How does a Thermos Flask Work?

            The thermos flask is a special kind of bottle in which hot liquids like tea and milk remain hot and cold items like ice and cold water remain cold for a long time. Sir James Dewar first invented this flask in 1892. That is why it is also known as the Dewar flask. It consists of a double walled glass bottle. These walls are silvered form inner side. The space between the walls is evacuated with the help of a vacuum pump and sealed.

Thermos Flask
Internal Structure of Thermos Flask

            Do you know how this bottle helps in keeping the hot things hot and cold ones cold? Normally, a hot substance becomes cold by losing its heat to the surroundings having a lower temperature. Similarly a cold substance becomes hot by absorbing heat from the surroundings having higher temperature. The function of the thermos flask is to prevent the flow of heat to and from the bottle. We know that heat can flow from one place to another by three modes- conduction, convention and radiation. The thermos flask prevents the flow of heat by any of these modes. The flask is made up of glass, which is a bad conductor of heat. Hence heat does not flow by conduction. Since there is a vacuum between the walls of the bottle, flow of heat by convection is also ruled out. Due to the silvering of the walls, heat is not lost by radiation. As a result, hot things kept in a thermos flask do not become cold and cold ones do not become hot for a long time.