# Surface tension doubt

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1. Dec 9, 2014

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?
$dW = Tdx$
$hence, T= dW/dA$

Last edited by a moderator: Dec 9, 2014
2. Dec 9, 2014

### Bystander

How many sides does the film have?

3. Dec 9, 2014

4 sides. 1 stuck withe the slider. 3 bounded by frame.

4. Dec 9, 2014

### Bystander

Not edges --- sides?

5. Dec 9, 2014

### Staff: Mentor

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

Chet

6. Dec 9, 2014

Understood. 2 surfaces.
Thank you

7. Dec 10, 2014

### gracy

T
One surface and then flipping this vertically

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