Hot Water Freezing?

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What about snap-freezing?

关于无脆化冻结的情况？

Another scenario suggested for the "hot water freezing" story is the possibility of supercooling and snap-freezing by the hot water.

另一种解释的‘热水结冰’故事是可能的：热水可能因过冷而迅速结冰。

The Mpemba effect

普拉托效应

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The Mpemba Effect

迈帕姆巴效应

There is in fact a large body of literature about initially hot water freezing faster than cold water and this phenomenon has been labeled the "Mpemba effect" (see references). The literature cited here takes the position that the phenomenon of supercooling is the main mechanism involved and that the history (dissolved gases, etc.) are not crucial to the phenomenon. Supercooling to -5 C or lower is cited and it is suggested that both the initially hot and the initially cold water supercool. If the cold water supercools more, then the the hot water may in fact freeze quicker with nothing more than the degree of supercooling to distinguish the two samples. It was also pointed out that the degree of supercooling is not easily predictable, since it probably depends upon tiny particles or bubbles which nucleate the crystal formation. Experiments showed different amounts of supercooling for the same sample when it was repeatedly liquified and refrozen.

事实上，有关初始温度较高的水比初始温度较低的水冻结更快的现象存在大量文献，这种现象被称为“Mpemba效应”（见参考文献）。此处引用的文献认为，过冷是主要的机制，而历史因素（如溶解气体等）对这一现象并不关键。文中提到过冷可达到-5°C或更低，并建议初始热水和初始冷水均会过冷。如果冷水过冷程度更高，那么热水实际上可能因过冷程度的不同而更快冻结。也指出，过冷程度难以预测，因为这可能取决于促成晶体形成的微小颗粒或气泡。实验显示，同一样品在反复液化和重新冻结过程中表现出不同的过冷程度。

Index

Heat questions

References

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Cooling Coffee

冷却咖啡

Leave it black for a while, then add cream.

放一段时间让它保持黑色，然后加奶油。

The intent is to get the coffee cool enough to drink in the shortest possible time. The heat loss rate by conduction, convection, and radiation all depend upon the difference in temperature between the hot object and its surroundings. Therefore the cooling rate is greater if you don't immediately add cream.

意图是尽快将咖啡冷却到可以饮用的温度。通过传导、对流和辐射的散热速率都取决于热物体与其环境之间的温度差。因此，如果不立即加奶油，冷却速率会更大。

An often cited but bogus reasoning in this example is to say that black objects radiate better than white ones. This is true for visible light, but this radiation is in the infrared where the visible color is not a factor.

在这个例子中，有一种常被引用但错误的推理是说黑色物体比白色物体辐射得更好。这在可见光范围内是正确的，但这种辐射发生在红外线区域，而可见颜色在此并不起作用。

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Cooling on very hot day?

炎热的夏日如何降温？

The standard heat transfer mechanisms act against you, transferring heat into your body on a very hot day. The main cooling mechanism you have available is the evaporation of perspiration. Fortunately, it is a very effective mechanism because of the large heat of vaporization of water. Note that you must continue to have a net outflow of about 90 watts, even on a hot day.

标准的热传递机制在高温天会向你的身体传递热量。你主要的散热机制是汗水的蒸发。幸运的是，这是一种非常有效的机制，因为水的汽化热很大。请注意，即使在炎热的天气中，你仍需要保持大约90瓦的净散热。

What if ambient is below body temperature?

如果环境温度低于体温呢？

Cooling mechanisms for the body

身体的冷却机制

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Heat Flow to Hotter Region

热量流向更热的区域

Although internal energy will not spontaneously flow from a cold region to a hot region, it can be forced to do so by doing work on the system. Refrigerators and heat pumps are examples of heat engines which cause energy to be transferred from a cold area to a hot area. Usually this is done with the aid of a phase change, i.e., a refrigerant liquid is forced to evaporate and extract energy from the cold area. Then it is compressed and forced to condense in the hot area, dumping its heat of vaporization into the hot area.

尽管内能不会自发地从冷区域流向热区域，但它可以通过对系统做功的方式被强制地从冷区域流向热区域。制冷机和热泵是热机的例子，它们使能量从冷区域转移到热区域。通常这是借助相变来实现的，即一种制冷剂液体被强制汽化，从而从冷区域提取能量。然后它被压缩并在热区域强制冷凝，将汽化热释放到热区域。

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