The goal of any insulator is to limit heat
conduction. Gases are poor conductors (and
therefore good insulators) since the atoms that
make them up are so far apart. The foam
insulation that is used in houses is made up of
little "bubbles" if you look at it close up. The
outside of the bubbles are a poorly conducting
plastic, and inside the bubbles is air. The air
is also a poor conductor, as it's a gas.
If you want an even better insulator than a
gas, try a vacuum. In a pure vacuum there are
no atoms, and therefore there can be no heat
transfer. A perfect vacuum is a perfect
insulator! A thermos works this way. A thermos
has two containers (one inside the other): a metal
or plastic container on the outside, and a glass
container on the inside. Between these containers
is a near vacuum (no perfect, though). There's a
little bit of gas in there, so some heat
conduction will occur. But since there are so few
atoms to transmit heat between the containers,
your items will be insulated for a long time. And
that's how a thermos can keep cold stuff cold and
hot stuff hot!
A cheaper thermos bottle
probably won't have a vacuum between the
containers, just air. Sometimes they come apart,
too. So you could take it apart to show your
students how the design (with the insulating gas
layer) keeps stuff how or cold.
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