Thermos Flask

Thermos Flask

A thermos flask is a device where all the three modes of transfer of heat are applied. It consists of a double walled glass bottle. The inner surface of the outer wall and the outer surface of the inner wall are highly polished. The space between the walls is evacuated and scaled. The whole system is enclosed with in a metal case which is provided with a cork at the bottom and a pad of felt at the neck for safety. Glass is a poor conductor of heat where as air, cork, felt etc. which are used in thermos flask are all had conductors of heat. Hence they prevent any loss of due to conduction. When a hot liquid is kept in the bottle, it remains hot for a long time. Any heat radiation coming from the hot liquid is reflected back from the inner surface of the outer wall. Also the heat from the liquid can not flow out through conduction or convection because of the empty space between the walls. Thus all the three processes of heat-transfer fail to transfer heat. Any heat energy through radiation conduction, convection or coming from outside cannot reach the liquid because of the highly polished surfaces and the empty space between them.