[00:01] >> Hey, it's still going, isn't it? It's really just dribbling out, though. you'll know that I love siphons. I mean, look, water is climbing up this wall before flowing back down on the other side. It's really cool. But, here's my
[00:15] question. How tall would I have to build the wall before the siphon breaks? Look, it's still working at this height. It's still working at this height. Like, would it ever break, actually? Is there a maximum limit to the height of a
[00:31] siphon? Or, hypothetically, could I still siphon water from my neighbor's pool, regardless of how tall they built the wall? Also, why did they dye it blue? It's weird, isn't it? Now, if you've
[00:44] ever watched a video about the maximum height of a drinking straw, like Derek's from 13 years ago, you might have an idea of what the answer is. In which case, maybe pause the video and leave your guess in the comments, because I
[00:57] I did this. >> This is a scissor lift. This is a pool. Imagine this was my neighbor's pool, and I wanted to steal the water from there into my pool. >> We've got a tube in here that goes all
[01:11] there, all the way through these tubes, and then comes out into the pool. We'll keep raising this thing until the siphon breaks, if it ever breaks. There you go. It's working. So, what we
[01:25] running, couldn't we? And then just raise the scissor lift whilst it's raise the scissor lift whilst it's pouring.
[01:37] 7.2 m 8.7 m 9.8 m m >> Hey, it's still going, isn't it? It's
[01:51] >> Hey, it's still going, isn't it? It's really just dribbling out, though. >> 10 m, that's 10 m exactly. It's still going.
[02:05] that's still going. I can't lower it anymore cuz that's at the lowest thing. So, I'm just going to start scooping it out.
[02:20] 10.07 is where it stopped. 10.07 m, that's when it broke. But, why did it break at all? Well, clearly some sort of gas got into the top here. It didn't sneak in through either of the openings. So,
[02:36] what's happening? Well, actually bubbles started to appear at 9 m. So, I think what's happening is the water had some gases dissolved in it. And then as the
[02:48] pressure went down at the top, those gases fell out of solution and formed bubbles. But, those bubbles don't break the siphon because the water is traveling through the tube fast enough that it just flushes those bubbles out.
[03:03] an equal amount of gas coming out of solution on both sides. So, it doesn't change the tug-of-war equation. So, what changes at 10.07 m? Well, it's because a
[03:16] new gas appears. This time it's water vapor, water in the gas phase. And the reason it seems to have appeared from nowhere is because it's actually boiled off from the surface of the water up here. That's mad, isn't it? When you
[03:30] think about boiling water, you think about heating it in a kettle or on a stove. But, it turns out you don't actually need to heat water to make it boil. That's because water boils in a vacuum. In other words, the weight of
[03:42] the water in these two columns is just enough to counteract the atmospheric pressure pushing it back up. And so, you have zero pressure at the top. You have a vacuum. You can see why having gas at the top breaks the siphon, by the way.
[03:56] See how the level of water here is higher than the level of water here, and higher than the level of water here, and as a result, this column of water is as tall as this column of water, and so the tug-of-war is a draw. You can see the
[04:10] fountain breaks. The difference in height between here and here is the same two pools at the bottom. This is the same thing that limits the height of a drinking straw, as Derek demonstrated over a decade ago on Veritasium. Like,
[04:27] if you could suck a perfect vacuum, what you're really doing is unleashing the full potential of atmospheric pressure to push that water upwards. And when you do the calculations, it turns out that atmospheric pressure can push water to a
[04:40] height of 10.33 m. So, why did I only get 10.07 m? Well, to check in with someone first. >> What's up? because you're topless. >> What?
[04:55] >> You didn't want that as an opener? >> 13 years ago you made a video about sucking water up a straw. The maximum height you came up with, 10.33, and everywhere. That's the height of water
[05:08] that exactly cancels out atmospheric pressure. I get less than that, and I by you because you made this straw to call me about this, um, I have not thought about it at all. I have no idea.
[05:21] Now that I'm talking through it, is it the case that because the water is at like room temperature, 20°, has some extra, uh, kinetic energy, so as you start to pump it up, now that water is going to
[05:35] turn into gas even when the pressure's not perfectly zero. So, did I nail it? see you get there in about 20 seconds. You'll You'll mostly You'll mostly do that. The issue with the 10.33 m calculation is that it doesn't take into
[05:48] account the fact that water is boiling at the top here, creating water vapor that has a pressure of its own. So, you can't have a perfect vacuum up here. And can't have a perfect vacuum up here. And actually, I think the new calculation is
[06:03] quite easy because we know that water has a vapor pressure at different temperatures. So, for example, at 20° C, which is what it is here, which is what it is here, that's about 2.3%
[06:18] calculate the maximum height of a siphon don't use atmospheric pressure, use atmospheric pressure minus the vapor pressure of water. When you do that, you get an answer of about 10.07 m. So, I
[06:35] there. >> Actually, I think it'd be reasonable to say that this is the world's tallest siphon because if you try to make it any taller, it would break. >> I've made the world's tallest siphon.
[06:47] level. >> Oh, cuz of the pressure. Oh, before I fine, I guess I'll go to Brighton then. There you go. The tallest possible siphon at sea level. And by the way,
[06:59] we're just not measuring with that kind know if someone made a siphon taller than that. >> Someone made a siphon taller than that. >> What?
[07:11] >> What? 15 m? How? They got it to 15 m. >> And the way they did that is actually related to another video that you made. It's related to trees. So, the answer to that one should be if you could
[07:25] perfectly fill a tube so that there's no air bubbles, no nucleation sites, no get lower than vacuum pressure at the top. >> Okay, so the explanation for how siphons work is a little more nuanced, it turns
[07:39] out. So, strap in because this is the full explanation of exactly how siphons work with nothing missing. First, forget about atmospheric pressure entirely. We'll bring it back later, but it's not foundational to how siphons work.
[07:54] Instead, assume one thing. Water is really hard to pull apart. We'll figure out whether that's a reasonable assumption in a minute, but for now, you can just think of water in the tube like a chain. Something that's really strong
[08:08] holds together. It's really hard to snap, in other words. So, let's siphon this chain. This has nothing to do with atmospheric pressure, of course. This chain siphon works because this length of chain is longer and therefore heavier
[08:22] than this length of chain. And so, the tug of war that's happening here is won by this part of the chain, and so it flows in this direction. I mean, who chain, but that's a different story. The point is, siphons are driven by gravity.
[08:37] This column of water is heavier than this column of water, so it's able to pull this water over the hump. Now, if you pull too hard on a chain, it snaps, but is that also true of water? If you pull on water too hard, does it snap?
[08:51] Like, is that what's happening here? Is the water snapping at the top? Well, no, I don't think that's the right way to think about it. Like, the water sitting in this glass is only being held together because it's being squeezed
[09:04] together from every direction by atmospheric pressure. And if we weren't squeezing it from every direction, in other words, if we removed atmospheric pressure, well, it would just fly apart on its own. It would boil. Snapping is
[09:17] when you stretch something until it breaks, not when you reduce the squeezing on something until it breaks itself. So, this is where it makes sense to reintroduce atmospheric pressure to the explanation of how a siphon works.
[09:31] So, a siphon is driven by gravity, but you need atmospheric pressure squeezing the water from all sides to stop it just flying apart. And that's what puts a limit on the height of our siphon. The weight of all the water in these two
[09:46] tall columns pushing downwards is canceling out the atmospheric pressure at the bottom pushing back up. So, the water at the top isn't being squished, it isn't being held together by atmospheric pressure, and so it does fly
[10:00] apart. It boils. But earlier I said that atmospheric pressure wasn't fundamental to the explanation of a siphon. To see why, consider this footage from the Periodic Videos channel. It's a great video, by the way, link in the
[10:13] description to watch the whole thing. It shows a siphon working in a vacuum. Why isn't the water boiling in there? It should be. To find out what's going on, let's boil some water in a microwave. One glass is brand new and contains pure
[10:27] distilled water. The other is an older glass. I've also put some grit at the bottom and it contains tap water. When that glass starts boiling, we know we've reached 100°. But despite reaching 100°, the pure water in the clean glass never
[10:42] boils until I add a bit of grit. And look, a big chunk of that water instantly and explosively boils. That's why it can be dangerous to boil water in a microwave. The reason it's possible to superheat water beyond its boiling point
[10:57] power of surface tension. Surface tension tries to minimize the surface of water. That's what makes bubbles in water spherical. A sphere is the smallest surface area for a given volume. But it also means that those
[11:11] tiny bubbles are really being squeezed by a surface tension, really crushed. So, to create a bubble in the first place, you have to overcome the power of don't worry about that sort of thing when you're heating water in a pan or in
[11:26] a kettle because tiny imperfections or scratches in in surface act like nucleating sites that bubbles can form around that counteracts the power of surface tension. But, it is possible, even by accident, to create an
[11:40] environment with no impurities, no imperfections, where it's possible to get water above its boiling point without it actually boiling. What it means is, in principle, I could create a vacuum at the top of the siphon, so zero
[11:55] pressure, meaning the water definitely wants to boil, but it just doesn't because it's too hard to create those tiny bubbles. >> That's correct. >> But, we you know, I was I was not being
[12:08] >> And so, the weird thing is, it's like you think about, you know, we're all being squished all the time, including water, you know, and then as squished less and less. Then you get to vacuum, not being squished at all. Then
[12:21] you keep going, and you're not squishing it anymore, you're you're pulling it. So, it goes from being under compression to being under tension. And the idea that water has like a tensile
[12:33] strength, that like it can't be pulled apart, is kind of crazy. was making this video. Liquids and gases, you don't think about those as like, well, being under tension. Like, that's so weird. Oh, this call is great.
[12:47] me. You know. >> So, in conclusion, siphons are driven by gravity, but for it to work, the thing being siphoned needs to not fall apart. So, if it's water you're siphoning as opposed to a chain, then you also need
[13:03] atmospheric pressure to hold the water together and stop it falling apart. That puts a limit on the maximum height of a siphon, unless you can keep everything really clean, in which case it's actually the tensile strength of water
[13:19] that holds it together, and you can go much higher. So, if you did want to steal your neighbor's pool water, and your pool was lower than theirs, and you wanted to use a siphon, the limit is about 10.07
[13:32] m, unless their water happens to be incredibly pure with no dissolved gases, and you can find a really clean tube, in which case you can go much higher than >> So, you got direct, and you didn't care.
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