Is the Formula for Surface Tension Incorrect?

AI Thread Summary
The discussion centers on the formula for calculating surface area in relation to surface tension, specifically questioning why the increase in surface area is represented as 2lx instead of lx. Participants argue that a soap film has two sides, which justifies the factor of 2 in the equation. The initial poster expresses confusion over this reasoning, suggesting that the increase in surface area should only account for one side. However, others clarify that to find the total change in area, both surfaces must be considered, leading to the conclusion that the total increase is indeed 2lx. The conversation highlights the importance of understanding the physics behind surface tension and surface area calculations.
rajatgl16
Messages
54
Reaction score
0
In the image that I've attached with this thread you can see proof of serface energy+surface tension. This scaned image is from my school book. When I was going through this topic I wound that in this
increase in surface area = 2lx
which according to me isn't reasonable. I discussed about it mwith my teacher but I'm not satisfied with his answer.
According to me
increase in surface area should be = lx.
Please post your comments on this
 

Attachments

  • scan0003.jpg
    scan0003.jpg
    79.3 KB · Views: 537
Physics news on Phys.org
The soap film has two sides. You need to count both, thus the factor of 2.
 
my teacher gave me the same explanation. But the point is 2 sides of soap film is okay for surface tension. But this in no way associated with area.
 
rajatgl16 said:
But this in no way associated with area.
Why do you think that? What do you think you're finding the area of?
 
well i think increase in surface area is denoted by LL'M'M in figure
 
rajatgl16 said:
well i think increase in surface area is denoted by LL'M'M in figure
Surface area of what?
 
rectangular frame
 
rajatgl16 said:
rectangular frame
Why would you care about the area of the rectangular frame? That's not a surface. (Remember that this is a physics issue, not just geometry.)
 
Then you tell me what is this?
 
  • #10
rajatgl16 said:
Then you tell me what is this?
Your trying to find the surface area of the soap film. It has two sides.
 
  • #11
Now check this one. iN THIS for a liquid bubble which too have 2 sides increase in surface area is taken as 4pi*r^2 not 2*4pi*r^2
 

Attachments

  • scan0001.jpg
    scan0001.jpg
    28.1 KB · Views: 462
  • #12
rajatgl16 said:
Now check this one. iN THIS for a liquid bubble which too have 2 sides increase in surface area is taken as 4pi*r^2 not 2*4pi*r^2
I see a factor of 2.
 
  • #13
I too can see. But Thats for surface tension according to me.
As you can see in figure this topic is part b). Actually part one is for liquid drop. In that surface tension is taken to b 'T'. But as for a liquid bubble there are to effective surfaces, surface tension comes out to b '2T'.
 
  • #14
rajatgl16 said:
I too can see. But Thats for surface tension according to me.
As you can see in figure this topic is part b). Actually part one is for liquid drop. In that surface tension is taken to b 'T'. But as for a liquid bubble there are to effective surfaces, surface tension comes out to b '2T'.
OK, so what's the issue?
 
  • #15
Issue is still same In my thread I've asked why increase in surface area is taken as 2lx instead of lx.
 
  • #16
rajatgl16 said:
Issue is still same In my thread I've asked why increase in surface area is taken as 2lx instead of lx.
I answered that (twice): There are two surfaces. You need the total change in area of the surface.
 
  • #17
i'm not getting what you mean.
 
  • #18
please elaborate how increase in surface area= 2lx. It simply makes no meaning that due to 2 surfaces total area becomes twice or 2lx
 
  • #19
rajatgl16 said:
please elaborate how increase in surface area= 2lx. It simply makes no meaning that due to 2 surfaces total area becomes twice or 2lx
I'm not sure what to say without repeating the same words over again. A soap film has two sides. You want the change in total surface area. Each side increases by lx, thus the total is twice that.
 
  • #20
can you mark that area it in figure?
 
  • #21
rajatgl16 said:
can you mark that area it in figure?
No. The figure only shows one side of the film.
 
  • #22
this wire system is 2dimensional or 3d? And sliding has occurred from LM to L'M' or reverse?
 
  • #23
Do you agree that the soap film has two sides? Yes or no.
 
  • #24
yes. And by it i mean that the wire system LM on both (left and right) side is coated with soap film.
 
  • #25
rajatgl16 said:
yes. And by it i mean that the wire system LM on both (left and right) side is coated with soap film.
Not sure what you mean by left and right, since the diagram shows only one side of the film. I'd call the two sides front (shown) and back (not shown).
 
  • #26
ok i got your this point, and my point of left and right is wrong. Now after this what?
 
  • #27
rajatgl16 said:
ok i got your this point, and my point of left and right is wrong. Now after this what?
As the area of the frame increases by lx, the total surface area of the film increases by 2lx, including both surfaces.
 

Similar threads

Surface Tension - Using hot soapy water to wash clothes
Replies
1
Views
2K
Confusion regarding surface tension direction and force balance
Replies
39
Views
2K
Effect of temperature on surface tension
Replies
2
Views
1K
What Drives Surface Tension in Liquids?
Replies
3
Views
2K
How does water rise along a glass plate? (surface tension question)
Replies
2
Views
1K
What Is Surface Tension and How Does It Form?
Replies
5
Views
5K
Correct definition of surface tension
Replies
1
Views
2K
I The surface area of an oblate ellipsoid
Replies
8
Views
2K
Tension, FB diagrams and some formula questions.
Replies
6
Views
3K
Is Hydrogen Bonding the Sole Factor Affecting Viscosity and Surface Tension?
Replies
2
Views
16K

Hot Threads

Recent Insights

Back
Top