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Name:   Daniel
Status:   student
Age:   13
Location: N/A
Country: N/A
Date: 2000-2001


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


Replies:
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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