Thermos Flask Class 7 Heat Insulation.pptx

Class 7 Science

1. THERMOS FLASK A thermos flask is a special kind of flask or bottle It is used to keep a hot liquid hot and a cold liquid cold for a long time.

2. It consists of a double-walled glass bottle . There is a vacuum i n between the walls. Both the walls are made shiny on the vacuum side. The mouth of the bottle is fitted with a cork (or plastic) stopper . The bottle is enclosed in a plastic case with the help of cork pieces or spring as shock absorber. The case is provided with a cap over the stopper.

3. The vacuum in between the walls of the bottle prevents any transfer of heat by conduction and convection . The shining surface on the outer side of inner wall and inner side of the outer wall reduces the transfer of heat by radiation . The cork stopper prevents the transfer of heat by convection . 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.