Clever thermos flask

kelfoan | 23:38 Tue 11th Apr 2006 | How it Works

11 Answers

I know you can keep hot liquids hot and cold liquids cold in a thermos flask. But how does it know which is which?

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ACB312

It doesn't, the contents are simply insulated from the outside world and there is little thermal conduction from the hot or cold liquid.

Mattk

Put simply it retains the temperature be it hot or cold.

rojash

Why giants can never trust men.
One winter's day a giant saw a man blowing on his hands. When asked why, the man explained that he was warming them.
Some time later, he saw the man blowing on his soup, and the man explained that he was cooling it. The giant then realised that men a liars and cannot be trusted.

Sparky1979

So...did the soup come from a flask rojash?????? ;-)

Space

you can keep even cold stuff cold by wrapping it in a thick blanket.

brachiopod

A block of ice sitting on a table at room temperature will melt because it gains heat from it's surroundings.
Similarly, a cup of hot water will lose heat to its surroundings.

A 'thermos�' or vacuum flask works by isolating the contents from the surroundings by means of a vacuum, The smaller, inner skin sits inside a larger, outer skin separated by a vacuum. Heat cannot conduct through a vacuum. ie heat cannot pass into or out of the inner container, so the contents remain at their respective temperature, be it hot or cold.

In reality, since the inner and outer skins are joined at the neck of the flask, some conduction of heat occurs at this point, and also through the lid - this is why your contents do eventually cool down / warm up.

heathfield

The vacuum reduces conductive heat loss, but not heat loss through radiation. To reduce the latter, the glass in a flask is silvered so that it reflects heat, either back into the flask for hot contents, or back into the surroundings for cold contents. Glass-lined flasks are better at this that metal ones, but they're more fragile since the vacuum is already putting a strain on the glass.