Question

The silver coating on the inner bottle of a thermos flask
(a) prevents heat transfer by conduction
(b) prevents heat transfer by convection
(c) prevents heat transfer by radiation
(d) prevents heat transfer by absorption

Answer 1

Hint: Use the information that the vacuum exits between the two glass shells of the thermos flask, so that the heat transfer cannot takes place with the outer surrounding of the thermos due to this reason, condition of the fluid in the thermos remains same.

Complete step by step answer:
We generally use the thermos to keep the liquid in the same thermal condition for a long period of time. In thermos, the hot liquid remains hot and cold liquid remains cold for a long period of time. To keep the same thermal condition of the liquid we need to stop the heat transfer from the thermos into the surrounding. Conduction, convection and radiation all three modes of heat transfer can occur in the thermos.
To stop the conduction heat transfer, we generally use special types of material like glass, cork, and stopper because properties of these materials are desirable to stop conduction heat transfer and we can stop the convection heat transfer by using the tapered air spaces and generating vacuum inside the thermos. We know that heat transfer through radiation can occur even after using the special materials inside the thermos, so to stop the radiation heat transfer we can apply silver coating on the inner bottle of the thermos flask.
Therefore, the silver coating on the inner bottle of a thermos flask is used to prevent heat transfer by radiation and option (c) is correct.

Note: The silver coating reflects the heat inside the thermos so that radiation heat transfer cannot take place. In thermos, the thickness of glass walls stops the heat entering or leaving the flask and the function of the tight stopper is to stop the air entering or leaving the flask, so that the convection heat transfer cannot take place.