Thermos

OK, tell me how a thermos works!

Thermos is a brand name, but now all manufacturers call their insulated containers 'thermos'.

The principle of a thermos is insulation of whatever is inside of it. This is what a thermos looks like:

                  /   \
                 /|   |\
                | |   | |
                | |   | |
                | |   | |
Outer case  ----| |   |-----  Inner case
                | |   | |
                | |   | |
                | |   | ----- Vacuum
                | |___| |
                ---------

Heat can be transferred in three different ways:

• Radiation: energy being emitted as infrared light because of the atom vibrations and rotations, due to changes in the atomic states.
• Conduction: also caused by atomic movements, but this is energy transferred between atoms due to collisions.
• Convection: Warm gas (or liquid) will rise above colder gas, due to how density is dependent on temperature. This makes it so colder gas will be close to the heat source

The reason for having vacuum between the outer and inner case, is that it's the best possible insulator there is. Theoretically there are no atoms at all in vacuum, meaning that heat cannot be transferred in any way between the hot inside and the cooler outside of the thermos. Vacuum will minimize the conduction and convection losses. Infrared light can still travel in vacuum, though.

The inside of the thermos is usually covered with a thin silver film. This is to decrease radiation.

So, that's how all ways of heat transfer is taken care of in a thermos. And that's why you burn your tongue on the coffee even though it's been in there for hours.