The family said: after the hot water in the kettle is cold, the kettle will not keep warm. Is it true?/You don't say.

Indeed, my family often says so! I don't know why this matter is so unified. My mother said the other day that I must use up the hot water in the kettle, otherwise the kettle will not keep warm after the hot water in the kettle is cold! When I heard that, I was confused again. I thought about the heat preservation principle of the kettle and its possible influence. This is totally unreasonable. I really don't know how the family summed up this rule. Let's analyze the heat transfer process and the heat preservation principle of the kettle.

Let's talk about how the heat is lost.

Heat is the internal energy of molecules in an object, but we usually don't say how much heat or energy an object contains, but describe the average kinetic energy of molecules in an object through the physical index of temperature. Then the higher the temperature, the greater the internal energy/heat contained in the object, and the higher the average kinetic energy possessed by the molecule; The lower the temperature, the lower the internal energy/heat of the molecule and the lower the average kinetic energy.

According to the theorem of thermodynamics, we know that heat will spontaneously transfer from a high-temperature object to a low-temperature object. Therefore, after we boil water, if we don't keep it warm, the heat will soon dissipate into the atmosphere. There are three ways of heat transfer, and we can keep an object warm by blocking these heat transfer processes.

The first is the process of heat conduction. As mentioned above, molecules/atoms in high-temperature objects have higher kinetic energy on average than those in low-temperature objects. When a low-temperature object and a high-temperature object are in contact with each other, molecules/atoms on their surfaces will frequently collide mechanically, so the molecules/atoms moving faster will transfer kinetic energy to the molecules/atoms moving slower during the collision.

In this way, the molecular average kinetic energy of high-temperature objects decreases, while the molecular average kinetic energy of low-temperature objects increases. Until two objects have the same molecular internal energy, the conversion of kinetic energy realizes the transfer of heat, and the temperatures of the two objects also rise and fall.

This kind of contact heat conduction is the most common way in life. For example, cold air molecules stop colliding with your body in winter, and then gain kinetic energy on your body surface and take away heat, which makes us feel very cold. In summer, on the other hand, hot air transfers kinetic energy to our body surface, so I feel very hot. However, the medium for transferring heat can also be divided into: good conductor and bad conductor of heat. For example, metal is a good conductor of heat, while wood and glass are poor conductors of heat. That's why metal things feel cooler at the same temperature, because metal can make heat dissipate faster. The above is contact heat conduction, and there must be medium.

Then there is thermal convection.

Do you think air is a good conductor of heat? Actually, it's not. You can imagine the following situation. It's really sultry in a room where the air is completely stagnant. It's like condensation around. The stagnant air can't take away the heat from you well. But if you turn on the electric fan at the same temperature, is it much cooler? This is that the flow of air can exchange heat well.

There is also thermal convection around the heating in our house in winter. The rise of hot air takes away heat, while the fall of cold air continues to be heated, which makes the whole house price rise quickly. If there is no air convection, it is difficult for the room to get hot. Then when boiling water, you can observe the movement of water in the kettle and exchange heat through continuous rising and falling. These processes belong to thermal convection. Of course, the way of thermal convection heat transfer must first have medium and fluid.

The last way of heat transfer is thermal radiation. Thermal radiation is the only way to transfer heat without any medium, because thermal radiation is an object releasing heat in the form of electromagnetic waves, and electromagnetic waves do not need medium in the transfer process. We know that there is a vacuum in the universe, and vacuum is a perfect adiabatic state. The heat of the sun will not be transmitted to the earth through any medium, and the only feasible way is sunlight, that is, electromagnetic radiation.

But thermal radiation is the slowest way of heat transfer, and because the temperature of boiling water is at most 100 degrees Celsius, the radiation emitted also belongs to the infrared band.

How does the kettle isolate the above heat transfer?

I believe that when you broke a thermos bottle as a child, you usually heard a bang. I believe you picked it up and saw the structure of the thermal insulation liner. This structure is actually not complicated, you can understand it at a glance. Double-layer glass bottle liner, vacuum in the middle, silver inside and outside, very bright. There is also a small tail at the bottom, and the bottle mouth is a cork. This is the structure of the whole thermal insulation liner.

As mentioned above, vacuum is adiabatic, that is, it is a poor conductor of heat and will not conduct any heat with any substance, because vacuum has no medium at all and no molecules will take away the heat of water. Without medium, thermal convection will not occur. Silver plating on the inner container of the bottle is to reflect infrared radiation, although some energy will still leak out, but the heat emitted by infrared radiation is very low, which is not the only reason for the heat dissipation of the kettle.

There is also a cork in the bottle mouth, which can be well insulated, but the place where the hot water kettle radiates the most heat is the bottle mouth. Because there is air leakage at the cork, because the inner and outer layers are connected around the neck of the bottle mouth, a lot of heat will be lost here through heat conduction. Therefore, when making bottles, try to shorten the bottleneck and increase the capacity of the kettle, so that the heat preservation effect is better.

Above, we have analyzed all the reasons of heat transfer in physics, and also analyzed the heat preservation principle of the kettle, but we have never mentioned the reason why the heat preservation effect of the kettle will become worse after water cooling. Think about it. Will the cooling water from the hot water cause any damage to the kettle liner? Or what effect did it have on the inner container of the kettle?

No, not at all! The hot water kettle can not only keep hot water warm, but also keep cold water cold, and the effect is the same. It won't have any influence on the kettle because of the temperature change, at least scientifically. As for why mom said so, we can buy a hot water kettle and do experiments to dispel the rumor.