3.2K views Jun 2009
A physicist explains how a drinking bird uses evaporative cooling, vapor pressure, and shifting balance to repeatedly dip its felt head into a glass of water.
0:00 Tell me how this thing works. 0:05 Right, um, sorry, Brady. You're looking at, um, a wonderful little duck, um, variously called a wonderful little bird, variously called the drinking bird, the drinking duck, the dunking duck. Um, used this, uh, a lot in my f- I used to do a a first year module on thermal and kinetic physics, and I used to bring this in at the start, and um, I'd ask how many of you've seen one of these birds before, and maybe a hundred odd hands would go up. I'd then say, um, apart from on The Simpsons, and all the hands would go down. So they're a pretty rare breed. They they tur- they crop up all the time. Anybody who's seen Alien will have seen one of these towards the start of Alien as well. Um, uh, Homer used it, uh, to um control a nuclear plant, disturbingly. Um, but it is just a fantastic demonstration of so much physics. First thing, you look at it, and uh, it appears like a, a perpetual motion machine. It just keeps going. And the first question you ask, well, hang on, um, first law of thermodynamics says that we've got to conserve energy. Where is the energy source? Where, where, how is it being powered? And there aren't any batteries, and there aren't any electrical connections. We're not cheating. This water, I should also point out, um, yeah, is at room temperature. And that int- that raises another important question, because the second law of thermodynamics basically says that you can't have a, a an engine just working off a single temperature, loosely speaking. And so the question is, you've got generally with any engine, you have a hot reservoir and a cold reservoir, and you in- you re- reject some heat into the cold reservoir. And um, this is a wonderful example of a sophisticated, um, heat engine. What, uh, is also extremely interesting, and I only found out about this recently, is that Einstein, um, couldn't work out how it worked. Um, so he was, he was at a dinner, um, uh, party, and one of these little creatures was, was there, and Einstein spent a lot of time trying to figure out how, how it worked and apparently couldn't. Now, I thought, I thought that was an urban myth, but actually, um, the son of the person who invented this, which was a guy called Sullivan, and he d- he invented it back in the 40s, um, has has said that that's absolutely correct. Einstein couldn't work out how it how it worked. I'm not surprised if somebody was to give this to to many physicists, including particularly myself, and say, well, how does it work? You go. So, how does it work? 2:30 Well, if Einstein doesn't know, surely Phil Moriarty doesn't know. 2:33 Oh, well, it's interesting. Phil Moriarty has access to Google and a huge number of resources that Einstein definitely didn't have access to. The other thing, as I said, Brady, is that, um, I had to carry out some emergency surgery on it this morning and actually fix the pivot, but, um, I'm glad to see that it's back to to working order. So we've got the the head, which consists of felt, and the important thing that you need to do is to get that head wet. So what I did before Brady came in, before you came in, Brady, is to put the head in in the water, make sure it's nice and wet. And then we have this, basically we have a process which is equivalent to the duck sweating. Uh, we've got evaporative cooling. That means we get a temperature difference, and that's key. Now we've got a temperature difference, we can start to see how this thing starts to act like a heat engine. Um, cools it down, and the important thing is that this blue stuff, it's colored blue, inside is not water. It's actually something called dichloromethane, a relatively na- well, quite nasty solvent. The important thing with dichloromethane is that at room temperature, it's got a very high vapor pressure, which means that there's lots of gas molecules streaming off from it. And so what happens is when you cool here, the temperature in the head goes down, and uh, the, uh, the dichloromethane vapor condenses in the head, okay, which means that you get a pressure difference between the head and the bottom. That pressure difference, because there's now a s- a smaller pressure here, you actually, um, drive, uh, the solvent, the dichloromethane, up through this inner tube. Sorry, I didn't mean to hit it. And, um, that changes because the the solvent is moving up, the dichloromethane is moving up the tube, that changes the center of gravity. The center of gravity moves, and it means that the head pops into the, um, water. And then, what's important, this is, this is crucial as it comes around, the bottom end of the tube comes out, momentarily out of the liquid, and that allows a pressure equalization, and you start the cycle all over again. Remarkable, wonderful, wonderful piece of, of physics. 
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