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Surface tension in capillaries

  1. Sep 15, 2013 #1
    The figures below represent capillaries of varying construction and arrangement. The diameter of the capillary portion is the same in each case, and all of the capillaries are constructed of glass, except where otherwise indicated. The equilibrium rise for water is shown at the left. Draw meniscuses in each figure to correspond to (a) the level reached by water rising up the clean, dry tube and (b) the level to which the water would recede after having been sucked up to the end of the capillary. The meniscuses in the capillary may be assumed to be hemispherical in shape. Pay attention to curvatures of meniscus as well.

    assign3.gif

    a) I think that they the water would just take up the same amount of volume in each tube? I'm confused about the paraffin one though.

    b) I don't get this at all.
     
  2. jcsd
  3. Sep 16, 2013 #2

    CWatters

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    First do you understand why the water goes up the left hand tube?

    If you do then see

    http://simple.wikipedia.org/wiki/Surface_tension

    and compare the contact angle for various materials such a water and glass (0 degrees) and water and paraffin wax (107 degrees).

    What happens to the surface tension when the water rising up the glass tube reaches the paraffin wax?

    a) Asks about water rising up the tubes. How far does it go? What does the meniscus look like when it stops?
    b) Asks what happens if you suck water up the tube and then stop sucking. Clearly the water will sink back down but how far and is the end result the same as in a)? What does the meniscus look like then?

    So the answer will be two sets of drawings. Each showing the curve of the meniscus.
     
  4. Sep 16, 2013 #3
    But where I'm confused is that, for example, the water rises up through the paraffin wax tube. But the amount of water makes the paraffin wax contact angle irrelevant because of the wax's location in the middle of the tube. The water rises above it.

    Is my reasoning wrong?

    For b, I feel like the end result will be the same as in a except opposite (from the other direction).
     
  5. Sep 17, 2013 #4

    CWatters

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    Does it? Best start at the beginning..

    What makes the water rise up the plain glass tube on the far left at all?
    Why does it only rise to that point and no higher?
     
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