​​How does Vacuum or Dewar Flask work? A vacuum flask or a de | NCERT NOTES

​​How does Vacuum or Dewar Flask work?

A vacuum flask or a dewar flask works on the concept of insulation.

A vacuum flask or dewar flask is designed in a way to maximise insulation and limit heat transfer.

Most vacuum flasks have an inner glass chamber, a middle glass or stainless steel layer, and an outer plastic or metal casing.

A tight plastic stopper than can be screwed in or an airtight lid is also provided.

The inner glass chamber is lined with reflective layers and there is a vacuum separating the middle and the outer chambers.

The vacuum flask prevents all three modes of heat transfer by its design, the inner chamber is separated from the middle chamber with a vacuum, preventing convection and conduction, the mirrored layer within the inside chamber reflects any heat being radiated.
Since there is two degrees of separation between the inner and outer layer, there is no conduction.

For example, Let us say boiling water (100°c) is kept in an open vessel in a hotel at room temperature (15-20°c), due to the difference in temperatures, heat travels from the water to its surrounding, and hence the water cools down by losing heat to the environment.

However when the same water is stored in a vaccum flask or a dewars flask, heat transfer is inhibited hence keeping the water hot.