# Problem understanding liquid pressure

• dalin mald
In summary, the speaker is trying to understand the concept of pressure, specifically in relation to liquid moving in a long tube. They question which pressure is being referred to and wonder if the pressure is the same on all particles. They seek a visual representation to aid in their understanding and thank others for their assistance.
dalin mald
TL;DR Summary
liquid in mouvement, pressure , Bernoulli equation,
Good morning.

I joined this forum to improve my self and give back what i will learn here for the members of this forum.

I m confused about the notion of pressure and i will explain the point that i don't understand.
since we know that the pressure is the force over surace. and since we know that the liquide pressure is depending to the depth . all is easy now.

so the hard side i did not know is when the liquide starts to move in a long tube for example.
Can you tell me which pressure we mean by the pressure on the tube. i ork in the ship and i always hear that the inlet pressure in fresh water cooler is 4 bars for example , is it the pressure of the up side water or the bottom water on the tube of cooler ?

than what is the pressure we mean in the equation of bernuli. they say the pressure energy exists so i agree because we have a pressure on a surface but is that pressure force is the same on all particuls , why they fix it ?

i really need a help.
i looked

I think I understand your question, but am not sure. Please post a diagram showing the four pressures in a liquid to liquid heat exchanger, and indicate which ones you want to know about.

dalin mald said:
Can you tell me which pressure we mean by the pressure on the tube. i ork in the ship and i always hear that the inlet pressure in fresh water cooler is 4 bars for example , is it the pressure of the up side water or the bottom water on the tube of cooler ?
Just to repeat: we really need to know the system configuration. It's unlikely that at a pressure of 4 bar this has anything to do with Bernoulli's Principle/hydrostatic pressure.

Lnewqban
dalin mald said:
Summary:: liquid in mouvement, pressure , Bernoulli equation,

so the hard side i did not know is when the liquide starts to move in a long tube for example.
Can you tell me which pressure we mean by the pressure on the tube.
A tube placed in a horizontal position will always have a higher pressure at the bottom than at the top.

The difference will be ##\rho g D## where ##\rho## is the fluid density, ##g## is the acceleration due to gravity (9.81 m/s²) and ##D## is the tube diameter.

But if we take the example of water (##\rho## = 1000 kg/m³), the difference is in the order of 10 000 Pa/m or 100 Pa/cm. Thus, for a tube with a 15 cm diameter, the maximum pressure difference is 1500 Pa, which is usually negligible compared to, say, atmospheric pressure (101 000 Pa; about 1.5 % difference). So the pressure is an average value. Just like the velocity of the fluid is an average value as well.

If the difference would be non-negligible, then you would probably not be able to make such simplification to study a liquid in motion.

Lnewqban
thank you friends for help

Last edited by a moderator:
berkeman

## 1. What is liquid pressure?

Liquid pressure is the force exerted by a liquid on an object or surface. It is caused by the weight of the liquid above the object or surface, and is measured in units of force per unit area, such as pounds per square inch or pascals.

## 2. How is liquid pressure calculated?

Liquid pressure is calculated by dividing the force exerted by the liquid by the area over which the force is applied. The formula for liquid pressure is P = F/A, where P is pressure, F is force, and A is area.

## 3. What factors affect liquid pressure?

The factors that affect liquid pressure include the density of the liquid, the depth of the liquid, and the gravitational force acting on the liquid. The shape and size of the container and the surface area of the object in contact with the liquid can also affect liquid pressure.

## 4. How does liquid pressure change with depth?

As depth increases, the weight of the liquid above the object or surface also increases, resulting in a higher liquid pressure. This is because the deeper the object is submerged, the more liquid there is above it, and therefore, the greater the force exerted by the liquid.

## 5. What are some real-world applications of understanding liquid pressure?

Understanding liquid pressure is important in many practical applications, such as designing and maintaining water systems, calculating the buoyant force on objects in water, and predicting the behavior of fluids in hydraulic systems. It is also crucial in fields like engineering, architecture, and oceanography.

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