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Homework Help: Working of a straw paradox?

  1. Jan 1, 2015 #1
    1.The problem I have been facing is understanding how a straw exactly works ?? I know the general, conventional solution given to this problem by the principle of working of a barometer, but I think it has a serious loophole.

    3.They say as a vacuum space is created in the straw due to our breathing actions, hence an extra amount of fluid is required to fill that space up so that the pressure exerted by the fluid given by the equation of {h*density of fluid*acc. due to gravity}, is equal to the atmospheric pressure and the whole condition is in accordance to Pascal's law. But if Mercury lies inside the container and I have a straw some 100 cm long, then will the mercury column rise and stop at 76 cm only, and not above that ???

    Infact any liquid will do same, as its density is less than air density, a lesser height of the column of liquid will be required to provide same amount of pressure, so practically, every liquid should rise only a fraction of height of straw. Please explain ??

    Any kind of help is appreciated.Thank You
  2. jcsd
  3. Jan 1, 2015 #2

    Doug Huffman

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    You are confusing the effects of gauge pressure and absolute pressure.
  4. Jan 1, 2015 #3
    m simply sayin, if in barometer, the height of liquid column produces same effect ( which is just the gauge pressure, for there is vacuum above the rest of the barometric tube), as the atmospheric pressure outside the tube produced on the liquid free surface, then if its the same principle in use in straw, then theres my question.
  5. Jan 1, 2015 #4


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    Correct. The exact height depends on local air pressure that day.

    I don't understand. Most liquids are denser than air.

    Edit: Worlds longest straw. Watch from 2min 45sec
    Last edited: Jan 1, 2015
  6. Jan 1, 2015 #5

    Doc Al

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    I don't quite understand what you are asking here.

    Does sucking liquid through a straw work with the same principle as a barometer? Sure. The air pressure pushes the fluid up the straw. Where's the loophole?
  7. Jan 1, 2015 #6
    Yes, assuming the atmospheric pressure that day is 76 cm Hg. Also, don't try to do this by sucking on a straw with mercury below. Mercury is very poisonous. Also, you can't apply a vacuum approaching 76 mm Hg by mouth; you just can't suck that hard.

    I think you meant to say that the density of other liquids is less than that of mercury. So the rise for other liquids would be greater than for mercury. For example, in the case of water, the maximum rise would be about 10 m. Do the math.

  8. Jan 1, 2015 #7


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    Assuming you mean that the liquid has a greater density than air, you seem to be thinking that the air pressure applied at the surface of the liquid outside the straw is only that arising from the layer of air from that level to the level of the top of the straw. No, there's kilometres of air above that contributing.
  9. Jan 1, 2015 #8
    i am extremely sorry abt the last lyn, i wanted to say jst the same, most fluids are denser than air, and thats my question is that if it is so , then any fluid can create same pressure at the required height inside the straw with a shorter column than that of air, hence any straw won't be working at all ??

    P.s. i made the necessary changes in the question.
  10. Jan 1, 2015 #9
    thanks that helped me....i ws missin on that point.
  11. Jan 1, 2015 #10
    thank you sir, this helped me a lot
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