What is the working principle of thermos flask?

The working principle of a thermos flask, also known as a vacuum flask or a Dewar flask, is based on the principles of thermal insulation and the reduction of heat transfer through conduction, convection, and radiation.

A typical thermos flask consists of two layers of glass or plastic with a vacuum-sealed space between them. The inner and outer layers are usually made of glass, while the vacuum-sealed space between them is partially evacuated to reduce the transfer of heat.

Here's a step-by-step explanation of how a thermos flask works:

Double-Walled Construction: The flask is made of two layers of glass with a vacuum-sealed space between them. The inner layer holds the liquid you want to keep hot or cold, while the outer layer provides structural support and insulation.

Vacuum Insulation: The vacuum-sealed space between the inner and outer layers is essential for thermal insulation. Since a vacuum contains no air or other matter, it minimizes heat transfer through conduction and convection, as there are no particles to transfer heat energy. The absence of air prevents heat from being conducted or convected through the walls of the flask.

Reflective Coating: The inner surface of the outer layer is often coated with a reflective material, such as silver, to minimize heat transfer through radiation. This reflective coating helps to reflect back any radiant heat that tries to enter or escape the flask.

Stopper and Lid: The thermos flask is equipped with a stopper or a lid that is tightly sealed to prevent air from entering or leaving the vacuum space. The stopper is typically made of plastic or rubber and often contains an additional insulating layer to further reduce heat transfer.

Overall, the combination of the double-walled construction, vacuum insulation, reflective coating, and sealed stopper effectively reduces heat transfer by conduction, convection, and radiation. This allows the thermos flask to maintain the temperature of the liquid inside, keeping it hot or cold for an extended period of time.