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A Thermos flask has two walls with a vacuum between them. How does this help in keeping tea
or milk hot?
Answers
The thermos flask is a special kind of bottle in which hot liquids like tea and milk remain hot and cold items like ice, cold water etc. remain cold for a long time. It was invented by Sir James Dewar in 1892. It consists of a double-walled glass bottle. The walls are silvered. The space between the walls is evacuated with the help of a vacuum pump and sealed. This bottle is protected by metal case. A cork is fitted on the mouth.
It is interesting to know how this bottle helps in keeping the hot liquids hot and cold liquids. Normally a hot substance becomes cold by loosing its hear to the surroundings. Similarly a cold substance becomes hot by absorbing heat from the surroundings. 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 processes: conduction, convection and radiation. The thermos flask prevents the flow of heat by any of these processes. The flask is made up of glass which is a bad conductor of heat. Hence heat does not flow by conduction. Since there is 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