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Homework Help: Surface tention problem

  1. Dec 26, 2011 #1
    Hello ,

    A Problem I found In a book , although I have the answer , I cant understand it

    the problem is

    Calculate the force required to separate two glass plates of area 10 cm^2 each , with a film of water .5mm thick between them . take the coefficent of surface tention (T)to be 72 dyne/cm and theta = 0

    The Answer written

    since theta = 0 so the radius of curvature of the cylindrical surface between the plates is r = d/2 =.025 cm

    ((note : I Understand this but what If theta was another angle ??))

    Delta P = T/r = 72/.025 = 2880 dyne/cm^2

    the Normal force required = Delta P * Area = 2880*10 =2.88*10^4 dyne = .288 N #

    I nead some explanation for this

    Another _Identical problem_ says

    two circular plates of radus 5 cm with 5 mm thick film of water betweem them , If the surface tention = 72.7*10^3 N/m , the normal force required to pull them Apart is what ?

    it is the same I know , but in the solution ((the solution this time is not in the book , it is an assistant Engineer at the college)) said that :

    Delta P = T(1/R1-1/R2) , R2 = infinity , so Delta P = T/R1

    My problem is Why did he write a negative sign ?? I know it won't change the answer but why is it minus or it is just a writing error ??
  2. jcsd
  3. Dec 27, 2011 #2
  4. Dec 27, 2011 #3


    User Avatar
    Homework Helper

    Read http://en.wikipedia.org/wiki/Surface_tension

    Zero contact angle means that the wall is tangent to the liquid surface. There are two parallel walls at distance d, therefore the radius of curvature is r=d/2. This is not true for other angles.


    Attached Files:

  5. Dec 27, 2011 #4
    Yes I do know that and I wrote that above , I asked what would be (mathematically) the radius curvature if theta was 30 degrees for example

    Any way it is not my basic question here :D
  6. Jan 6, 2012 #5
    I found some answers , but what I can't well understand is

    how the pressure difference between the water and the air due to the curviture is multiplies by the (whole area) of the sheet , why ?
  7. Jan 7, 2012 #6

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