Hi Vivien,
A dewar, named after Sir James Dewar, a Scottish physicist who
invented it, is a double-walled container with a vacuum between the
walls. The walls are made of or are coated with a highly reflective
material. This container is designed to be a good thermal insulator for
keeping the contents hot or cold. A thermos is an example of a dewar
for keeping coffee hot, and scientists and engineers use them often to
keep liquid nitrogen cold so that it can be used in Physics Van
demonstrations. There's a picture of one holding liquid nitrogen on our
web site
here.
The dewar covers all the bases in order to stop heat energy from flowing into or out of it.
1) Conduction: Dewars have two walls and they are not in contact,
so there is no heat conduction (except perhaps at the lip where the
walls are joined together to hold the dewar in one piece.
2) Convection: There is a vacuum between the two walls and so
flowing air or other fluid cannot convey heat energy from one place to
another. Cheap plastic thermoses may have double walls but with
styrofoam or polyester fill or something else as an insulator.
Styrofoam insulates because it keeps the air inside from flowing,
transferring heat by convection. These aren't technically called
Dewars, however, because they lack a vacuum.
3) Radiation: Here's the really clever part. A shiny surface
reflects back all the light you shine on it (in this case, the infrared
part of the spectrum is the important part). But more importantly, it
also emits infrared radiation at a much lower rate than an object which
is colored, say, black. The reason for this is that an object which is
in equilibrium with a surrounding light radiation field emits and
abosorbs radiation at the same rate. A black object absorbs everything,
and so a warm black object must re-radiate the heat away. On the flip
side, a shiny object absorbs almost nothing, and so it also emits
almost nothing when warm. So the walls are made of shiny material so
they will not radiate their energy away with infrared light.
Tom J.
(published on 10/22/2007)
