman



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