...straw containing liquid, even if only one side of the straw is blocked and the other side is open, the liquid doesn't come out of the open end? for example when you're drinking with a straw it's filled(with water for eg) and you block only one open end of the straw even if the other end is open the liquid doesn't fall/come out.
What is the reson for this?
2007-05-16 14:50:15 · 8 answers · asked by ♫ Melody♫ 3 in Science & Mathematics ➔ Physics
That's simple enough. This is due to two things. I won't get technical on you. The first is vaccum force for lack of a better term. The second is the attraction of water to itself and surroundings.
Ok Vaccum Force - This though really is just pressure inbalance in reality. But anyways if you believe that the water should just flow out due to gravity you would be right IF something else flowed in to take the place of the water molecules that vacated. If not then the vaccum that would be left would tend to want to "suck things in"
So then the obvious question is why won't the air just take it's place. It is lighter than water so water should fall out. Well that is exactly what happens if you take tall thin glass for example, filled it with water, covered it up with your hand, turned it over, then uncovered it. One side is blocked but the other is not and water does go out. However notice that the water kinda does a "glunck" or a slosh sound" Okay now imagine the diameter of the tall glass is getting smaller and smaller while repeating the same process above. Soon this will give you the effect you saw with the straw.
The other thing is the attraction of water molecules to themselves and their surrounding. There comes a point when the force from gravity is not enough to overcome the force that binds water molecules to their surroundings. Air simply can't find a hole to come up the straw to replace that water that would leave, if air can't replace it the vaccum force won't let it go.
That "slosh" sound I wrote of earlier is the sound of air making it's way through water that wants to stick together.
I can be more technical if you want.
Cheers
2007-05-16 15:18:28 · answer #1 · answered by LCD 1 · 2⤊ 0⤋
I'm assuming you have a genuine interest in the answer so I'll try a basic explanation.
Pressure is applied force most easily recognized in the weight of an object. Gravity is exerting a downward force, applying pressure on any object that tries to resit the force.
We have atmospheric pressure due to the weight of air from our reference point to the boundary of the atmosphere. This can actually be measured at about 14.7 pounds per square inch (psi). Meaning a one inch square tube extending to the atmosphere would contain air weighing 14.7 pounds.
Pressure can be positive, + or negative, -. When you use a vacuum cleaner it demonstrates negative pressure. If you put this against a measuring device it would read the negative pressure as - psi. You can get a rough idea of the power but creating the vacuum on objects of different weight.
Assuming the vacuum created was - 1 psi; if the tube area is one square inch, you can pick up a one pound object. If the tube is 1.5 square inches, you can pick up a 1.5 pound object.
Back to your straw question. When you put your finger on the straw you seal the top. Air or pressure can only enter through the bottom. If you seal the empty straw and push it into a glass of water you will notice the water level in the straw is lower than in the glass.
That is because the water entering the straw has to compress, pressurize, the trapped air. That's why I said you push the straw into the glass. When they are balanced the psi of the air is the same as the psi of the water at the bottom of the straw. If you release your finger you will see the water jump up to the same level as the psi equalizes.
If you plug the top again and lift the straw, air cannot enter to equalize the trapped pressure. Some water will initially drip out until the - psi, (negative in relation to 14.7 atmosphere) of the air equals the + psi (equal to the force of gravity on the given water in the straw). When these two forces are equal the water will not leave the straw.
Surface tension would have a minuscule affect in aiding the - psi as it is trying to hold the water molecules together.
I hope this helps.
2007-05-16 15:38:33 · answer #2 · answered by Caretaker 7 · 2⤊ 0⤋
The previous users are incorrect. No vacuum is created by placing your finger over one end of a straw.
The inside the straw is at the same pressure as the air at the open end of the straw (initially).
The small amount of distance that the liquid moves down the straw causes the volume that the air occupies in the straw to slightly increase, which because of the ideal gas law, causes a decrease in pressure inside the straw.
Because of this pressure differential, the air outside the straw is now effectively pushing the liquid back up the straw, and not allowing it to drip out.
2007-05-16 15:07:45 · answer #3 · answered by pickapop85 2 · 1⤊ 0⤋
When you cover the top side, you don't allow air to enter. If you don't allow air to enter, then nothing can use the space that would be left if the liquid goes, just vacuum.
But the atmosphere has pressure, and it pushes the liquid up to fill that vacuum.
Since the atmospheric pressure is not enough to pull all the liquid up, there is a bit of vacuum in the top (you can check with a transparent straw!)
So it's two things working at the some time: there is no air to fill the vacuum and the atmospheric pressure is pushing the liquid so it fills the vacuum.
2007-05-16 14:58:23 · answer #4 · answered by marcos_r_c 2 · 1⤊ 0⤋
The above answers are OK as far as they go. But surface tension is also key to this phenomenon. Surface tension tries to minimize the free surface area of the liquid. (The free surface is the surface not touching the container.) This means trying to keep the bottom surface of the water as close to perpendicular to the long axis of the straw as possible. This tendency is opposed by the weight of the liquid trying to pour out, as it does from a wider closed-end container such as a glass, which would greatly extend the free surface. With a wide enough straw the trick won't work because the weight of the additional water overcomes the surface tension.
Edit: The answerer who described surface tension's effect as "minuscule" doesn't get it. Of course the air pressure difference above and below the water column is preventing the water from shooting out the bottom in uniform tubular flow, just as it does with an inverted bottle, but try it with a wide enough straw, slightly tilted, and that pressure difference can't stop the water finding another way to escape, spilling out as its unbalanced weight breaks the meniscus at the bottom, just the same as with the bottle. In a narrow enough straw, surface tension dominates over any weight imbalance and this doesn't happen.
2007-05-16 15:07:37 · answer #5 · answered by kirchwey 7 · 1⤊ 0⤋
It is due to negative pressure above the water column in the straw, combined with capillary action. If you take a straw and simply place it in a cup of water you will upon close examination see the water inside the straw slightly higher than the water in the cup. The smaller the diameter of the straw the higher the water will go up in the straw. (this is how plants take up water). This combined with negative pressure above will keep the water in the straw. If you have the patience and hold the water in the straw it will leave the straw by evaporation.
2007-05-22 18:29:30 · answer #6 · answered by brandon l 2 · 0⤊ 0⤋
trapping air in the straw causes pressure in the straw to be lower than the outside pressure, and so the pressure outside the straw keeps the liquid in.
2007-05-16 14:55:52 · answer #7 · answered by jimbothe_smartguy 2 · 1⤊ 0⤋
you create a vaccum under your hand, and finger and then the pressure keeps it in
2007-05-16 14:58:05 · answer #8 · answered by Anonymous · 1⤊ 0⤋