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 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

In a thermos flask ,

(a) a thin layer of vacuum is provided between the two walls of flask, to minimise the heat loss by conduction , because for conduction medium (molecules) is required , but now medium is absent .

.(b) the two walls are silvered to minimise the heat loss by radiation , because a shiny surface is a bad absorber and bad emitter of radiation i.e. good reflector of heat , so prevents the heat exchange between content of flask and surrounding .

(c) the lid of flask is provided with a tight cork stopper to minimise the heat loss by convection , so that convection currents cannot go out .

(d) a plastic (or metal) case is provided with a flask , to protect the flask from physical damage .