how does the thermos keep my tea hot for several hours?
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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.
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