Q:

How does a thermos keep things hot?

A:

Quick Answer

A thermos is made from insulating materials that keep hot things hot and cold things cold. These materials are selected to minimize the transfer of heat between different objects or environments.

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Full Answer

Objects that are hotter have atoms that are moving faster than objects that are colder. When a hot object is placed into a colder environment, the hot object loses energy to its surrounding environment, causing its atoms to slow down. The colder environment warms up, and its atoms speed up due to the energy gained from the hot object. If everything was left the same, the temperature would equalize between the object and the environment. Even the most perfect thermos cannot stop heat transfer completely; the goal is to design the thermos so heat transfer slows as much as possible.

There are three ways heat can be transferred. First is conduction, which is when two surfaces are in contact with each other, this is the most common reason for cooled drinks. The second is convection, which is when a liquid or gas comes in contact with the object and moves across the surface taking heat away with the flow of liquid or gas and another major reason drinks may cool. The last is radiation, which is direct heat transfer due to photon emission.

First, the material is encased in very good insulation, which also prevents direct conductive contact with the surrounding environment. Second the middle portion of the thermos is constructed of materials that eliminate airflow, preventing any convection heat loss. Third, the thermos is coated to reflect radiation heat.

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