Grades 3–5 · Everyday Life
How can an almost empty gap keep a drink warm?
A vacuum flask can keep a warm drink warm and a cold drink cold. It does not make heat or cold. It slows the transfer of energy between the drink and its surroundings.
In the diagramAn almost empty gap slows heat transferA vacuum flask has two walls, not a source of heatDrinkVery little gasHeat can still crossthe lid and neckReflective surfacesGap between wallsReflective walls also reduce transfer by radiation.
The bottle has an inner wall and an outer wall. Most of the air has been removed from the space between them. With very little gas there, much less energy can cross the gap by gas conduction, and gas cannot circulate around the gap to carry heat by convection.
Energy can still travel across empty space as infrared radiation. Reflective surfaces reduce that transfer. Heat can also pass through solid connections at the neck and through the lid. A good lid reduces those remaining paths, but no flask keeps a drink at exactly the same temperature forever.
This explains why the same bottle helps in both directions. A hot drink loses energy more slowly. A cold drink gains energy more slowly. The almost empty gap slows the journey; it does not choose a favorite temperature.
About this explanation
Starting knowledge: Recognize that heat transfers from warmer to cooler places.
Sources
- Thermos — How Vacuum Insulation Works