how thermoses (vacuum flasks) work
Take a jar like a container and wrap it in a foam insulation.
There are two principles for insulation.
First of all, the plastic in the foam is not a good thermal conductor.
Second, the air in the foam is a worse thermal conductor.
So the conduction is reduced.
Because the air is broken down into tiny bubbles, another thing the foam insulation does is to largely eliminate the convection inside the foam.
Therefore, the heat transfer through the foam is very small.
As it turns out, there are better insulators than foam: vacuum.
Vacuum is the lack of atoms.
The \"perfect vacuum\" contains zero atoms.
It\'s almost impossible to create a perfect vacuum, but you can approach it.
Without atoms, you completely eliminate conduction and convection.
What you found in the thermos is a glass envelope with a vacuum.
There is glass inside the thermos bottle and vacuum around the glass.
The glass envelope is fragile, so it is wrapped in a plastic or metal case.
In many thermostats you can actually screw down and remove this glass envelope.
Then take another step forward.
The glass is silver (like a mirror)
Reduce infrared radiation.
The combination of vacuum and silver greatly reduces the heat transfer of convection, conduction and radiation.
So why does the hot stuff in the thermos cool down?
You can see two heat transfer paths in the picture.
The big one is the hat.
The other is the glass, which provides a conduction path at the top of the flask, where the inner and outer walls meet.
Although the heat transfer through these paths is small, it is not zero.
Do you know if the liquid inside is hot or cold? No.
Everything the thermos does is to limit the heat transfer through the walls of the thermos.
This makes the fluid in the thermos bottle keep the temperature almost constant for a long time (
Is the temperature hot or cold).