Surface tension

[rajatgl16]

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

 

[Doc Al]

The soap film has two sides. You need to count both, thus the factor of 2.

 

[rajatgl16]

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.

 

[Doc Al]

But this in no way associated with area.

Why do you think that? What do you think you're finding the area of?

 

[rajatgl16]

well i think increase in surface area is denoted by LL'M'M in figure

 

[Doc Al]

well i think increase in surface area is denoted by LL'M'M in figure

Surface area of what?

 

[rajatgl16]

rectangular frame

 

[Doc Al]

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.)

 

[rajatgl16]

Then you tell me what is this?

 

[Doc Al]

Then you tell me what is this?

Your trying to find the surface area of the soap film. It has two sides.

 

[rajatgl16]

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

 

[Doc Al]

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.

 

[rajatgl16]

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'.

 

[Doc Al]

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?

 

[rajatgl16]

Issue is still same In my thread I've asked why increase in surface area is taken as 2lx instead of lx.

 

[Doc Al]

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.

 

[rajatgl16]

i'm not getting what you mean.

 

[rajatgl16]

please elaborate how increase in surface area= 2lx. It simply makes no meaning that due to 2 surfaces total area becomes twice or 2lx

 

[Doc Al]

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.

 

[rajatgl16]

can you mark that area it in figure?

 

[Doc Al]

can you mark that area it in figure?

No. The figure only shows one side of the film.

 

[rajatgl16]

this wire system is 2dimensional or 3d? And sliding has occurred from LM to L'M' or reverse?

 

[Doc Al]

Do you agree that the soap film has two sides? Yes or no.

 

[rajatgl16]

yes. And by it i mean that the wire system LM on both (left and right) side is coated with soap film.

 

[Doc Al]

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).

 

[rajatgl16]

ok i got your this point, and my point of left and right is wrong. Now after this what?

 

[Doc Al]

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.