Question

The figure shows a cut-away drawing of a thermos bottle (also known as a Dewar flask), which is a device designed specifically to slow down all forms of heat transfer. Explain the functions of the various parts, such as the vacuum, the silvering of the walls, the thin-walled long glass neck, the rubber support, the air layer, and the stopper.

Answer 1

Dewar Flask:

The vacuum flask consists of two flasks, placed one within the other and joined at the neck

A vacuum flask is a bit like a super-insulated jug. Most of them have an inner chamber and an outer plastic or metal case separated by two layers of glass with a vacuum in between.

The glass is usually lined with a reflective metal layer. Unbreakable flasks do away with the glass. Instead, they have two layers of stainless steel with a vacuum and a reflecting layer in between them. There's also a tight, screw-down stopper on the top.

These few, simple features prevent virtually all heat transfer by either conduction, convection, or radiation.

Different Parts and Role of them::

The vacuum prevents conduction.

The tight stopper prevents air from entering or leaving the flask, so convection isn't possible either.

When infrared radiation tries to leave the hot liquid, the reflective lining of the inner chamber reflects it straight back in again.

Since there is virtually no way heat can escape from a vacuum flask and a hot drink stored inside will stay steaming hot for several hours.

(The gap between the two flasks is partially evacuated of air, creating a vacuum which prevents heat transfer by conduction or convection. Heat transfer by thermal radiation may be minimized by silvering the flask surfaces facing the gap)