Question

Draw a neat labelled diagram of a thermos flask. How does it prevent transfer of heat by conduction, convection or radiation?

Answer 1

Thermos flask consists of double glass or metal walls with a space in between them. Air is completely sucked out of this space, and after that the walls are sealed at the neck or at the bottom of the flask. This vacuum is created to prevent heat loss due to conduction and convection, as both these methods require a medium to transfer heat. Then, the walls are silvered. This silver polish prevents the heat loss through radiation, as heat from inside the flask is radiated back to it and the external heat is reflected back to the surrounding. Finally, the vessel is put inside a metallic case with a plastic or cork support on both sides so as to protect the glass from breaking. At last, the mouth of the vessel is closed by a stopper made from an insulating material like plastic or cork.