Surface tension doubt

  • #1
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If I have a U-shape wire frame and it contains a straight sliding wire and I dip this in soap, a film forms between the sliding wire and the boundary of U shape frame. Why is the force due to surface tension F = 2Tl where l is length of sliding wire?

They say there are 2 layers in contact. Which 2? There is only one film.

Also
##dW=Fdx=(2Tl)dx##
They book says 2ldx is the change in area dA. Why?
##dW = Tdx##
## hence, T= dW/dA##
 
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Answers and Replies

  • #2
Bystander
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How many sides does the film have?
 
  • #3
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How many sides does the film have?
4 sides. 1 stuck withe the slider. 3 bounded by frame.
 
  • #4
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Not edges --- sides?
 
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  • #5
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The film has two surfaces. There is surface tension on each surface.

Chet
 
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  • #7
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Understood. 2 surfaces.
Thank you
exper2_04.jpg
T
One surface and then flipping this vertically
upload_2014-12-11_10-6-36.png

in this way two surfaces.Each surface has surface tension so F = 2Tl.
 
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