A vacuum flask is an insulated bottle which helps to keep the liquid inside it at the same temperature at which it was added for an extended period of time.
Also known as Thermos, it works by minimizing the heat transfer between the inside and outside environment.
Sir James Dewar, a Scottish scientist, invented the Vacuum flask in 1892. Hence it is also called as Dewar flask.
How it works?
Vacuum flask consists of two walls, usually made of stainless steel or glass. In between the two walls, air is evacuated to create a vacuum so that heat cannot be transferred due to conduction or convection.
Additionally the surfaces of two walls are often coated with a reflective material such as silver which helps reduce the heat transfer through radiation.
On top, the neck of the bottle is usually reduced so that energy loss is reduced due to large surface area.
Finally, a tight-sealing lid prevents heat loss through evaporation and stops warm or cold air from escaping.
Why it works?
- If you pour hot liquid inside, the vacuum prevents heat from escaping, keeping the drink warm.
- If you pour a cold drink, the vacuum prevents external heat from entering, therefore keeping the drink cool.
By blocking heat transfer in all three ways that is through conduction, convection, and radiation, a vacuum bottle efficiently maintains the temperature of its contents for hours.