Working principle of thermos flask

We often use a thermos flask to carry coffee or tea while travelling.

A thermos flask keeps the tea, coffee or water hot.

The thermos flask can keep the liquids inside it hot for a long time.

This thermos prevents contact between the system and surroundings.

Let’s study about system and surroundings.

We will concentrate on the system which is a part of the universe.

And rest of the universe that interacts with the system is called surroundings.

Now let’s have a look at the structure of the thermos flask.

A thermos flask contains silvered double walls that are evacuated, an outer case and a plastic cap.

Let us now study the different parts of a thermos flask.

The cap of the thermos flask is attached with insulation to lock the heat.

The vacuum flask is double-walled and consists of an evacuated tube.

This tube has a layer of silver inside.

Now, let’s study the working principle of a thermos flask.

To understand the working principle of thermos we have to first understand conduction, convection, and radiation.

The heat from one body is transferred to another body when they are in contact. This is conduction.

Example: When you heat one end of a wire the other end also gets heated.

When the heat transfers without any direct contact it is convection.

Example: Transfer of heat between liquid molecules when heated.

Radiations are energies traveling in the form of rays, waves, and particles.

Example: Rays coming from the sun.

Now, let’s understand the working principle of the thermos flask.

The structure of the thermos flask prevents heat transfer by conduction, convection, and radiation.

The silver coating inside the tube prevents the heat transfer by radiation.

The vacuum tube between its double-walls prevents the heat transfer by convection.

The thin glass wall stops heat from entering or leaving from the flask.


Thermos flask is used to keep hotter things hot and colder things cold.

Thermos flask doesn’t allow conduction, convection, and radiation of heat between system and surroundings.

Because of no conduction, convection and radiation things kept inside thermos flask retain their heat.

The end