Why are vacuum (Thermos) flasks effective at keeping liquids hot?
AThe vacuum traps heat like a magnet
BThe vacuum increases radiation losses heavily
CThe vacuum acts as a chemical insulator only
DThe vacuum stops conduction and convection of heat
Answer & Solution
Correct answer: D. The vacuum stops conduction and convection of heat
1. A Thermos has a double-walled glass vessel.
2. Between the walls is a vacuum.
3. Vacuum has no particles, so no conduction or convection.
4. Silvered walls also reflect radiation, reducing losses.
_Source: Selina Concise Physics Class 8, Ch 5 Heat (Mistral OCR'd PDF), section on Thermos flask_
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