This is mpemba effect.
The principle of science originates from the world of experiment and the field of observation. Observation is the first step. Without observation, there will be no subsequent progress.
-Mendeleev (Russia)
First, the careful observation of Mpamba, a middle school student, challenges the authoritative Newton's cooling law.
When I (Mpamba) was in the third grade at Makamba Middle School in Tanzania, the children in the school always boiled the milk before cooling it, then poured it into the ice tray and put it in the refrigerator. In order to win the last ice tray of the refrigerator, I decided to put hot milk in it at the risk of damaging the refrigerator. More than an hour later, we opened the refrigerator, and there was an amazing miracle: the hot milk in my ice tray had formed hard ice, but their ice was still thick liquid. I ran to ask the physics teacher quickly, and he replied faintly, "Such a thing will definitely not happen."
After entering high school, while studying Newton's cooling law, I asked my physics teacher again, and he also rashly denied my observation. I continued to tell my reasons, but the teacher didn't want to listen. The students on the side also helped the teacher ask me, "Do you believe in Newton's cooling law?" I had to defend myself: "but the law doesn't match the facts I observed!" " In the laughter of the students, the teacher said with a helpless look: "What you said is called physics in Mpamba!" From then on, "Mpamba's Physics" became my nickname. As long as I made a mistake, my classmates immediately said, "What is this in Mpamba ..." Nevertheless, I still firmly believe that my observation is correct, which may contain more profound truth.
In this year, Dr. Osborn, director of the Department of Physics at Dar es Salaam University, Tanzania's highest institution of learning, visited our school. I decided to turn to the doctor and told him about my adventure. He smiled at first, and then listened carefully to my retelling. When the doctor returned to school, he did it himself and observed the same fact. He spoke highly of my observation. He said: "Mpamba's observation actually raised the danger that authoritative physicists may encounter, and also raised an interesting question for physics teachers."
The doctor invited me to jointly publish a paper published in British Education, which introduced and explained the phenomenon that hot milk was frozen first in the refrigerator. Its main contents are:
1.After changing the milk into water, we observed that the hot water in the refrigerator was still frozen into ice before the cold water.
2. When hot water is put into the refrigerator for cooling, there is a significant temperature difference between the upper surface (S) and the bottom (B) of the water. The temperature difference during slow cooling is almost unobservable. Figure 1- 1 is an observation record of the S-B temperature difference of water with initial temperatures of 70℃ (solid line) and 47℃ (dotted line) respectively. As can be seen from the figure, there is no temperature difference between the upper surface and the bottom at the beginning, but once it is rapidly cooled, the temperature difference will appear immediately. The highest temperature difference in water with an initial temperature of 70℃ is close to 14℃, while the highest temperature difference in water with an initial temperature of 47℃ is only about 10℃, which is what we have observed for cold and hot water.
The related pictures of this topic are as follows:
On the basis of the above quantitative observation, we explain the phenomenon that hot milk (or hot water) freezes first as follows:
1.The cooling speed is not determined by the average temperature of the liquid, but by the temperature difference between the upper surface and the bottom of the liquid. When hot milk is cooled sharply, this temperature difference is large, and the temperature difference of hot milk is always greater than that of cold milk during the whole cooling process before freezing.
2. The higher the temperature of the upper surface, the more heat is emitted from the upper surface, so the faster the temperature drops.
Based on the above two reasons, hot milk is cooling at a higher speed, which is the secret that hot milk is frozen first.
In addition to the explanation that hot milk freezes first, we boldly deduce an interesting "conjecture": on days of severe freezing, hot water pipes should freeze before cold water pipes, right? Because we live in Tanzania near the equator, the climate here is hot all the year round, so it is difficult to observe this very interesting phenomenon. Welcome friends from middle schools who can observe this phenomenon to provide us with information and discuss it with us.
Since the publication of our article, many scientific magazines in the world have published this natural phenomenon, which is considered to be a severe challenge to Newton's cooling law. But also named this natural phenomenon "Mpamba effect" after me.