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.
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. Athermos 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
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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