# Understanding Surface Tension: Laplace's Law and Balloon Experiment Explained

• seraphodiabolus
In summary: So when you open the valve and allow the air to flow between the balloons, the big balloon will have a lot of air in it and the small balloon will have less air in it.
seraphodiabolus
Having trouble to understand a classical example of surface tension:
Two balloons are connected to each other with a valve. If the surface tension of the two balloons is the same but one balloon is bigger than the other, when the valve is lifted open so the air in the two balloons is now connected, what will happen to the two balloons?
The correct answer is the small balloon will get smaller and the big balloon gets bigger. This answer follows the application of Laplace's law where it says (something like the following) T(surface tension) = P(pressure) x R (radius). So the bigger the radius, the smaller the pressure. Thus when the valve is open, air will flow from high pressure (small balloon) to low pressure (big balloon), as always.
The confusion I have is, I assume a bigger balloon will have MORE air inside. If two balloons are made of the same material (so as to have the same surface tension) and I pump air into the balloons, shouldn't the bigger balloon contain more air, which means higher the pressure. Then the logical conclusion will be opposite to what Laplace's law predict: if I open the valve, air will flow from big balloon (more air) to small balloon (less air). What's wrong with my assumption?

... it is possible to have the same tension and different materials.
Some rubber is stiffer than others so it requires a smaller expansion to build the same tension. You should know this from blowing up different brand/shape party balloons. In order that the tension is the same, the pressure in the big one must be smaller. Maybe there is less air in it and the material is very stretchy.

See: http://hyperphysics.phy-astr.gsu.edu/hbase/ptens.html

I think this question relates to soap bubbles not balloons.
The surface tension in a soap bubble is constant and this is why the pressure in a small soap bubble is greater than in a large bubble.
A balloon is made of rubber and the tension increases as the rubber is stretched as it gets bigger.

## 1. What is surface tension and how does it work?

Surface tension is a physical property of liquids that causes the surface of a liquid to behave like an elastic membrane. This is due to the cohesive forces between molecules on the surface of the liquid, which creates a tension that resists any force that tries to break or deform the surface.

## 2. What is Laplace's Law and how does it relate to surface tension?

Laplace's Law is a mathematical equation that describes the relationship between surface tension, pressure, and curvature. It states that the pressure inside a liquid droplet or bubble is directly proportional to the surface tension and inversely proportional to the radius of curvature.

## 3. How does the balloon experiment demonstrate surface tension and Laplace's Law?

In the balloon experiment, a balloon is partially filled with water and then placed in a container of water. As the balloon is submerged, the water inside the balloon will form a spherical shape due to surface tension. This is because the surface tension of the water molecules is trying to minimize the surface area of the water inside the balloon. The pressure inside the balloon can also be measured, and when compared to the radius of the spherical shape, it follows Laplace's Law.

## 4. Can surface tension be observed in everyday life?

Yes, surface tension can be observed in many everyday situations. For example, when you fill a glass of water to the brim, the surface tension of the water prevents it from overflowing. It can also be seen when water forms droplets on a surface, or when insects like water striders can walk on the surface of water due to surface tension.

## 5. How does understanding surface tension have practical applications?

Understanding surface tension is important in many fields, including physics, chemistry, and engineering. It is used in the design of various products such as soap, detergents, and oil dispersants. It also plays a crucial role in biological processes, such as the movement of water and nutrients in plants and the functioning of our lungs.

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