# Surface tension doubt

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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Bystander
Homework Helper
Gold Member
How many sides does the film have?

How many sides does the film have?
4 sides. 1 stuck withe the slider. 3 bounded by frame.

Bystander
Homework Helper
Gold Member
Not edges --- sides?

Chestermiller
Mentor
The film has two surfaces. There is surface tension on each surface.

Chet

Not edges --- sides?
Understood. 2 surfaces.
Thank you

Understood. 2 surfaces.
Thank you
T
One surface and then flipping this vertically

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