This is the mpemba effect.
Scientific principles originate from the experimental world and the observation field. Observation is the first step. Without observation, there will be no subsequent progress.
-Mendeleev (Russia)
First, the careful observation of middle school students Mpamba challenges the authoritative Newton's cooling law.
When I was in the third grade at Mpamba Middle School in Tanzania, the children in the school always boiled the milk first, then cooled it, then poured it into an ice tray and put it in the refrigerator. In order to get the last ice tray in the refrigerator, I decided to put hot milk in it at the risk of damaging the refrigerator. After more than an hour, we opened the refrigerator, and there was an amazing miracle: the hot milk in my ice tray had turned into hard ice, while their ice was still a viscous liquid. I hurried to ask the physics teacher, and he replied faintly, "Such a thing will definitely not happen."
After entering high school, when I was studying Newton's cooling law, I asked my physics teacher again, and he also rashly denied my observation. I went on telling my reasons, but the teacher didn't want to listen. The students on the side also helped the teacher ask me, "Do you believe Newton's cooling law?" I have to defend myself: "but the law doesn't match the facts I observed!" "In the laughter of the students, the teacher said helplessly:" What you said is called physics in Mpamba! "From then on," Baba's physics "became my nickname. As long as I made a mistake, the students immediately said, "What is this Mpamba ..." Nevertheless, I still firmly believe that my observation is correct, which may contain deeper truth.
This year, Dr. Osborne, head of the Physics Department of Dar es Salaam University, Tanzania's highest institution of learning, visited our school. I decided to go to the doctor and tell him about my adventure. First he smiled, and then he 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 might 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 freezes first in the refrigerator. Its main contents are:
1. After changing milk into water, we observed that the hot water in the refrigerator was still frozen into ice before the cold water.
2. When the hot water is put into the refrigerator for cooling, there is an obvious 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 the S-B temperature difference observation record of water with initial temperature 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 first, but once it is cooled rapidly, the temperature difference appears 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 we observed during the rapid cooling of hot and cold water.
The related pictures of this theme 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 rate 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 the upper surface emits, so the faster the temperature drops.
For the above two reasons, hot milk cools faster, which is the secret that hot milk freezes first.
In addition to the explanation that hot milk freezes first, we boldly deduce an interesting "guess": on days of severe freezing, should hot water pipes freeze before cold water pipes? 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 middle school friends who can observe this phenomenon to provide us with information and discuss with us.
Since the publication of our article, many scientific magazines in the world have published this natural phenomenon, which is considered as a severe challenge to Newton's cooling law. But also named this natural phenomenon "Mpamba effect" after me.