General Help in Understanding Pressure. PLEASE HELP

In summary, the conversation discusses the concept of pressure in fluids, specifically in the context of flow through a pipe. It is mentioned that the dynamic pressure increases and static pressure decreases when the flow goes from a wider section of the pipe to a skinnier one, but the reasoning behind this is not fully understood. There is also a question about the direction and action of static pressure, and how it changes in relation to fluid flow. The use of a manometer is brought up, and the comparison is made to the energy balance in objects like a ball or pendulum. The explanation is given that the force of a fast-moving stream of water can cause more harm due to the concentration of energy in one spot, while a slower stream of water
  • #1
Physicsman89
3
0
I thought I understood pressure in fluids, but it seems like its starting to make less sense to me.

Let's consider a flow through a pipe where the rho g h term drops out. Therefore, we would only have the dynamic and static pressure. This is where I start to lose it. I have been told that when the flow of water goes from a wider section of a pipe to skinnier one. the dynamic pressure increases and static pressure decreases. The dynamic pressure going up makes sense, but I don't understand why the static pressure goes down. I think it may be that I don't understand exactly what static pressure is.

Does the static pressure act equally in all three dimensional directions or does it act only horizontally and vertically? Does it act on the pipes pressing outward or does it act horizontally pressing the fluid forward? I don't understand why the pressure wouldn't increase if the fluid is moving faster because i think getting hit by something fast would hurt more than getting hit by something slow.

Could someone please give a good basic description of what it is and how it changes because I can't find any simple answers. I would really appreciate it!
 
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  • #2
To add to that, if you're using a manometer, why would the height change if the total pressure is constant throughout the entire flow?
 
  • #3
Anyone?
 
  • #4
It's energy balance.

You could compare it to dropping a ball from a height h.
At h the ball has a potential energy but no kinetic energy.
from the moment the ball is dropped, the potential energy decreases but since the ball is picking up speed, the kinetic energy increases.

Or a pendulum with rising and falling potential and kinetic energy as the bob sweeps along its arc.

The static and dynamic pressure work the same way to balance the energy equation.
As the diameter of the pipe cahnges from small to big or visa versa the dynamic and static pressures change also.

You get hurt from a fast moving stream of water because the water is being stopped by a particular small part of your body, and all that energy concentrated in one spot. A slow moving stream of water of the same amount of energy could possibly sweep you off your feet.
 
  • #5


It is understandable that you may feel confused about pressure in fluids, as it can be a complex concept to grasp. Let's start by clarifying what static pressure is. Static pressure is the measure of the force exerted by a fluid on its surroundings when it is not in motion. In other words, it is the pressure that is present in a fluid at rest.

When a fluid is in motion, it also has a dynamic pressure, which is the force exerted by the fluid due to its motion. In the scenario you described, where the flow of water goes from a wider section of a pipe to a narrower one, the dynamic pressure increases because the water is being forced to move faster through a smaller space, resulting in a higher force.

At the same time, the static pressure decreases because the water is no longer at rest and is now experiencing the force of its own motion. This decrease in static pressure is due to the conservation of energy, where the increase in dynamic pressure is balanced by a decrease in static pressure.

To answer your question about the direction in which static pressure acts, it acts equally in all three dimensions. However, its effects may be more noticeable in the horizontal and vertical directions, depending on the specific situation.

In regards to your confusion about why the pressure wouldn't increase if the fluid is moving faster, it is important to remember that pressure is not solely determined by the speed of the fluid. Other factors such as the density and viscosity of the fluid also play a role in determining pressure.

I hope this explanation has helped clarify some of your questions about pressure in fluids. If you need further assistance, do not hesitate to ask for more information or clarification. As a scientist, it is my goal to help others understand complex concepts in a clear and concise manner.
 

What is pressure?

Pressure is a measure of the force applied on a surface per unit area. It is typically denoted by the symbol "P" and its SI unit is Pascal (Pa).

How is pressure measured?

Pressure can be measured using various instruments, such as a manometer, barometer, or pressure gauge. These instruments typically use a liquid or gas to exert a known force on a surface, which can then be converted to a pressure measurement.

What causes pressure?

Pressure is caused by the collision of gas or liquid molecules with a surface. The more collisions that occur, the greater the pressure will be. Other factors that can affect pressure include temperature, volume, and the amount of substance present.

What are some real-world examples of pressure?

Some examples of pressure in everyday life include the air pressure in car tires, the water pressure in a garden hose, and the blood pressure in our bodies. Pressure is also important in many industries, such as aviation, where changes in air pressure affect flight conditions.

How does pressure affect the behavior of gases and liquids?

The behavior of gases and liquids is greatly influenced by pressure. For gases, an increase in pressure will decrease the volume and increase the density of the gas. For liquids, an increase in pressure will increase the density and decrease the volume slightly. Pressure also plays a role in determining the boiling and freezing points of liquids.

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