Question

In a thermos flask, give reason for the following:
a) A vacuum is created between the two walls of the glass bottle.
b) The two walls of the glass bottle are highly silvered on their vacuum side.
c) The glass bottle is covered with a cork stopper.
d) The glass bottle is placed in a plastic case by surrounding it with cork.

Answer 1

Hint:The vacuum flask consists of two flasks, which are put within each other and joined at the middle. There is a gap in the middle where air is not present and thus a vacuum is created. Heat can travel in three ways- radiation, conduction and convection. The vacuum present in the thermos flask does not allow heat to escape in any of the mentioned ways.

Complete answer:
The heat is prevented from escaping the inside part of the flask by trough radiation due to the inner lining of the vacuum.The heat is prevented from escaping the walls by the process of conduction due to vacuum present between the walls of the thermos. The heat is prevented from escaping through the air by convection due to the tight bottle cap.
In the thermos tank,
a) Between the two walls of the flask, a thin layer of vacuum is given to reduce the heat loss by conduction, since the conduction medium (molecules) is needed, but now there is no medium.
b) The two walls are silvered to reduce radiation heat loss, since a polished surface is a poor absorber and bad radiation emitter, i.e. a good heat reflector, because the heat interaction between flask material and the surrounding environment is avoided.
c) The flask cap has a strong cork stopper to prevent convection heat leakage, so that convection currents do not go out.
d) A flask is fitted with a plastic (or metal) case to shield the flask from physical harm.

Note:Although conduction, convection and radiation may appear to be similar they are completely different processes. Energy transfer through conduction takes place by touching. Energy transfer through convection occurs through air. Radiation is totally different from conduction and convection as energy transfer takes place through atoms and molecules.