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I Water in capillary tube

  1. Feb 9, 2017 #1
    When we put an empty capillary tube in a water container, the water goes up a little bit by itself untill the surface tension (ST) force is equal to its weight .This shows that unlike friction, this ST force acts individually .
    So if we take out the tube out of the container, and drain a little water (in which case there's less water in it), the ST force would be more than the weight of water. So why doesn't it go up continuously in the tube?
     
  2. jcsd
  3. Feb 9, 2017 #2

    Bystander

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    Think about it for just a little bit longer than it takes to ask the question. What are all the forces acting on the water mass?
     
  4. Feb 9, 2017 #3
    Well,
    1- wight
    2- ST on the top side
    3- ST on the bottom side
    If by the question you mean that the sum of first & third force cancle the 2nd force I think it's wrong. I believe that both STs are in the same direction (upwards) since the tube can hold more water while out of the container. I encountered this point in a book and experienced it myself.
    By the way the P_atm is also cancled obviously since it affects both sides.
     
  5. Feb 9, 2017 #4

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  6. Feb 9, 2017 #5
    Yes they're in the same direction so they both help the water go up, but this actually won't happen. My question is why?!
     
  7. Feb 9, 2017 #6
    Here is a picture of it. f057311016.jpg
     
  8. Feb 9, 2017 #7

    A.T.

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    For the reason you stated:
    But note that only the vertical component of ST is relevant for balancing weight.
     
  9. Feb 10, 2017 #8
    Please read the question carefully !!! Seems you didn't get the point.
    First of all it doesn't act like friction forces, in other words F_capillary is not always equal to weight of column of water necessarily! (if it was equal, the water wouldn't be sucked up in the first place)
    Then we have h_1 as the height of water when the tube is in container. So : (there's ST only on top side)
    F_capillary_top = Gamma * A * h_1
    Then we take it out and drain some of it so it would be h_2 < h_1
    Then obviously (according to the picture above and my explanation) we have:
    F_capillary_top + F_capillary_bottom > Gamma * A * h_2
    So the water must go up!
     
  10. Feb 11, 2017 #9

    A.T.

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    The shapes of the water surfaces adjust, until the forces balance.
     
  11. Feb 11, 2017 #10
    This could be the answer except one problem. In that way when the tube is completely empty, there's no need to be any ST (or as you say change in water sarface). Nevertheless the water is sucked from container when we put the tube in it. How do you explain this?
     
  12. Feb 13, 2017 #11

    A.T.

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    The adhesive and cohesive forces exist, regardless if there is any "need" for them. The changes in the surface shape occur, if those forces are not balanced with other forces (like weight).
     
  13. Feb 13, 2017 #12
    I got it completely thanks to your explanation and some more little experiments. Thank you so much.
    This was also very useful.
     
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