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Homework Help: Fluid pressure in different test tubes

  1. Aug 2, 2007 #1
    1. The problem statement, all variables and given/known data

    okay, so what i've been asked to do is explain why a liquid reaches a higher altitude in a skinnier test tube than in a wider test tube.

    i've brainstormed ideas...but i can't find the one that deals with pressure and tension and stuff like that...stuff that deals with bernoulli's equation. these are my ideas.

    -capillary action inside the tubes that allows the water to easily climb up walls on a thinner tube than a thicker one.

    -the weight of the water has to be equal to the atmospheric pressure outside the tube to balance it out. so the thinner the tube, the less water it has in a specific area than a larger tube does in that same area. so water rises more in a thinner tube (to increase the weight) so that the atmospheric pressure outside is equivalent to the pressure inside.

    -then i was thinking. the area of a thinner tube is less than the area of a thicker tube. so if the area is smaller that means that the pressure is larger. and if the pressure is larger in a skinnier tube, it travels upwards to even out with the lower atmospheric pressure. (at high altitudes, the atmospheric pressure is lower)

    can anyone help?
  2. jcsd
  3. Aug 2, 2007 #2


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    Stick with the capillary action explanation. The others are silly.
  4. Aug 2, 2007 #3
    Probably the tube isn't high enough for atmospheric density to change significantly.
  5. Aug 2, 2007 #4
    okay...so the other thing i thought of last night was that the weight of the fluid inside the tube has to equal the atmospheric pressure outside the tube. if you have a smaller tube, the weight of the fluid inside of it is different than the weight of the fluid inside a larger tube... so in order for the fluid to equal the atmospheric pressure outside the tube, more water is pulled up through the tube.
  6. Aug 2, 2007 #5
    Is the tube sealed from atmosphere? If not, then the pressure in the tube IS the atmospheric pressure, which is the same as outside the tube.
  7. Aug 2, 2007 #6
    no...the tube is not sealed...so should i just stick with the capillary action, cohesiveness, and adhesiveness to describe the water being able to easily climb up a skinnier tube than a thicker tube?
  8. Aug 2, 2007 #7
    What?! Water is adhesive?

    As already told, the main cause is capillary action. Everything else are minor corrections, which can in most circumstances be neglected.
  9. Aug 2, 2007 #8
    alright...i'll stick to that explanation then. THANKS SO MUCH. take care...and my fingers are faster than my brain....but just to clarify...water is NOT adhesive...my mistake on that slip.
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