Which one of the following statements regarding a thermos flask is NOT correct?
[amp_mcq option1=”The walls of flask are separated by vacuum and made of glass which is a poor conductor of heat” option2=”The glass walls themselves have shiny surfaces” option3=”The surface of inner wall radiates good amount of heat and the surface of outer wall absorbs some of heat that is radiated from the inner wall” option4=”The cork supports are poor conductors of heat” correct=”option3″]
This question was previously asked in
UPSC NDA-1 – 2019
– To minimize heat transfer by radiation, the surfaces of the inner and outer glass walls are made shiny (often silvered). Shiny surfaces are poor emitters and poor absorbers of thermal radiation.
– Statement A is correct: Vacuum between walls minimizes conduction and convection; glass is a poor conductor.
– Statement B is correct: The glass walls have shiny surfaces to reduce radiation.
– Statement D is correct: Cork or plastic supports minimize conduction through the neck.
– Statement C is incorrect because the shiny inner wall surface is designed to radiate a *poor* amount (low emissivity) of heat, not a “good amount”. While the outer wall would absorb some radiated heat, the primary mechanism to reduce radiative transfer is the low emissivity/absorptivity of the surfaces.