Why does hot water thermos retain heat?

A thermos bottle is a household appliance used to store boiling or warm water.

Thermos were invented by physicist and chemist Sir James Dewar (1842-1923), Scottish chemist and physicist, in 1892 thanks to an improvement from Newton's calorimeter barrel.

Pour a cup of boiling water, leave it in the air, and soon it will cool down. But, if boiling water in a thermos bottle can maintain the temperature of boiling water for a long time.

The thermos of hot water retaining heat is determined by the structural characteristics of the gut.

The thermos of hot water retaining heat is determined by the structural characteristics of the gut . The inner core is formed by two thin glass shells, drawing air between the two shells and coating a thin layer of mercury on one side of the plug. The thermosus has a much smaller mouth than its "body" . The top of the thermos can be covered with a soft wooden button. It is thanks to such a structure that makes the thermos bottle into a thermos bottle "hot intestine, cold appearance".

When the boiling water is finished in the thermos, the structure of the thermos makes the heat of the water cannot be transmitted by normal methods.

1. One is convection heat is cut off . The heated air inside the thermos will find a way to escape, while the cold outside air will also find any gaps to get inside the thermos. But because the neck was relatively small, it was covered by a soft wooden cork, so the only path of thermal convection was cut off.
2. The other is thermal conductivity is hindered. Although the air conducts less heat than metal items, the heat inside the thermos can still pass through the outer glass casing, which transmits air outside the thermos. But because the thermos have two glass shells, in the middle are drawn into a vacuum, so the air - the medium conducts heat, becomes very thin, the heat path is also blocked.
3. The third is radiant heat suppression. Winter, under the sun, we will feel a little warm. It is caused by the heat radiation of sunlight. Because the plug is coated with a thin layer of mercury, the radiation of the heat is reflected by the mercury layer and must be left inside the plug. That makes the path of heat radiation is also completely blocked.

Ideally, after the gut plugs off three types of heat transfer methods, the hot water in the thermos can retain heat forever. But in fact, the insulation effect of hot water thermos is not so complete, so the heat retention of thermos always has a time limit. Beyond that limit, the thermos will no longer retain heat.