To solve the problem, all you have to do is think of heat as a kind of liquid that flows only “downhill” from high temperatures to low temperatures.
The thermos bottle acts like a dam that blocks the flow of heat. It won’t let heat flow “down” from your hot coffee inside to the lower-temperature air outside. But by the same token, it won’t let heat flow “down” from the outside air to your lower-temperature iced tea inside.
Another way of saying this is that the walls of the thermos bottle are a very effective heat insulator, a substance or arrangement of substances that retards the flow of heat. We’re most familiar with using insulators to keep heat from flowing out of our warm bodies and houses into the cold outdoors; ski jackets, sleeping bags, and attic insulation come readily to mind. But our refrigerators are also insulated, in this case to keep heat from flowing in. Insulators work both ways.
Heat, of course, isn’t a liquid, even though it does flow from one place to another.
It moves in three ways: by conduction, by convection, and by radiation. Let’s take them one by one and see how a thermos container foils them all.
Put a cool object in close contact with a warm one and you know what will happen: The warm object surrenders some of its heat to the cool one, so that the cool one becomes warmer and the warm one becomes cooler. Some heat has been transferred, or conducted, from the warmer object to the colder one.
But what is heat, anyway? It is the agitation, or movement, of an object’s molecules. The more vigorously its molecules are moving, the warmer it is. So when you place a warm object (having rapidly moving molecules) in close contact with a cooler object (having slowly moving molecules), some of the faster molecules will collide with the slower molecules, transferring some of their energy to the slower molecules and speeding, warming, them up. That’s conduction: direct molecule-to-molecule energy transfer.
When you touch a hot frying-pan handle, your skin molecules are speeded up by collisions with the frying pan’s faster-moving molecules. When you touch an ice cube, your skin molecules lose some of their speed through collisions with the ice molecules. A thermos container hinders conduction because it has double walls with nothing—a vacuum—in between. Because there are no molecules in a vacuum to collide with, heat conduction can’t take place through it.
Convection is the process whereby heat is transferred from one place to another by the actual bulk movement of a heat-containing gas or liquid. You’ve heard people say that heat rises? What they really mean is that hot air rises, and along with it goes the heat it contains. That’s convection. A convection oven is simply an oven with a fan in it that assists the circulation of hot air. In that case, the process is called forced convection.
A thermos bottle hinders convection simply by being a closed container; warm air can’t pass through its walls. Any kind of closed container would stop convection.
Finally, heat can be radiated from one place to another in the form of infrared radiation. These energy waves are emitted by warm objects. They fly through space and can be absorbed by cooler objects, transferring their energy to them and heating them up.
A thermos container hinders infrared radiation by deflecting it with a mirror. The double walls of the container are silvered on their inner (vacuum-containing) surfaces, so any infrared radiation that tries to get through from either direction is reflected right back to where it came from.
If you think radiation isn’t a serious contender for heat transmission, consider how you broil a steak underneath the heating element in an electric oven. Heat travels upward by convection, all right, but a lot of it also goes downward (and in all other directions) by radiation.
No thermos container is perfect, of course. Some heat is always being conducted or radiated out of your hot coffee or into your iced tea. But the thermos slows down the heat-transfer processes substantially, and your food or beverage stays hot or cold for hours, rather than minutes.
Incidentally, the name Thermos (it’s just the Greek word for “hot”) started out as a trademark in 1904, but it became so widely used that it’s now a generic term for any vacuum container. One manufacturer still uses it as a brand name, however.