My daughter asks why soup gets cold when you blow on it. I thought you blow the molecules apart and therefore more molecules can be released so that the structure changes. My daughter is 5 and how do I best tell her this? But above all: are those molecules true? I would be happy with any help.
Answer
Dear Anushka,
There are three ways to change the temperature: conduction (as with a hob for example), radiation (as with a radiator) and finally convection.
In the latter way, heat is exchanged via a liquid or gas. The principle is simple: a liquid or gas moves along a warm (or cold) surface and exchanges some heat so that the liquid or gas becomes somewhat warmer and the surface somewhat colder. It goes without saying that this heat exchange goes faster if there is a greater temperature difference between gas and surface and if the speed at which the gas moves is greater.
So if you blow on a plate of soup, you increase the speed at which the air moves over the soup, so that the heat exchange by convection increases and the soup cools down faster.
In other words: a thin layer of air is formed above the soup, which has been warmed by the warm soup. Under normal circumstances, this is a relatively thick layer that has an insulating effect, as it were. However, when you blow, you blow away this thin layer of warm air above the soup, so that the soup has to start heating up new air and thus more easily loses its heat to the air.
regards,
Lieven
Answered by
ir. Lieven Meert
Long term economic growth and employment.
http://www.ugent.be
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