32. The design features of the Thermos flask

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The design features in a Thermos Flask

An excellent example of how practical design can be used to reduce heat transfer is found in the Thermos Flask. Thermos flasks are able to keep a liquid hot (or cold) for long periods of time.

The plastic stopper is a poor conductor, and so this will reduce the amount of heat transfer via conduction. It also stops any gas from rising out of the flask and so it reduces heat transfer by convection. (It's also rather useful to stop the liquid from spilling out 😉 )

Secondly, there is a double-walled glass or plastic container. Both these materials are poor conductors of heat energy.

The real reason for this double-walled container is to hold a vacuum between the walls. A vacuum contains no gas or any material at all, and so we eliminate heat transfer via conduction and convection across this barrier because both these methods of heat transfer require a material to move through. However, radiation can still cross this gap...

Finally, the silvered surfaces reduce heat transfer by radiation because a shiny/light surface is a poor emitter of radiation (and also reflects the heat back into the hot liquid).

This is a classic exam question, so it's a really good idea to be able to describe the design features and how they work in a Thermos Flask to keep your coffee warm!

GCSE Keywords: Thermos flask, conduction, convection, radiation, vacuum, plastic, silvered walls, glass

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