Glass: Thermal Conductor or Insulator
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.
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.
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,
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Update: June 2012