Film title: Thermos flask
The Thermos flask has a brilliant design.
It keeps hot or cold contents at an almost constant temperature for a long time.
temperature
But how does it know whether to keep liquids hot or cold?
The Thermos Flask
The Thermos flask is a thermal insulator.
thermal insulator
It has an outer wall and an inner glass envelope.
Not only is glass a good thermal insulator, but all the air molecules have been removed from inside the envelope.
This creates what we call a vacuum.
vacuum
contains no air molecules
Air is a good insulator, but it's not a perfect insulator.
Its molecules still transfer a small amount of heat.
But if there are no molecules, heat can't flow in any direction.
The vacuum acts like a barrier.
The cap of the flask is also made of a thermal insulator, like cork or plastic, to keep the contents at a steady temperature.
With hot chocolate, heat can't flow out, so the hot chocolate stays hot.
With iced water the heat can't flow in and the water will stay cold.
So a Thermos flask is able to keep its contents hot or cold.
But it can't keep them at the same temperature forever.
Over time, heat flows through the cap and through the glass at the top of the flask where the inner and outer walls meet.
This transfer of heat will eventually change the temperature of the contents.
It doesn't matter if its contents are hot or cold ... all a thermos flask does is reduce the flow of heat energy through its walls.
Thermal insulators reduce the flow of heat energy.
How do these flasks work to keep drinks at a constant temperature?
Learning points
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