How Does a Thermos Work?

A Thermos is useful to keep any kind of liquid at the desired temperature which makes it the ideal item to take on a trip or to a picnic. Many people prefer to use it to take an extra cup of coffee or tea with them.

Also known as an isulation bottle or a vacuum flask, a thermos preserves a beverage, for a perioad of time at the same temperature, whether it is hot or cold . In fact, it thermally insulates a liquid: it prevents heat from leaving or entering. This is very effective in good quality thermoses.

The operating principle of the thermos

The liquid in the flask is kept at a constant temperature by limiting wall interaction with the external environment. A Thermos has two walls with a void area inbetween. The inner wall has a reflecting layer to reflect heat radiation from the recipient’s interior to the liquid.

thermos functioning

Using the vacuum space as a  heat-insulating zone is a good call in this case because in the vacuum area the only way to lose heat is through body heat radiation.

Normally, objects or bodies lower their temperature by loosing heat coming in contact with the environment (phenomena of conduction and convection).

If we create a vacuum environment, then that body will take much longer to lose heat.

We should bear a clear image of the difference between heat and temperature. The temperature is a state parameter, a number that expresses the sum of all the kinetic energies of the atoms that make up that body. The heat, however, is a transfer of energy, the amount of heat moved from one place to another.
The operating principle of the Thermos is therefore based on the avoidance three phenomena:

  • Conduction: through the use of a bottle nested within another (see dewar) separated by vacuum. The inner cylinder is not resting on another, but merely suspended to avoid as much contact.
  • Convection: always with the above system, fluid movement is almost impossible with the gap between the two containers.
  • Electromagnetic radiation: the walls of the inner and outer container are reflective. Thus the radiation coming from the outside is reflected, and the radiation emitted by the liquid can not escape from the inner container.


Limiting to the maximum these three phenomena allows the thermos to keep the liquid at the desired temperature.

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