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Thermal insulation principle of vacuum flask


There are three ways of heat transfer: thermal radiation, thermal convection, and thermal conduction.

When people are exposed to the sun's rays, they will feel hot on their bodies. This is because the sun's heat reaches our bodies. This is called thermal radiation.

The best way to prevent heat radiation is to block it back. The best material to reflect heat is a mirror.

Pour a cup of boiling water and place it on the table. Due to the flow of the water in the cup and the surrounding air, the temperature of the water gradually becomes the same as the temperature of the surrounding environment. This is heat convection.

If you add a lid to the cup, it will block the path of convection. But the glass of water will still get cold, but it will take longer. This is because the cup has the property of conducting heat, which is called heat conduction.

The thermos bottle is made of double-layer glass, and both layers of glass are coated with silver. Like a mirror, it can reflect heat rays back, thus cutting off the path of heat radiation. Evacuating a vacuum between the two layers of glass in a thermos bottle destroys the conditions for convection and conduction. The hot water bottle cap is made of cork, which is not easy to conduct heat, and blocks the path of convection heat transfer. By perfectly blocking all three paths of heat transfer, heat can be retained for a long time. However, the heat insulation of the thermos bottle is not that ideal, and some heat can still escape. Therefore, the heat preservation time of the vacuum flask has a certain limit.

The function of the thermos is to maintain the temperature of the hot water inside the bottle and cut off the heat exchange between the inside and outside of the bottle, so that the "hot" inside the bottle cannot go out and the "cold" outside the bottle cannot come in. If you put popsicles in a vacuum flask, the "heat" from the outside will not easily escape into the bottle, and the popsicles will not melt easily. Therefore, it is scientific to call a thermos a thermos, because it can keep both "hot" and "cold". The thermos bottle used at home can maintain the temperature of hot water very well. What is its principle? The cooling of hot water is caused by heat convection, heat conduction and heat radiation. The thermos bottle bladder is made to solve the above three problems. A cork is used at the mouth of the bottle to prevent the convection of hot and cold air; the gap between the double-layered bottle bladder is evacuated to solve the problem of heat conduction; a thin layer of silver is applied to the bottle bladder to make it reflect light and reflection. A hot mirror uses the silver layer to block the heat radiation. In this way, heat will not be lost and it will act as a vacuum flask.

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