How thermos flask works to keep thing hot or cool?

A thermos flask, also known as a vacuum flask, operates based on the principle of minimizing heat transfer through conduction, convection, and radiation. Here's how it works to keep contents hot or cool:

1. Double-Walled Structure: A thermos flask consists of two layers of glass or stainless steel with a vacuum-sealed space between them. This double-walled structure prevents heat transfer through conduction and convection.

2. Evacuated Space: The vacuum between the inner and outer walls eliminates air molecules that would otherwise transfer heat through convection. Without air, there's no medium for heat to move from the inner surface to the outer surface.

3. Reflective Coating: The inner surface of the outer wall is often coated with a reflective material, usually silver or aluminum. This coating acts as a mirror, reflecting heat radiation back into the flask. This helps maintain the temperature of the contents inside.

4. Tight Sealing: The flask is equipped with a tight-fitting stopper or lid that prevents the exchange of air between the inside and the outside environment. This prevents heat loss or gain through convection and reduces heat transfer through conduction and radiation.

5. Insulating Materials: In addition to the vacuum, some thermos flasks also incorporate insulating materials, such as polyurethane foam, between the inner and outer walls. These materials further enhance the flask's ability to minimize heat transfer.

By combining these design features, thermos flasks effectively inhibit the transfer of heat through conduction, convection, and radiation, thereby maintaining the temperature of their contents for extended periods. Hot liquids remain hot, and cold liquids stay cold for several hours or even days.