Which one of the following statements regarding a thermos flask is NOT correct ?
- A.The walls of flask are separated by vacuum and made of glass which is a poor conductor of heat
- B.The glass walls themselves have shiny surfaces
- C.The surface of inner wall radiates good amount of heat and the surface of outer wall absorbs some of the heat that is radiated from the inner wall
- D.The cork supports are poor conductors of heat
Correct answer
Explanation
The correct answer is C. The statement is wrong because the inner wall of a thermos flask is silvered so that it radiates very little heat; the shiny outer surface also reflects heat back rather than absorbing it.
A thermos (vacuum) flask, invented by James Dewar in 1892, is built to stop all three modes of heat transfer. The vacuum between its double glass walls stops conduction and convection, because both need a medium. The walls facing the vacuum are silvered: shiny surfaces are poor radiators and poor absorbers, and they reflect heat radiation back, which cuts down loss by radiation. The cork or plastic stopper and supports, being poor conductors, stop heat leaking through the neck and the base.
A is a correct statement: the vacuum and the glass both resist conduction. B is a correct statement: the shiny glass walls reduce radiation. D is a correct statement: the cork supports are poor conductors of heat. Since the question asks for the incorrect statement, these three are not the answer.
Exam tip: vacuum stops conduction and convection; silvering stops radiation; good absorbers are good emitters (dull black), shiny surfaces are poor at both.
Practice Questions
View allWhich one of the following is monatomic?
- A.Hydrogen
- B.Sulphur
- C.Phosphorus
- D.Helium
Show answer
A monatomic element exists as single, separate atoms. Helium is a noble gas of group 18: its only shell, the first, is full with two electrons, so a helium atom has no tendency to share or transfer electrons and does not bond even with another helium atom.
Read the full explanation →The normal systolic pressure in human beings is :
- A.80 mm of Hg
- B.100 mm of Hg
- C.120 mm of Hg
- D.140 mm of Hg
Show answer
Blood pressure is written as two numbers, and normal adult blood pressure is about 120/80 mm of mercury.
Read the full explanation →'Study of human societies and cultures from the oldest times to the present' is the work of
- A.Archaeologists
- B.Anthropologists
- C.Palaeontologists
- D.Geologists
Show answer
Anthropology, from the Greek 'anthropos' (human) and 'logos' (study), is the science of human beings: how societies and cultures began, how they changed and how people live today.
Read the full explanation →Which of the following catalytic systems is used for the reduction of unsaturated hydrocarbon to saturated hydrocarbon?
- A.Copper and H2
- B.Iron and H2
- C.Zinc and H2
- D.Nickel and H2
Show answer
Adding hydrogen across the double or triple bond of an alkene or alkyne turns it into a saturated alkane; this reaction is called hydrogenation and it needs a catalyst.
Read the full explanation →Which one among the following is not a plant hormone ?
- A.Gibberellins
- B.Abscisic acid
- C.Auxins
- D.Thyroxine
Show answer
Thyroxine is an animal hormone, made by the thyroid gland in the neck, and it controls the rate of metabolism in the human body.
Read the full explanation →