Qunmei Catalog

"Ahem, I told you to read the question again, but I didn't hear clearly just now," Li Mulin replied awkwardly.

"A foreign junior high school student discovered a strange phenomenon. He put twice the same volume of milk into the refrigerator at the same time, one of which was at 20 degrees and the other at 60 degrees. The result was surprising: the milk at 60 degrees was the first to freeze. Bing. Please explain why!" Yang Nihong repeated the previous question in a blunt tone.After speaking, he gave Li Mulin a glance, and said, "Can you answer the question? If you can't answer, I will announce the examiner's mark."

Li Mulin said with a slight smile on his face: "Actually, this question is very simple. Do you want a simple answer or a complicated answer?"

"Of course I answered carefully!" Wang Kai said disdainfully.

"In fact, this phenomenon is called the Mpemba phenomenon physically. From a physical point of view, there are four co-existing mechanisms for cooling: radiation, conduction, vaporization, and convection. Through experimental observation and comparison of the results, it is found that The reason why hot water freezes earlier than cold water is mainly due to the combined effect of conduction, vaporization, and convection. A cup filled with cold water at an initial temperature of 4°C will take a long time to freeze because water and glass are materials with poor heat conduction. , it is difficult for the heat inside the liquid to be effectively transferred to the surface by conduction. As the temperature drops, the volume of the water in the cup expands, the density becomes smaller, and it gathers on the surface. Therefore, the water freezes first on the surface, and then flows to the bottom and Extending around, and then forming a closed "ice shell". At this time, the inner water is isolated from the outside air, and can only rely on conduction and radiation to dissipate heat, so the cooling rate is very small, preventing or delaying the inner water temperature. Continue to descend normally. In addition, because the volume of water will expand when it freezes, the "ice shell" that has formed also plays a certain role in restricting or inhibiting further freezing. A cup filled with hot water with an initial temperature of 100°C, Relatively speaking, the time is much less. The phenomenon seen is that the ice layer on the surface can never be connected into an ice sheet, and the phenomenon of "ice shell" cannot be seen, but the needle-shaped growth along the ice-water interface into the liquid. Ice crystals (this phenomenon cannot be seen when the initial temperature is lower than 12°C). As time goes by, the ice crystals change from thin to thick. This is because the hot water with high initial temperature will become denser after the upper layer of water cools down. The flow forms the convection inside the liquid, making the water molecules freeze around their respective "crystallization centers". The higher the initial temperature, the more intense the convection and the greater the energy loss. It is this convection that makes the upper layer The water in the water is not easy to form an ice sheet. Due to heat transfer and latent heat of phase change, the internal energy loss per unit time is relatively large, and the cooling rate is relatively large. When the water surface temperature drops below 0°C and there is a sufficient low temperature, the water surface begins to Ice crystals appear. Water with a higher initial temperature grows ice crystals at a faster rate. This is due to the fact that the ice sheet has not formed and the convection is severe. Finally, it can be observed that the ice sheet is still formed, and the cooling rate becomes smaller. However, due to the water The internal ice crystals have grown and become thicker, with greater surface energy, and the growth rate of ice crystals is proportional to the unit surface energy, so the growth rate is still much faster than that of water with a lower initial temperature."

"Stop stop!!!" Wang Kai was overwhelmed by Li Mulin's long speech. He didn't know anything about refrigeration mechanism or cooling rate, so he interrupted Li Mulin's answer unceremoniously.

"You should answer simply." Wang Kai wiped the sweat from his brow, and if Li Mulin continued, he would probably go crazy.

Li Mulin took out his hands from his trouser pockets, spread them out and said, "I've finished speaking."

And in his trouser pocket, an extremely small voice said: "The cost of breaking into the Physics Data Center of the Chinese Academy of Sciences is 880. It is your friendship price, [-]. Remember to call it on my card."

Li Mulin pursed his mouth indiscriminately. The original price was 880, and he was charged [-]. This is also called a friendship price, but he has no chance to refute at this time. Heisi still worships him incomparably, he is even more powerful than Nobita's Doraemon, and he is omnipotent. He originally had the mentality of giving it a try, but he didn't expect Heisi to solve it for him, but the price is that Li Mu Lin had to borrow some money from a buddy again.

At this time, the surroundings were completely silent, and everyone was shocked by Li Mulin's wonderful answer. Although Wang Kai, the second generation ancestor, didn't understand, there were still some good students who could understand Li Mulin's exposition. The physics professor was sweating profusely at this moment. In fact, even he didn't know the answer to this question, so he concluded that Li Mulin couldn't answer it. But now Li Mulin not only answered it, but also almost leaked it. From a certain point of view, Li Mulin's answer is perfect. Of course, this is only the result of scientists' exploration. The real reason has yet to be explored. Miscalculation, because it is recognized by many scientists.

"Are you answering a question? I think you are talking nonsense!" Since there was no answer, Wang Kai didn't know whether Li Mulin answered correctly. He thought Li Mulin didn't know how to answer and was just talking nonsense, so he shot Drinking at the table.

Li Mulin just glanced at Wang Kai in disdain. For this kind of guy who is full of sperms but has no goods in his stomach, Li Mulin and Ben are too lazy to explain to him. Let him answer this question by himself. He really can't answer it. But he was able to tell the difference. Even if he answered this way, even if it was not correct, it was absolutely impossible to be wrong.

"Professor Liu Yichang, you come and tell him that his answer is completely nonsense!" Wang Kai was anxious to drive Li Mulin out of D University, and he was a little emotional. What he said was a bit commanding.

The displeasure on Liu Yichang's face disappeared in a flash, and he said blankly: "Counselor Li Mu's answer is completely correct, I will give you ten points." After finishing speaking, he picked up the scoring board in his hand and wrote a big ten.

Liu Yichang's move undoubtedly slapped Wang Kai severely. There are many people in D who dare not offend Wang Kai, but it is a pity that Liu Yichang is not in this rank. The old professor is very prestigious in the academy, even Wang Bifu treats him politely, so he is not afraid of offending Wang Kai at all.

"You!" Wang Kai wanted to say something, but Wang Bifu gave him a hard look. Wang Kai angrily sat back on the stool, with a gloomy face, without saying a word.

"Young man, you're doing well." Liu Yichang also gave Li Mulin a thumbs up, as if he didn't think it was enough to hit Wang Kai.

Li Mulin nodded immodestly, with a proud smile on his face, as if he had solved the problem himself.

"There are still five tests, I don't believe I can't kill you!" Wang Kai clenched his hands tightly into fists.