How does the layers of a thermos bottle help keep coffee hot?

Answer 1

A Thermos, or in its generic name a vacuum bottle, keeps hot drinks hot (or cold drinks cold) by not allowing heat to flow out (or in, as the case may be). That sounds like a stupid answer but it's really the heart of the matter.

Heat flows from hot surfaces to cold ones, and it can do so by three routes; conduction, convection, and radiation. Conduction is the flow of heat through solids. One example of this is that if you held a nail and put one end of it into a candle flame, the other end would rapidly become too hot to hold, even though it isn't in the flame itself. Convection happens when a fluid, such as water or air, is warmed against a hot surface and then flows around until it touches a colder one, cooling itself and warming the cold surface. Radiation occurs when hot atoms emit infrared radiation. The radiation can then "shine" away"from the object, carrying a bit of heat energy with it, thereby making the hot object that little bit cooler.

A vacuum bottle is designed to minimize all three kinds of heat flow. The "guts" of the vacuum bottle is a double-walled container with a vacuum, or in other words no air, between the container's inner wall and outer wall. The inner wall and outer wall don't touch everywhere, only in limited areas such as at the bottle's neck. The inner and outer surfaces are also polished or silver-plated to give them a high shine, like a mirror.

Convection could happen if the hot inner shell of the bottle heated air between the two shells, but there is no air between the shells, so that means of heat flow is eliminated. Heat could and does conduct from the inner shell to the outer shell and thus escape, but because the inner and outer shells only touch each other in very small areas this effect is far, far slower than it would be in ordinary containers. Heat does escape as infrared radiation, but a mirror will reflect infrared the same way it will reflect visible light, so most of the infrared that escapes the inner shell is reflected and sent back inside.

The effect of this is that heat flow still happens, but it is slowed down enough that it takes many hours for coffee inside a vacuum bottle will finally cool down to room temperature.