Two bottles are nested in one another, separated by a small space. This isolates the contents of the inner bottle, slowing down its cooling or heating.
A Thermos bottle keeps heat and cold. (Image: VIPDesignUSA /
Our site )
A Thermos is produced from plastic, metal or glass. Thanks to its specific structure, it is possible to keep drinks or food hot or cold for a long period of time in an insulating bottle.
If you put a bowl of snow or a cup of tea burning at room temperature, the snow melts and the tea cools. The ambient temperature, that of the snow and that of the tea are balanced. In a Thermos bottle, this heat exchange is slowed down.
How does the temperature balance?
Temperatures of several objects can be balanced either by direct contact or by thermal radiation. If one touches a cup of hot tea, the hand warms and the cup cools, because the movements of the molecules of the cup and the hand balance. This is called thermal conduction . The thermal radiation is radiation in the infrared range, which is issued by any material. If in a room everything is at the same temperature, a particle of matter receives as much heat radiation as it emits. The hot cup sends more heat radiation than it receives, so it loses heat and cools.
How is heat exchange prevented?
In our example of hot tea in the Thermos, as much heat as possible should be kept in the bottle and not lost in the environment. To do this, it is necessary to minimize the contact surface of the tea with the ambient air, which we try to achieve with the principle of the two nested bottles. To completely avoid the thermal conduction between the two bottles, it would be necessary to remove all the material in the intermediate space (thus the air also): one needs a vacuum. This is not possible at 100% because the inner bottle can not float in a vacuum, but must be attached to the outer bottle near the opening. Even if the vacuum is well made in the intermediate space, the insulation properties of the Thermos bottle are less good at the lid according to the water bottle choosing tips from INSURANCEJUST.
However, the vacuum does not prevent the propagation of the thermal radiation. This is why the surfaces inside the Thermos are covered with a special layer, which reflects the thermal rays and keeps a large part in the bottle. The tea does not remain indefinitely hot but at least for several hours.