My son just asked me at dinner and I do not have an exact answer.
In: 7
To get the water in the straw, the suction force you apply had to overcome the gravitational force pulling the water down (the obvious bit). When the suction stops (the one end of the straw is opened), gravity draws the water down again. The downward movement of the water in the straw creates a slight low pressure as the water flows down which sucks in air in the wake of the dropping water (the air replaces the water in the tube, almost like what happens when you drain a big bottle of liquid). However, when you block the top of the straw, you’re removing the flow of air from the equation. Now, the gravitational force is still pulling down on the water in the straw, but since the low pressure that forms as the water is pulled down cant be equalised by the air through the top, a vacuum forms between the top of the water and the blocked straw opening, which is equal to the gravitational force downwards (which is why the water in the straw stays at a constant level after adjusting).
It’s because the straw is narrow enough that water’s inherent “thickness” (I assume a child doesn’t know what viscosity means) stops it from just sliding out if it can’t be replaced by air. If you block one end of the straw, an air bubble cannot travel up through the water, and cannot enter through the top. So it just sits there.
But it’s important that it’s a straw. If you start increasing the diamater of the tube the water is in (to, say, bottle neck size), at some point the water isn’t “thick” (viscous) enough to overcome gravity anymore, and will buckle on one side, allowing a bubble of air to rise and replace it on the other. That’s why if you tip a bottle upside down, it doesn’t just all flow out at once either, but gurgles. It still has enough viscosity to kind of stick together. But it will flow of you spin it, because then the air enters through the centre of the whirlpool.
How does your son drink the water through the straw?
He SUCKS on the straw, creating a vacuum (suction), and the water comes up because it’s being suctioned in. So if he puts his finger there, the finger won’t let the air in, maintaining the suction.
What is suction? Well, all of the air that’s above our heads is also pulled down by Earth’s gravity like everything else. So it creates pressure, like the pressure in a balloon. It’s about 14 pounds of pressure per each square inch of surface. It’s about half of what car tires have in them (36 PSI).
We live inside a balloon of air. We don’t feel it because we’re used to it, but if you go up in space you have to have a space suit to keep yourself “inside a pressurized balloon” like you’re used to on Earth.
Anyway, suction is less pressure than this “normal” air pressure.
So basically, the air actually pushes the water INTO the straw, if you create suction (less pressure) at the other end.
Air pressure. All air likes to be at the same pressure. It takes a lot of energy to force a pocket of air to be at a different pressure than the other air around it. Think about how hard it would be to blow up a basketball with your breath and you’ll understand.
When you pick up a straw with one end covered, you have a straw that is filled with water at the bottom and air at the top. When water flows out the bottom of the straw, it leaves empty space behind, and that side has to be filled with something. Normally, that side would be filled by air that would get sucked in the top of the straw, but it can’t because your finger is covering it. The only other way the water could flow out is if the air in the straw agreed to lower its pressure so that the same amount of air filled a larger space. But, there’s no way in hell you’re gonna find a pocket of air that would agree to do that. So, the water just stays in the straw until it finds something that can take its place if it were to flow out.
Three things. Capillary forces, vacuum, and surface tension.
Capillary forces are the interaction between the water and the straw. The water is “holding onto” the sides of the straw.
Vacuum is created by gravity pulling down on the liquid in the straw when it is sealed at the other end with your finger.
Surface tension is the water holding onto itself in the bottom of the straw. It is the same thing that makes a drop of water sit in a half sphere when spilled on the counter.
If any of these things break down the water will not stay in the straw.
To answer the question of why the water doesn’t fall out (which is what I think you are actually asking): Just like water is a kind of stuff, air is stuff too. It’s actually pretty difficult to get rid of all the stuff in an area, so if you don’t put your finger over the straw when you lift it out of the water, the water will run out of it, but air will rush into the other end to fill the space it left behind. If you use your finger to stop the air rushing in, the water can’t leave because it would leave nothing behind, which takes a lot of work, so instead the water stays in the straw until you lift your finger and let the air in.
To answer why it doesn’t work if you have something larger than a straw (which is related but not what I think you are asking): So if air has to replace the space the water takes, it has to either enter the straw from the end your finger is on, or it has to go up through the water. If it goes up through the water your finger won’t stop it from falling out. Water likes to stick together though, so it’s hard for the air to make it through. But if the opening is narrow enough, the water can’t hold together as well, and some water can fall out and air can make its way in.
Air pressure is pushing it back up.
Air pressure is pushing on us from all sides, and it’s going to be pretty much the same at the top of the straw and the bottom since the straw us so short, so can ignore the difference. The air bubble at the top of the straw that’s trapped under your finger will be at that same pressure. If the water tries to run out, it will mean that same air will have to fill the new volume above the straw, reducing that pressure. The pressure at the surface of the water is going to be the same pressure at the bottom of air bubble, which means the same expansion would need to happen in the water. You may have heard that water is “incompressible” meaning that as water is put under pressure, it hardly changes in volume. The same applies in reverse, reducing pressure means very little expansion. If that expansion can’t happen, then that means that the system must already be in equilibrium, and therefore nothing should move. By removing the finger from the end of the straw, that air bubble no longer is under the reduced pressure, so the air pressure from above is pushing as hard as the air pressure from below, so gravity takes over and the water falls.
Here is a (https://youtu.be/jzZR0_NSUOg) of the same phenomenon happening in a jar.
The air bubble isn’t necessary for this to work, in fact it’s a stronger seal without that expansion of an air bubble because it’s only the water trying to expand, which it can’t do. I just used it to help explain, and it’s more similar to whats happening in your straw.
Surface tension is also the only force preventing air from bubbling up the water to equalize the pressure and allowing the water to pour out. In the jar video, you can see when the students poke the surface of the water, they momentarily break the surface tension, causing an air bubble to form.
I have a very fond memory of asking my dad the very same question about straws as a kid, and I think you have yourself a future scientist on your hands, and I hope you nurture this curiosity.
If the water should sink, something had to take its place. But since your’re holding your finger over one end, and the other end is sealed in water, no air can enter the straw. So the water cannot sink down to its normal level.
For the water to come out, air must come in its place. Your finger blocks the air path, air cannot come in so water cannot come out.
For water and most liquids to flow out of a container (a straw counts as a container in this context) air has to flow in to equalize the pressure.
Latest Answers