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Thread: Finger Over Straw Trick

  1. #1 Finger Over Straw Trick 
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    When you cover the top of a straw that is in a glass of water and pull it out of the glass, the water remains in the straw as if it were still in the glass of water. How exactly does this work? I would assume that it has something to do with the pressure inside of the straw pushing down on the water being less than the pressure outside of the straw pushing up on the water, but I'm not quite sure.
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    Yes it's do do with atmospheric pressure. You wouldn't be able to do on in the moon. This actually is how barometers measure the atmospheric pressure.
    Barometer - Wikipedia, the free encyclopedia
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  3. #3  
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    Capillary action
    torricellian vacuum and barometer
    siphon in a vacuum
    wetting contact angle and wikipedia contact angle

    I added some contact angle info. I worked in soldering and coating of materials and this stuff is need to know in certain tasks.
    Last edited by pikpobedy; 09-26-2014 at 05:31 PM.
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  4. #4  
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    Pickpobedy, do you think capillary action is what holds the water up when you lift the straw up?
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  5. #5  
    Senior Member Boing3000's Avatar
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    Quote Originally Posted by jilan
    and pull it out of the glass
    It's only because the water tension surface of the bottom part of the straw is bigger then the pressure of the water column inside the straw.
    A longer straw or a larger straw would empty itself.

    A straw out of a glass is no barometer
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    Ah completely out of the glass. I still think it's atmospheric pressure.
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    Atmospheric pressure is what keep the water liquid at atmospheric temperature. It is required for the water to stay in that glass anyway.

    The atmosphere pressure is part of the experiment, but the width and height of the straw are the most important factor (and even material of the straw (hydrophobic or hydrophile), and the type of liquid)

    For a straw small with very small width you don't even have to cover the top of the straw, it will just stay there or climb "all by itself".

    For a straw big enough, the water will get out anyway.

    For normal size straw, what keep the liquid to get out is the surface tension of the liquid (which depend of the pressure on each side of the surface) that binds molecule more strongly then the pull of gravity.

    Pipobedy's links are well worth looking at to understand that situation.
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  8. #8  
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    These children's games questions of which "the straw" is but one of many are often inaccurately explained.

    The upside down glass is another one.
    "What happens when we burn a candle in an upside down glass dipped in bowl of water? How (why) does this happen?".
    Typical answers involve the consumption of oxygen but this is inaccurate.
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    Quote Originally Posted by pikpobedy
    "What happens when we burn a candle in an upside down glass dipped in bowl of water? How (why) does this happen?".
    Typical answers involve the consumption of oxygen but this is inaccurate.
    You mean when the candle dies down ?

    I always thought it was because the rarefaction of O2 slowly replace be CO2, making the chain-reaction that is burning less and less likely to happens...
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  10. #10  
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    burning of carbon O2 +C gives CO2 plus heat, one to one ratio of gas volume. NO GAIN, NO LOSS in volume
    burning of hydrogen O2 + 2H2 solid gives 2 H2O vapor plus heat, Two to one INCREASE in volume.

    The general explanation that is confirmed by observation involves heat, expansion, condensation and gas volumes. The air gets hotter, expands, increases in pressure, plus the solid hydrogen forms water vapor doubling the volume of some O2 and forces a few bubbles out of the glass. The air subsequently cools and contracts at the same time the water vapour condenses.
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  11. #11  
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    That's clear that heat increase in the glass, which in my opinion will only favor the combustion.
    Water vapor will maybe temper the combustion, although I have made enough fire under the mist to know if the wood is dry, it'll burn all the same.

    Really, what stop the combustion is just that oxygen gets less and less available.
    Other factors also count, but secondary gazes are just neutral by-stander.
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    I do not write an explanation to a problem that I gave as an example if I do not know the explanation. I simply gave significant details that are lacking in the overly simple explanation. This is a physics site. Elaboration for the sake of being exact without losing clarity... that's a good thing. Would you not agree?
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    Indeed, details are important. Especially that those examples are often used in school.
    When I did that experiment, I think we where supposed to note the presence of vapor.

    Your description is more complete, but the reason the combustion stop is oxygen consumption. You said is was inaccurate, I would have said incomplete. But let's not split hairs
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  14. #14  
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    I should have said "incomplete". I laboured over the choice of "inaccurate" versus "wrong". Wrong was definitely inappropriate. Incomplete slipped my mind.


    I agree. At a certain oxygen level combustion stops. A question of rate of heat output, rate of oxygen transfer inwards and the cooling effect of the inert gas (nitrogen in air).
    Interestingly, a match does not stay lit in zero gravity.
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  15. #15  
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    Quote Originally Posted by Scheuerf View Post
    When you cover the top of a straw that is in a glass of water and pull it out of the glass, the water remains in the straw as if it were still in the glass of water. How exactly does this work? I would assume that it has something to do with the pressure inside of the straw pushing down on the water being less than the pressure outside of the straw pushing up on the water, but I'm not quite sure.
    I guess the same. There is no air pressure at the top, but there is at the bottom.
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    It is atmospheric pressure. Gravity causes the water to want to flow down the straw but, to do so, would leave a vacuum at the top of the straw. That would mean zero pressure at the top of the column of water (neglecting the vapor pressure of water so the purists on the site won't jump my bones). The bottom of the column will have atmospheric pressure acting upon it and will support it as long as the straw is less than about 34 feet. If the straw is longer than 34 feet you have a barometer.

    To make it work at 34 feet you probably need the bottom end to be in a pool of water. For short straws surface tension probably is needed to keep the water from flowing out. So, for whoever mentioned surface tension, it does play a role in the straw experiment.
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    It might be useful to imagine what happens when you remove your finger. With a drinking straw the liquid will run out, but with a coffee stirrer it might stay in.
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  18. #18  
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    Quote Originally Posted by Jilan View Post
    It might be useful to imagine what happens when you remove your finger. With a drinking straw the liquid will run out, but with a coffee stirrer it might stay in.
    Try it and see if it works. I don't have any drinking straws and I drink coffee black, so let us know how the experiment works out. Observe the experiment carefully since the volume of water in one is much more than in the other, but in each will be some water droplets clinging to the side walls I am sure.
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    I don't need to try it, years of experience!
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    Sufficiently smaller diameter than a straw water is drawn in by capillary action (aka wicking). The water creeps in and stays in.

    A straw diameter can filled, blocked on top with a thumb and the miniscus surface tension on the bottom is strong enough to handle the the air pressure upwards and the load of water downwards without disrupting.

    Sufficiently larger than a straw the bottom miniscus surface tension cannot handle the stress. The water column collapses.

    So putting a thumb on a 50 mm pipe will not hold water in it.
    The general children's trick is to fill a glass with to the brim with water, cover it with cardboard, carefully hold them together, turn it upside down and the water and cardboard will all stay in place. We have replaced a delicate surface tension miniscus with robust cardboard.
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    Quote Originally Posted by cincirob
    It is atmospheric pressure. Gravity causes the water to want to flow down the straw but, to do so, would leave a vacuum at the top of the straw.
    Well, atmospheric pressure do not prevent water to flow down. It would be considering the water as solid.
    This experiment is not about a solid piston in a cylinder. In such case, yes, pressure differences will hold the piston in place. In vacuum the piston will go down anyway.

    Water is a liquid and should flow. It does not because of the surface tension, which somewhat transform the water in solid piston.
    The root cause is surface tension. In vacuum a special liquid (stable in vacuum) with surface tension will also not go down (again under certain boundary condition)
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  22. #22  
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    Boeing3000: Well, atmospheric pressure do not prevent water to flow down. It would be considering the water as solid.

    cinci: Why does the water flow down when you remove your finger? I noted that surface tension plays a role, but it obviously is not sufficient in a soda straw to prevent the water from flowing.
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  23. #23  
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    Interestingly in a mercury barometer, the partial vacuum created in the head space is refered to as torricelli's vacuum. And of course the bottom of the tube is immeresed in mercury otherwise the mercury would fall out and a sufficient volume is required to allow bak and forth (in and out) mercury exchanges between the tube and the bottom reservoir.

    One of the reasons mercury is used is its exceptionally high specific gravity of 13.5 requires a shorter column than water for barometry.

    One can get a feeling for this using the 34 feet of water that cincirob stated in a prior post. Scuba divers know that 34 feet of fresh water represents about one atmosphere of pressure.
    Using imperial units: water density is 62.4 pounds per cubic foot water = 62.4 pounds per foot of water per 144 square inches = 0.433 pounds per foot of water per square inch.
    One atmosphere is 14.7 psi: 14.7 psi / 0.433 pounds per foot per square inch = 34 feet of water
    This depth or height of water is often cited as the limit of certain pumps and siphons. I tried looking up stuff on this a few days ago but I lost interest. I was also concerned that some of the explanations may invoke incomplete or even erroneous explanations. Water is not a perfect liquid. It has hydrogen bonding, cohesiveness, viscosity and wetting ability.

    34 feet of water can be said to be equivalent to 2.52 feet of mercury = 767 mm Hg

    But let's not split straws. er hairs. or break the camel's back. snort.
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  24. #24  
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    Quote Originally Posted by cincirob View Post
    Boeing3000: Well, atmospheric pressure do not prevent water to flow down. It would be considering the water as solid.

    cinci: Why does the water flow down when you remove your finger? I noted that surface tension plays a role, but it obviously is not sufficient in a soda straw to prevent the water from flowing.
    The surface tension is the same thing that holds the water together and makes it a liquid rather than a gas. The interaction between the liquid and the straw is what supports it under gravity (without the vacuum). This is like the capillary action that plants use.
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  25. #25  
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    Quote Originally Posted by cincirob
    I noted that surface tension plays a role, but it obviously is not sufficient in a soda straw to prevent the water from flowing.
    It may or may not. Nor you finger still on the top will avoid the liquid to flow down. Just set the straw to 45% or more and the water will flow anyway, because it is a liquid, and liquids flow. That is the "bizarre" thing in the O.P. question.

    And of course atmospheric pressure can help pushing water out of the straw, and break the surface tension.

    We can also note that no vacuum exist under the finger (for very long straw). Water will evaporate and equalize the pressure of the atmosphere minus the surface tension resistance.



    P.S. You can use standard quoting in the editor in non-WYSIWYG mode (top left icon) and by using those tags
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  26. #26  
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    cinci: Why does the water flow down when you remove your finger? I noted that surface tension plays a role, but it obviously is not sufficient in a soda straw to prevent the water from flowing.

    Jilan: The surface tension is the same thing that holds the water together and makes it a liquid rather than a gas. The interaction between the liquid and the straw is what supports it under gravity (without the vacuum). This is like the capillary action that plants use.

    cinci: The size of the soda straw determines the load that the surface tension at the bottom of the straw must support. The circumference increases linearly with diameter, the area by the square. So for a very small straw, surface tension is sufficient to support the water. As the diameter increase, the surface tension will fail. The finger on the top of the straw and the associated vacuum reduces the load on the surface tension so that larger straw will work for the trick. If you have a 10 foot in diameter straw, the surface tension will fail and the finger on top won't hold it either.
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  27. #27  
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    cincirob: I noted that surface tension plays a role, but it obviously is not sufficient in a soda straw to prevent the water from flowing.

    Boeing3000: It may or may not. Nor you finger still on the top will avoid the liquid to flow down. Just set the straw to 45% or more and the water will flow anyway, because it is a liquid, and liquids flow. That is the "bizarre" thing in the O.P. question.

    cinci: The surface tension is needed (see note above to Jilan). When you angle the straw, the surface tension is required to operate over a larger area and a less symmetrical one. So it fails.

    An interesting experiment would be to add detergent to the water which reduces surface tension and see if the trick still works.
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  28. #28  
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    Quote Originally Posted by cincirob View Post
    cinci: The size of the soda straw determines the load that the surface tension at the bottom of the straw must support. The circumference increases linearly with diameter, the area by the square. So for a very small straw, surface tension is sufficient to support the water. As the diameter increase, the surface tension will fail. The finger on the top of the straw and the associated vacuum reduces the load on the surface tension so that larger straw will work for the trick. If you have a 10 foot in diameter straw, the surface tension will fail and the finger on top won't hold it either.
    Indeed, or even a cup turned upside down for that matter.
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  29. #29  
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    Quote Originally Posted by cincirob View Post
    An interesting experiment would be to add detergent to the water which reduces surface tension and see if the trick still works. [/B]
    Detergent? Doesn't that increase the surface tension, like when you make soap bubbles?
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    Boeing3000: Detergent? Doesn't that increase the surface tension, like when you make soap bubbles?

    cinci: Detergents generally don't bubble much. We use them in dishwashers and laundry machines to keep them from making too many suds. Redcuing surface tension makes the water "wetter" so that it infiltrates better.
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