Question

Draw a neat, labelled diagram of a thermos flask. Explain how hot tea kept it it remains hot for a long time.

Answer 1

Hot tea can transfer its heat to its surrounding by three ways, i.e., conduction, convection or radiation. Thermos flask has vacuum space between double wall of glass. This vacuum restricts the transfer of heat through conduction and convection mode because these two modes require molecules of medium to transfer heat. And the silvered surface reflects again the radiated heat into the flask. Hence, it provides restriction in transfer of heat through radiation mode. As all three modes of transfer of heat are restricted, the tea remains hot for a long time.