Why doesn't the tea in the thermos cool for a long time?

Greetings my dear friends and subscribers ✌

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It's all about the design of the thermos. Let's see, how does hot tea cool down in general? Pouring it into a cup, it will cool down pretty quickly, because it will give off its heat energy to the environment.

There are only three ways in which heat can be released:

- directly through the thermal conductivity of the substance

- heat transfer by convection

- heat transfer by radiation

Now let's look at how a thermos is made. Inside the thermos is a two-layer flask of glass or metal, with a vacuum between the walls. As we know, a vacuum has no thermal conductivity, i.e. it does not transmit heat at all. Therefore, direct heat transfer is not possible. Heat transfer by means of convection is also excluded, because this requires gas or liquid, the molecules of which are able to move, and we have vacuum behind the wall... You need a totally black surface for radiation to occur - and the surface of the thermos is mirror-like, on the contrary reflecting the heat.

Thus, the thermos blocks all three ways of heat transfer, keeping the tea hot for a long time. But the tea will not stay hot forever, in a few hours the tea will get cold, why? The problem lies in the tightness and thermal conductivity of the thermos cap.

By the way, do not forget that the thermos holds not only high temperature, but also low, for the same reasons. Cold tea in a thermos will also remain cold for a long time, even on a hot day.

 

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Sincerely Eduard!