Problem
Explain in five lines why a good-quality thermos flask keeps coffee hot for twelve hours. Consider the three heat transfer mechanisms (conduction, convection, radiation) and how the thermos acts on each one.
Cross-section of a thermos flask: the double wall encloses a vacuum gap around the hot liquid.
Solution
The thermos flask has a double wall with a gap in which a vacuum has been created. The vacuum almost entirely eliminates conduction and convection, both of which require a material medium: with no molecules between the two walls there is nothing that can transport or transmit heat by contact or by convective motion. The surfaces facing the gap are silvered and mirrored: they reflect back inward the infrared radiation emitted by the coffee, thus blocking radiation. Only small losses remain through the cap and the contact points between the walls, which are kept to a minimum. By acting simultaneously on all three mechanisms (the vacuum against conduction and convection, the mirrors against radiation) the thermos reduces heat losses almost to zero, keeping the coffee hot for many hours.
Collegamenti
Argomenti: Termologia Concetti: Trasmissione del calore