A) Vacuum between the double walls
B) Presence of air, which is a bad conductor of heat in between the double walls
C) Silver coating on the inside of the double walls
D) combined effects of A and C
Correct Answer: D
Solution :
The thermos flask consists of a double-walled glass bottle. The walls are silvered. The space between the walls is evacuated with the help of a vacuum pump and sealed. This bottle is protected by metal case. A cork is fitted on the mouth. The function of the thermos flask is to prevent the flow of heat to and from the bottle. We know that heat can flow from one place to another by three processes: conduction, convection and radiation. The thermos flask prevents the flow of heat by any of these processes. The flask is made up of glass which is a bad conductor of heat. Hence heat does not flow by conduction. Since there is vacuum between the walls of the bottle, flow of heat by convection is also ruled out. Due to the silvering of the walls heat is not lost by radiation. As a result, hot things kept in a thermos flask, do not become cold and cold ones do not become hot for a long time.
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