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Contradiction regarding capillarity

  1. Oct 8, 2011 #1
    I am currently studying surface tension. I came across a topic named "capillarity" where it is said that mercury falls down in capillary tube. But it rises in a barometer . Why is it so?
     
  2. jcsd
  3. Oct 8, 2011 #2

    xts

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    In barometer?
    Hmmm... what 'barometer' means?
    Maybe it is forced to enter the pipe by atmospheric pressure?

    For classical (Pascal/Torricelli) barometers pipes have usually diameter of 3mm or so. Surface tension is negligible for so wide 'capillares'.
     
  4. Oct 8, 2011 #3
    This is surely a question of language.

    When one end of fine (capilliary) glass tube is dipped into liquid mercury and open to atmosphere at the other the meniscus (surface) of the mercury within the tube is observed to be lower than the general surface of the liquid.

    However when the same experiment is performed in water the meniscus is found to be higher than the general surface.

    This is because water 'wets' the glass, whereas mercury does not.

    This is the origin of "the mercury falls down"

    A barometer, however, is not an open ended tube, dipped into mercury or whatever. It is essentially a manometer comparing the pressure in an enclosed bulb with atmospheric pressure. Often the enclosed bulb actually has a vacuum in it so the implementation is a straight tube, closed and evacuated at the top and dipped into mercury at the bottom open end.
    As the atmospheric pressure changes it is observed and said that the mercury rises or falls in the tube.

    I have tried to make this clearer in the attachment.
     

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    Last edited: Oct 8, 2011
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