Which one of the following statements regarding a thermos flask is NOT correct?
The walls of flask are separated by vacuum and made of glass which is a poor conductor of heat
The glass walls themselves have shiny surfaces
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
The cork supports are poor conductors of heat
Answer is Right!
Answer is Wrong!
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.