Question

What is the construction and working of thermos flask

Answer 1

Apparatus


good quality thermos flask with silvered lining filled with hot water

Action

The students examine the flask and explain how the three processes of heat transfer are affected by the flask. They should note that while the liquid inside is hot, and hence the inner wall is also hot, the outer wall is at approximately room temperature.


The Physics


A thermos flask has double walls, which are evacuated and the vacuum bottle is silvered on the inside. The vacuum between the two walls prevents heat being transferred from the inside to the outside by conduction and convection. With very little air between the walls, there is almost no transfer of heat from the inner wall to the outer wall by convection. Conduction can only occur at the points where the two walls meet, at the top of the bottle and through an insulated support at the bottom. The silvered walls reflect radiated heat back to the inside, the same way a space blanket does. The diagram below shows a cross section through a thermos.

Answer 2

Construction and working of thermos flask:

Explanation: It is constructed in such a way that heat loss due to conduction,convection and radiation is minimized. It is a double walled bottle of thin glass and the space between the walls is evacuated. The absence of a material medium between the walls reduce the loss of heat due to conduction and convection. The mouth of the bottle is closed by a cork or plastic stopper. The bottle is enclosed in a plastic case supported on all sides by non conducting plastic mouldings. The stopper, the inward case and inner supports are bad conductors of heat. Thus, Loss of heat due to construction is further reduced. The outer surface of the inner wall and the inner surface of the outer wall are silvered. This reflects heart back into the chamber, minimising loss of heat by radiation. This the heat loss due to conduction, convection and radiation is very little, as a result of which the liquid in the flask retains it's temperature for a considerably very long time.