The principle of insulation of thermos
There are three ways of heat transfer: heat radiation, heat convection, and heat conduction.
Under the sunlight, people will feel hot on their bodies, because the heat of the sun hits us, which is called hot radiation.
The best way to prevent heat radiation is to block it back, and the best material for reflecting heat is a mirror.
Pour a cup of boiling water on the table, due to the water in the cup and the flow of the surrounding air, the water temperature gradually becomes the same as the temperature of the surrounding environment, which is the convection of heat.
If you put a lid on the cup, you block the path of convection. But this glass of water will still get cooler, just for a longer time. This is because the cup has the property of heat transfer, which is called heat conduction.
The thermos gallbladder is made of double glass, both of which are plated with silver, like mirrors, which can reflect heat rays back, which cuts off the path of heat radiation. By vacuuming between the two layers of glass of the thermos, the conditions for convection conduction are destroyed. The thermos cap is made of cork that is not easy to transfer heat, which isolates the path of convective heat transfer. By perfectly blocking the three roads of heat transfer, the heat can be retained for a long time. However, the thermal insulation of the hot water bottle is not so ideal, and there is still a part of the heat that can run out, so the holding time of the hot water bottle has a certain limit.
The function of the hot water bottle is to maintain the temperature of the hot water in the bottle, cut off the heat exchange between the bottle and the outside of the bottle, so that the "heat" inside the bottle cannot go out, and the "cold" outside the bottle cannot come in. If you put popsicles in the hot water bottle, the "heat" outside is also not easy to run into the bottle, and the popsicles are not easy to melt. Therefore, it is scientific to call a thermos a thermos because it can keep both "hot" and "cold". The warm water bottle used at home can maintain the temperature of hot water well, what is the principle? The cooling of hot water is caused by convection of heat, conduction of heat, and radiation of heat. Warm water bottle is made to solve the above three problems. Cork at the mouth of the bottle to prevent the convection of hot and cold air; The gap between the double-layer bottle bladders is pumped into a vacuum, which solves the heat conduction; A thin layer of silver is applied to the bottle to make it a mirror that reflects light and heat, thus using a silver layer to block back the heat radiation. In this way, the heat is not lost, which plays a role in heat preservation.
