how does the thermos keep my tea hot for several hours?
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For something to cool off, the heat has to be transferred to something else. Energy can’t be created or destroyed – it can only change forms.
Your thermos is double-walled with a hollow cavity in between the two walls. That cavity is a vacuum, meaning that the inner wall will heat up from the hot liquid, but that heat can’t go anywhere (there is nothing in the vacuum for the heat to transfer into). This means the heat stays in the liquid (for the most part – eventually the heat will transfer out via contact with the air when you open the thermos and the small welds that connect the inner and outer walls).
One thing that hasn’t been mentioned yet is that there’s a lot of energy in hot water. So it takes a lot of energy to get it hot, but also takes a lot to get it cold again. That helps a lot with keeping water warm for a very long time.
The main way objects lose heat is that they transfer it to cooler objects next to them. The tea will lose heat to the inner layer. There is a vacuum between the inner and outer layers. Because the inner and outer layers aren’t in physical contact, the heat can not be transferred to the outer layer. Thereby keeping the inside hot. (There are other heat loss methods like radiation that do transfer heat. Plus, the inner and out connect at the neck. Eventually, the heat will be lost but it takes a while)
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When a drink gets cold, you can think of what’s happening like the “hotness” of the drink playing a game of Frogger where the starting side is your drink, and the finishing side is the air.
A glass is like a game of Frogger with tons of logs in the river part. A thermos has almost no logs, so the “hotness” has a hard time getting through.
Heat transfers out of the tea through conduction (colder thing touching the hot tea), convection (air moving over the hot tea), and radiation (hot tea sends out heat energy as light in frequencies too low for our eyes to see).
The thermos tries to reduce all three of these by having two metal walls (to reduce radiation) with all the air sucked out of the space in between (to prevent convection), and they’re only touching each other at the mouth of the thermos (to reduce conduction). The lid is also usually pretty thick with hollow parts inside it to act as insulation.
A thermos has two “walls” separated by a layer that has been evacuated (had all the air sucked out).
Vacuums are extremely bad at transferring heat because of the lack of particles; they can only do so by a process called radiation.
Imagine you are standing at the front of a classroom and want to give a book to someone at the back of the room. There are three ways to do this.
1) You can pass the book to the person next to you and have them do the same until it gets to the back. This is *conduction* – passing heat from particle to particle.
2) You can walk to the back of the room yourself (or someone else can). This is *convection* – the heat moves through a fluid but remains “attached” to one particle (there will then be some conduction transferring heat to different particles)
3) You can throw the book to the back of the room. This is *radiation* – heat transfer without using particles at all. It’s how the sun’s energy reaches earth.
Radiation is the least efficient of these three processes so by minimising conduction and convection the flask slows a change in the temperature of its contents.
The tea is hot because it has extra energy. In order to cool down, it needs to lose that energy. In a mug, that heat can be transferred into the mug and into the sir through conduction, and then the air mixes around enough that the extra energy isn’t noticeable.
In the thermos, there’s an outer wall, a vacuum, an inner wall, and then your tea. The inner wall can absorb some of that heat, and it can transfer to the top of the thermos where it can get to the outside air, but that’s a very small area, so it’s a lot less heat loss.
The vacuum, since there’s no matter there, there’s nothing to conduct the heat away. Heat can only escape through the vacuum via infrared radiation, which is a very slow process. Since it’s so hard for the tea to lose heat, the tea stays warm for a long time.
The thermos is double walled. This means that it’s basically a bottle within a bottle. The outer layer and the inner layer are separated and the gap is either filled with an insulating material or it’s a vacuum with no air inside. Of course the two bottles are still connected at the neck but it’s by a thin layer of material and it’s usually insulated. What this means is that while it does not completely prevent heat from escaping through conduction it severely limits it which makes the whole process a lot slower. The actual surface area of material that is in contact with the outside layer and the air is very small. The majority of the container is surrounded by a vacuum and heat can’t transfer through a vacuum, at least not through conduction or convection.
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