Ask A Scientist© Physics Archive

name         Daniel
status       student
age          13

Question -   How does conduction, convection, and radiation preserve
in a thermos?


>Hello,
>
>As you know, there are three modes of heat transfer, conduction,
>convection, and radiation.
>
>When there is a temperature difference in a medium, heat is transferred
>by conduction  from warmer to colder regions.
>
>Convection heat transfer occurs when heat flows between a surface and a
>moving fluid when they are at different temperatures.
>
>Radiation heat transfer is the heat (or electromagnetic radiation)
>emitted by all objects having temperatures above absolute zero. If an
>surface is surrounded by suface(s) at a different temperature, there
>will be a net transfer of heat in the form of radiation from the warmer
>surface to the colder surface.
>
>In most thermal problems all three modes of heat transfer are present
>but the radiation heat transfer share is often low unless temperatures
>involved are over several hundreds degrees.
>
>A thermos cuts down on the CONDUCTION of heat between the liquid in the
>thermos and the outside.  This is done by thermally insulating the glass
>or metal container. If the heat cannot get out or in, the liquid
>temperature in the thermos is maintained.
>
>Is convection also present given the fact that the liquid in a thermos
>can and does move?  Yes, but the amount is small because the insulation
>makes the temperature difference between the liquid and the surface in
>contact with the liquid small.
>
>Radiation heat transfer is negligible because the temperatures involved
>are rather small.
>
>
>AK
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