How does a thermos keep hot things hot and cold things cold?

03 Feb How does a thermos keep hot things hot and cold things cold?

In today’s physics tuition, we will cover an interesting story that may have come into your mind. We all use thermos flasks in our house to keep hot liquids hot and cold ones cold for relatively long duration. How this is possible? Do you know the answer? In today’s physics tuition, we will find answers to this interesting question. 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. 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 a metal case. A cork is fitted in the mouth.

It is interesting to know how this bottle helps in keeping the hot liquids hot and cold liquids cold. Normally a hot substance becomes cold by losing its heat to the surroundings. Similarly, a cold substance becomes hot by absorbing heat, from the surroundings. The purpose of a thermos flask is to stop heat flow to and from the thermos flask. We all know that the flow of heat from one place to another is possible by three processes: conduction, convention and radiation. The thermos flask prevents the flow of heat by any of these processes. The flask is made up of glass. Now glass does not conduct heat well as it is a bad conductor of heat. Thus heat does not flow by conduction. Since there is a vacuum between the walls of the bottle, the 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. We hope you enjoyed this lesson from physics tuition. We will be back with more such interesting findings in our next physics tuition.

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