Question

Draw a labelled diagram of a thermo flask. Explain how the transfer of heat by conduction, convection and radiation is reduced to a minimum in it.

Answer 1

Part-1: Labelled diagram of a thermo flask

Part-2: Explanation

Following are the factors which minimised the transfer of heat by conduction, convection and radiation in a thermo flask:

• The vacuum between the two walls, rubber, glass, cork and air do not allow the loss of heat by conduction.

• Cork in the neck of the flask and the cup over it prevent loss of heat by convection.

• Heat cannot be lost by conduction or convection because of the vacuum between the two walls.

• Heat loss is also minimised by radiation, by making the outer surface of the inner wall and inner surface of the outer wall silvered. The inner wall is a bad radiator and the outer wall is a good reflector of radiation.

• Thus, the transfer of heat either from inside to outside or from outside to inside is very little and the contents of the bottle can remain at the same temperature for a long time.