Physics Help Forum Drinking bird temperature
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 May 15th 2019, 06:07 AM #1 Junior Member   Join Date: May 2019 Posts: 5 Drinking bird temperature In a drinking bird, water from the wet on the outside of the head evaporates, causing the vapor inside the head of the bird to cool. This reduces the pressure of the vapor at the top, allowing the higher pressure in the lower bulb to push up the column of liquid. But the bird's body (including the rump and neck) is also a bit colder than the ambient temperature (but not as cold as the beak). Why is that? My idea is that when the liquid rises in the bird, the vapor pressure in the rump decreases. As a result, a part of the liquid evaporates. The necessary entropy is removed from the remaining liquid, which thus cools. Is that a conclusive argument? Are there any other reasons why the drinking bird is colder than the ambient temperature? On the other hand: Shouldn't there be a temperature compensation by heat absorption from the environment? Why is the bottom bulb colder than the environment at all? Another idea: The liquid might cool when it rises to the head. So when the liquid flows back into the bottom bulb, as the bird goes back into the verticale, it's colder than originally. I would be very grateful if somebody who understands this better than I do could explain it to me. Thanks in advance! PS: English is not my mother tongue, so please excuse my linguistic mistakes.
 May 15th 2019, 08:05 AM #2 Senior Member     Join Date: Jun 2016 Location: England Posts: 933 The drinking bird works by creating a temperature difference between the bulb and the head. If it is in direct sunlight, the birds rump might actually be hotter than ambient. but the key is that the head must be colder than the rump for the nodding bird to work. __________________ ~\o/~
May 15th 2019, 09:29 AM   #3
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Join Date: May 2019
Posts: 5
 Originally Posted by Woody The drinking bird works by creating a temperature difference between the bulb and the head. If it is in direct sunlight, the birds rump might actually be hotter than ambient. but the key is that the head must be colder than the rump for the nodding bird to work.
It's clear that the temperature difference is decisive for the bird to work.

I was only asking out of interest, because with a thermal camera you can see that the bulb is colder than anbient (ok, of course not in sunlight).
Is this simply because of convection or because the liquid in the bulb evaporates, when liquid rises, as the vapor pressure decreases?

May 15th 2019, 07:34 PM   #4
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Posts: 494
 Originally Posted by NPhantom In a drinking bird, water from the wet on the outside of the head evaporates, causing the vapor inside the head of the bird to cool. This reduces the pressure of the vapor at the top, allowing the higher pressure in the lower bulb to push up the column of liquid. But the bird's body (including the rump and neck) is also a bit colder than the ambient temperature (but not as cold as the beak). Why is that?
As you say the bird moves by the cooling effect of evaporation. The head is the source of the reduction in temperature, but this will inevitably be transferred to the lower regions , liquid from there moves up and down , reducing the temperature of the whole volume of fluid in the bird.

 Tags bird, drinking, drinking bird, entropy, temperature

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