Very confused about surface tension

In summary, the article discusses the concept of surface tension and its application in various scenarios. It explains that surface tension is the force working to minimize the surface area and that it is always tangential to the surface. The article also clarifies the confusion surrounding the formula γ = F/2l and why there are two surfaces involved. It also provides an example and explanation for the force exerted by surface tension on a needle resting on the surface.
  • #1
dEdt
288
2
See link: http://www3.interscience.wiley.com:8100/legacy/college/cutnell/0471713988/ste/ste.pdf. When defining surface tension, the article makes reference to a C-shaped apparatus. Why is γ = F/2l and not F/l? It says something about there being two surfaces, but it seems to me that because there's only one surface touching the wire, it should be F/l. Second question: In example one, surface tension is applying a force outwards on the needle. But based on the introductory explanation of surface tension, I thought surface tension was only inwards. I can see why compressing the liquid will produce some outward force against the needle, but why would that equal γL? It seems like those are two different phenomena.

I am obviously well aware that I probably have a deep missunderstanding of what's actually going on. The way I see it, any molecule on the surface will be 'sucked' in, and that's the force which is pulling the wire in in the first question. Is this at all correct?

Thanks a lot!
 
Last edited by a moderator:
Physics news on Phys.org
  • #2
dEdt said:
It says something about there being two surfaces, but it seems to me that because there's only one surface touching the wire, it should be F/l.

There are two sides.

I can see why compressing the liquid will produce some outward force against the needle, but why would that equal γL? It seems like those are two different phenomena.

Think in terms of surface area, rather than directions. The surface tension is the force working to minimize the surface area. When you place a needle on the surface, it's "stretching" the surface, increasing the surface area. The force isn't from compressing the liquid- Pour soap in it to disrupt the surface tension and the needle will sink, even though the compressibility of the water doesn't change at all.
 
  • #3
dEdt said:
Why is γ = F/2l and not F/l? It says something about there being two surfaces, but it seems to me that because there's only one surface touching the wire, it should be F/l.
The sheet of liquid has two surfaces. See Fig 3, which calls them the upper and lower surfaces.

Second question: In example one, surface tension is applying a force outwards on the needle. But based on the introductory explanation of surface tension, I thought surface tension was only inwards. I can see why compressing the liquid will produce some outward force against the needle, but why would that equal γL? It seems like those are two different phenomena.
The surface is always under tension, just like a piece of taut rope. (Imagine it as a stretched rubber sheet.) The tension is always tangential to the surface. The needle rests on the surface (which bends around it), and the surface exerts a tangential force, which in this case has a vertical component that balances the weight of the needle.

Part of the confusion may be the diagrams in Fig 2, which are inaccurate. Fig 2a implies that the inside of the liquid is under tension, when it's really under compression; Fig 2b implies that there's a net inward force at the surface, which would produce an inward acceleration.
 
  • #4
Doc Al said:
The tension is always tangential to the surface.

I didn't realize that. Fig. 2 made me believe that force was perpendicular to the surface, but now I see why that's wrong. Thanks a lot.
 
  • #5


Surface tension is a complex concept and it is understandable that you may have some confusion about it. Let me try to clarify some of your questions.

First, the reason why γ = F/2l and not F/l is because surface tension is a force per unit length, and in the C-shaped apparatus, there are two surfaces that are being pulled in opposite directions. Therefore, the total force is divided by the length of the wire on each side, resulting in F/2l. This is because the surface tension is acting on both surfaces, not just one.

In regards to your second question, surface tension can act both inwards and outwards depending on the situation. In the example given, the liquid is being compressed by the needle, causing an outward force on the needle due to the surface tension of the liquid. This is because the liquid molecules are being pushed together, creating tension on the surface. However, in other situations, surface tension can indeed act inwards, such as when a drop of liquid forms a sphere due to the cohesive forces of the liquid molecules.

Your understanding of surface tension pulling molecules towards the surface is correct. This is due to the cohesive forces between the molecules at the surface, which creates a stronger attraction towards the surface compared to the bulk of the liquid. This is what creates the surface tension and allows the liquid to resist external forces, such as the weight of an object placed on its surface.

I hope this helps to clarify some of your confusion about surface tension. It is a complex concept, but with some further reading and understanding, you will be able to grasp it better. Remember, surface tension is a result of the cohesive forces between the molecules at the surface, and it can act both inwards and outwards depending on the situation.
 

1. What is surface tension?

Surface tension is a physical property of liquids that measures the force needed to break the surface of the liquid. It is caused by the attraction between molecules at the surface of the liquid, which creates a sort of "skin" or tension on the surface. This allows some insects, like water striders, to walk on the surface of water.

2. How does surface tension work?

Surface tension is the result of the cohesive forces between the molecules of a liquid. These forces cause the molecules at the surface of the liquid to be more strongly attracted to each other than to the molecules below them, creating a tension on the surface. This tension acts like a thin film, allowing the liquid to resist external forces and maintain its shape.

3. What factors affect surface tension?

Surface tension is affected by several factors, including the type of liquid, temperature, and the presence of other substances (like surfactants) in the liquid. The strength of the cohesive forces between the molecules also plays a role in determining the surface tension of a liquid.

4. Why is surface tension important?

Surface tension has many important applications in daily life and in various industries. It allows insects and some small animals to walk on water, and it helps plants to transport water from their roots to their leaves. In industry, surface tension is used in processes like coating, printing, and cleaning. It also plays a crucial role in some medical procedures, such as the formation of droplets for medication delivery.

5. How can surface tension be measured?

Surface tension can be measured using a variety of methods, including the drop weight method, the capillary rise method, and the tensiometer method. Each of these methods involves measuring the force required to break the surface of the liquid or to lift an object from the surface. Surface tension can also be calculated using mathematical equations that take into account the properties of the liquid and the environmental conditions.

Similar threads

Replies
7
Views
2K
  • Mechanics
Replies
4
Views
1K
  • Introductory Physics Homework Help
Replies
1
Views
2K
  • Mechanics
Replies
5
Views
6K
Replies
5
Views
3K
  • Introductory Physics Homework Help
Replies
4
Views
4K
  • Introductory Physics Homework Help
Replies
4
Views
2K
  • Introductory Physics Homework Help
Replies
1
Views
1K
  • Introductory Physics Homework Help
Replies
31
Views
5K
Back
Top