Question

What is the working principle of thermos flask?

Answer 1

Hint: The thermos flask consists of a container whose walls have layers and we have a vacuum between these walls. The purpose of the vacuum is to prevent the transfer of heat through conduction and convection which can quickly turn the contents of the thermos flask cold.

Complete answer:
Heat can be transferred from one plane to another through conduction, convection and radiation. The conduction takes place when two surfaces at different temperatures come in contact with each other. The convection takes place in fluids like liquids and gases where heat is transferred by actual motion of particles of the medium from one place to another. Radiation transfers heat through radiative processes and does not require a medium to take place.
A thermos flask is used to keeping a liquid hot for a long duration of time. But the above processes between thermos flasks and surroundings lead to loss of stored heat. To prevent these processes, the walls of the thermos flask are double layered with a vacuum between them. The vacuum does not allow the heat transfer between the hot liquid inside and the cold air outside. The processes of conduction and convection are unable to occur because of this insulation. This is the basic working principle of a thermos flask by which the contents of the thermos flask remain hot for a long duration of time.

Note:
It should be noted that the insulation is not able to prevent heat transfer through radiation since it does not require a medium to occur and can also occur through the vacuum between the walls of the thermos flask. But the process of radiation does not transfer heat at a very large rate and is slower compared to conduction and convection. Conduction is the fastest process of transfer of heat. The contents of thermos flasks eventually lose their heat over a long duration of time through radiation.