# What is Pressure and surface tension in fluids

• member 428835
In summary, during a conversation with a professor about fluids, you were asked to explain what pressure and surface tension are. You defined pressure as force per unit area acting normal to a surface, and surface tension as a force per unit length between molecules. The professor then asked about cuts through a control volume and whether pressure and surface tension depend on them. It was suggested that the professor was trying to emphasize that pressure exists everywhere inside a fluid, while your answer focused on a specific surface.
member 428835
Today I was speaking with a professor and he asked me, regarding fluids, what is pressure. I said force per unit area acting normal to a surface. He then asked what I meant by surface. I wasn't really sure how to respond.

He also asked me what surface tension was. I said it is a force per unit length between molecules, like a cohesive force and that if we are given a square surface, surface tension acts to pull the sides of the square, ultimately minimizing surface energy.

He then said the explanation he was looking for referred to cuts through a control volume, and whether pressure and surface tension depended on such cuts. Can someone finesse all of what I've said, whether I'm right or wrong, and explain the cuts he was referring to?

hani14
I'm not a fluid specialist (as are @Chestermiller and @boneh3ad), but I think that the point the professor was trying to get you to see is that pressure exists everywhere inside a fluid, while your answer involved a surface.

Yeah, I was thinking he might have been looking for the surface to be real or virtual, located anywhere in the fluid and spun in all 3 axes, thus providing pressure at every point and in all directions.

Chestermiller
Any ideas anyone? If not I'll stop posting on this thread.

joshmccraney said:
Any ideas anyone? If not I'll stop posting on this thread.
Huh? You posted this thread three weeks ago and got some responses. Did you not see them?

Yea sorry, I totally missed the above two comments. Thanks!

## What is pressure in fluids?

Pressure in fluids is the force exerted by the weight of a fluid per unit area. In other words, it is the amount of force that is applied to a given area of the fluid's surface. This is typically measured in units of Pascals (Pa), which is equivalent to one Newton per square meter.

## How is pressure related to depth in fluids?

In fluids, pressure is directly related to depth. This means that as depth increases, pressure also increases. This is because the weight of the fluid above a certain point exerts a greater force on the fluid below it. This relationship is described by the equation P = ρgh, where P is pressure, ρ is the density of the fluid, g is the acceleration due to gravity, and h is the depth.

## What is surface tension in fluids?

Surface tension in fluids is the force that acts on the surface of a liquid, causing it to behave as if it has a thin film over it. This is due to the cohesive forces between the molecules of the liquid, which causes them to stick together and resist being separated. Surface tension can be observed in everyday phenomena such as water droplets forming on a surface or insects walking on water.

## How does surface tension affect the behavior of fluids?

Surface tension plays a significant role in the behavior of fluids. It is responsible for the formation of menisci (curved surfaces) in liquids, capillary action (the ability of liquids to flow against gravity in narrow spaces), and the shape of liquid droplets. It also affects the movement and flow of fluids, as it can create barriers and resistance at the surface.

## What factors can affect pressure and surface tension in fluids?

There are several factors that can affect pressure and surface tension in fluids. These include temperature, type of fluid, presence of impurities or additives, and external forces such as gravity or electric fields. Changes in these factors can alter the strength of the cohesive forces between molecules, thus affecting the behavior of fluids.

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