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J. Perelman
"Entertaining physics". Book 2.
Chapter 7. Thermal phenomena

COOLING JARS

If you have never seen such jugs, then you've probably heard or read about them. These vessels are made of unfired clay has the curious feature that poured into them, the water becomes cooler than the surrounding objects. The jars in a large spread among the southern peoples (among other things, and we in the Crimea) and have different names: in Spain "ALCATRAZZ"Egypt "Hola" etc.

The secret to the cooling action of these jugs is simple: the liquid seeps through the clay wall outside and there slowly evaporates, taking the heat ("latent heat of evaporation") from the vessel and the prisoner of fluid in it.

But it is not true that the liquid in such vessels is very cool, as you have read in books of travels in the southern countries. Cooling may not be great. It depends on many conditions. Than trainee the air, the sooner and more abundantly, evaporates the liquid, moisturizing vessel from the outside, and therefore, the more water is cooled inside the jug. Depends on cooling and on the humidity of the air: if it has a lot of moisture, evaporation is slow, and the water is cooled slightly; in dry air, by contrast, is a vigorous evaporation, causing more noticeable cooling. Wind speeds evaporation and promotes cooling; it's all well know the feeling of cold, which is necessary to test in wet dress in warm but windy day. The temperature drop in the cooling jugs does not exceed 5°. In hot southern day, when the thermometer shows sometimes 33°, the water in the cooling jug like a warm bath, 28°. Cooling, as you can see, almost useless. But the pitchers well retain cold water; for this purpose they are primarily used.

We can try to calculate the amount of cooling water in aliceazzo".

Suppose we have a jar containing 5 l of water; assume that 0.1 l has evaporated. For evaporation of 1 l of water (1 kg) is required when the temperature is hot (33°) of the day about 580 calories. We evaporated 0.1 kg, therefore, took only 58 calories. If all this heat was taken only from the water that is in the jug, the temperature dropped at least 58/5, i.e., C 12. But a large part of the heat required for evaporation is taken from the walls of the jug, and from the surrounding air; on the other hand, near the cooling water in the pitcher happens and heating it with warm air adjacent to the pitcher. Therefore, cooling is barely half of the figures obtained.

It is difficult to say where the jar is cooled more - in the sun or in the shade. The sun accelerates evaporation, but at the same time increases and the supply of heat. The best bet is probably to keep cooling the jars in the shade on a light breeze.

Entertaining physics J. Perelman

 




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