how does the thermos keep my tea hot for several hours?
In: 30
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).
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