How Does A Thermos Flask Keep A Hot Thing Hot And A Cold One Cold?

Thermos flak is also called a vacuum flask. It is used for keeping the hot liquid like milk and tea hot for long time and cold items like water and ice cold for long time. It was invented in 1892 by Sir James Dewar. It consists of double walled glass bottle. The inner side of outer wall and the outer side of inner wall are silvered. The air is pumped out between the two walls of bottle to create a vacuum and then sealed it. The sealed bottle is enclosed in a metal case, where it is supported on a cork pad. It protects the glass bottle from breaking. It is very interesting to know how thermos flask helps in keeping the hot liquid hot and cold liquid cold for long time. Generally, a hot substance gets cold by releasing its heat to the surroundings.

Likewise a cold substance gets hot by absorbing the heat from the surroundings. The most important function of the thermos flask is to prevent the flow of heat to and from the bottle. Conduction, convection and radiation are the three processes by which heat can flow from one place to another. Thermos flask doesn’t allow heat exchange to take place. Glass is used to make up a thermos flask which is a bad conductor of heat; hence heat is prevented from escaping out by conduction. Due to the presence of vacuum within the walls, heat loss due to convection is also ruled out. Since the walls are also silvered heat is also not lost due to radiation. Due to all these reasons mentioned above, a hot thing remains hot and a cold thing remains cold for many hours.

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