A cup of boiled water will be cold soon. Why is a cup of boiling water still hot in a thermos for a day or two?

There are three ways of heat transfer: radiation, convection and conduction. People will feel warm under the sunshine, because the heat of the sun hits us, which is called thermal radiation. The best way to prevent thermal radiation is to block it back, and the best material to reflect heat is a mirror. Pour a cup of boiling water on the table. Due to the flow of water in the cup and the surrounding air, the water temperature gradually becomes the same as the temperature of the surrounding environment. This is thermal convection. If you put a lid on the cup, it will hinder the path of convection. But this glass of water will still be cold, just for a longer time. This is because the cup has the property of heat transfer, which is called heat conduction. The thermos container is made of double-layer glass, and both layers of glass are silver-plated, which can reflect heat rays back like a mirror, thus cutting off the path of heat radiation. The vacuum between two layers of glass in a thermos bottle destroys the conditions of convection and conduction. The thermos bottle cap is made of cork, which is not easy to transfer heat, thus blocking the passage of convective heat transfer. If the three paths of heat transfer are completely blocked, heat can be preserved for a long time. However, the insulation of thermos is not so ideal, and some heat can still escape, so the insulation time of thermos is limited. The function of a thermos bottle is to keep the temperature of hot water in the bottle and cut off the heat exchange inside and outside the bottle, so that the "heat" inside the bottle can't get out and the "cold" outside the bottle can't get in. If you put the popsicle in a thermos, the "heat" outside will not easily run into the bottle, and the popsicle will not easily melt. Therefore, it is scientific to call the thermos bottle a thermos bottle, because it can keep both "hot" and "cold". How to keep the thermos bottle warm The thermos bottle used at home can keep the temperature of hot water well. What is its principle? The cooling of hot water is caused by thermal convection, heat conduction and thermal radiation. The thermos bottle liner is made to solve the above three problems. The bottle mouth is blocked with a cork to prevent the convection of cold and hot air; The gap between the double-layer bottles and bladders is vacuumized, which solves the heat conduction; Coating a thin layer of silver on the bottle liner makes it a mirror for reflecting light and heat, thus using the silver layer to block the heat radiation from coming back. In this way, the heat will not be lost, and it will play the role of heat preservation.