# The Concept of Pressure in Bernoulli’s principle

• tasveerk
In summary: So, when someone says "pressure increases when velocity increases" they are saying that the pressure is increasing due to the increase in kinetic energy of the fluid, which is true. But when someone says "pressure decreases when velocity increases" they are saying that the pressure is decreasing due to the decrease in kinetic energy of the fluid, which is also true. In summary, the pressure is exerted by the constituent molecules of the liquid on the walls of the pipe or other material in it. If the velocity is greater the pressure FROM THE FLOWING FLUID is lesser and vice versa.When I think of pressure, I think that there is higher pressure acting on a liquid when a pipe is smaller rather than larger.This is true in the case
tasveerk
Hello,
I am somewhat confused about the concept of pressure in Bernoulli’s principle, which states that velocity and pressure are inversely related. When I think of pressure, I think that there is higher pressure acting on a liquid when a pipe is smaller rather than larger. I would appreciate it if someone could explain what sort of pressure is this principle referring to.
Thanks!

tasveerk said:
velocity and pressure are inversely related. When I think of pressure
what sort of pressure is this principle referring to.
Hi,
The pressure is exerted by the constituent molecules of the liquid on the walls of the pipe or other material in it. If the velocity is greater the pressure FROM THE FLOWING FLUID is lesser and vice versa.

tasveerk said:
When I think of pressure, I think that there is higher pressure acting on a liquid when a pipe is smaller rather than larger.
This is true in the case where there is a limited power source (as opposed to unlimited) driving the fluid through the pipe, such as the water in a hose from a fully open tap at your home. The pressure in the hose will be greater if the flow is restricted due to some constriction in the hose rather than letting the water nearly freely flowing through an unrestricted hose.

What Bernoulli principle is getting at is what happens to the fluid as it travels through a constriction in a pipe. The mass flow (kilograms per second) through any cross section of the pipe is constant or otherwise mass would be accumulating. Since the mass flow is constant, the velocity of the flow must be greater in narrower sections of the pipe. Since the pipe doesn't generate any forces that increase the fluid's velocity, the only remaining explanation is that a pressure differential within the fluid itself is responsible for the increase in velocity in the narrower sections of a pipe. This in turn means that the pressure in the narrower section of pipe must be less than the wider section of pipe leading into the narrower section of pipe.

Another term used for this reduction of pressure in a narrowing pipe is Venturi effect. It's used in carburetors (for engines that still use these), to assist in drawing fuel into the air stream. Wiki link:

http://en.wikipedia.org/wiki/Venturi_effect

Part of the confusion here is due to the fact that there is more than one type of pressure and people/articles are often sloppy about which they are referring to. Often when people say "pressure" they are referring to static pressure. But there is also velocity pressure and total pressure. In Bernoulli's equation, when velocity increases, velocity pressure increases and static pressure decreases, while total pressure remains constant.

Pressure is a measure of potential energy stored, per unit volume measured in J/m^3.

Now consider what Bernoulli says - all he says is that when a fluid gains kinetic energy it must loose some pressure energy, and/or vice versa, because energy cannot be created or disappeared in a smooth, lossless, fluid flow.

That's all Bernoulli's equation says. It states that the sum of pressure and kinetic energy of a fluid flow remains constant.

## 1. What is Bernoulli's principle?

Bernoulli's principle is a fundamental concept in fluid dynamics that states that as the speed of a fluid increases, its pressure decreases. This principle is based on the law of conservation of energy and is applicable to all fluids, including air and liquids.

## 2. How does Bernoulli's principle relate to pressure?

Bernoulli's principle explains the relationship between the speed and pressure of a fluid. As the speed of a fluid increases, the pressure decreases, and vice versa. This means that areas of high fluid speed will have lower pressure, while areas of low fluid speed will have higher pressure.

## 3. What is the significance of Bernoulli's principle?

Bernoulli's principle is essential in many real-world applications, such as airplane and car aerodynamics, fluid flow in pipes and channels, and even the flight of birds. It helps engineers and scientists understand and predict the behavior of fluids in various situations.

## 4. Can Bernoulli's principle be violated?

No, Bernoulli's principle is a fundamental law of fluid dynamics and cannot be violated. However, it is essential to note that this principle is only applicable to ideal fluids, which do not exist in the real world. In real-world situations, other factors such as viscosity and turbulence may affect the behavior of fluids.

## 5. How is Bernoulli's principle used in everyday life?

Bernoulli's principle is used in various everyday objects, such as carburetors in cars, airplane wings, and even the design of sports balls. It is also used in medical devices such as inhalers and nebulizers, where the principle is used to create a pressure difference to deliver medication to the lungs.

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