Fluid Velocity and Pressure

[reising1]

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Two questions that I don't understand:

569. When comparing two points of fluid flowing through the same horizontal pipe, if the fluid velocity is greater, then
A. the temperature is less
B. the temperature is greater
C. the temperature is unchanged
D. the pressure is greater
Answer: A

570. A decrease in which of the following fluid characteristics would increase the pressure at any given point in a moving ideal fluid?
A. temperature
B. density
C. cross-sectional area of pipe
D. velocity
Answer: D

Can anyone explain either of these questions/answers?

 

[HesitantManatee]

Two questions that I don't understand:

569. When comparing two points of fluid flowing through the same horizontal pipe, if the fluid velocity is greater, then
A. the temperature is less
B. the temperature is greater
C. the temperature is unchanged
D. the pressure is greater
Answer: A

You have to assume energy in the system is the same in both cases. Their is two types of kenetic energy discuss in this question. Transational KE (velocity) and random KE (temperature). If transational KE increases then ramdon KE much decreases so the energy of the system remains constant

570. A decrease in which of the following fluid characteristics would increase the pressure at any given point in a moving ideal fluid?
A. temperature
B. density
C. cross-sectional area of pipe
D. velocity
Answer: D

This question is asking about bernoulli's principle. You can get the answer by just looking at bernoulli's equations.

 

[Patassa]

569, is bogus. I wouldn't worry much less think about that question. This question loses the forest for the trees.
The first rule in doing fluid dynamic problems concerning flow through a pipe of unchanging volume or size is that velocity everywhere is equal. Conservation of mass, can't get around it.

 

[grburst]

569. When comparing two points of fluid flowing through the same horizontal pipe, if the fluid velocity is greater, then
A. the temperature is less
B. the temperature is greater
C. the temperature is unchanged
D. the pressure is greater
Answer: A

From Bernoulli's equation, you have

(1/2)(\rho)v^2 + \rho*g*h + P = constant

where \rho = density, v = velocity, g = acceleration due to gravity, h = height and P = pressure.

If the first term increases, one or both of the other two terms must decrease in order to satisfy the relationship. How can this happen?

(1) \rho*g*h decreases. This means either \rho, g, or h decreases. Since the fluid is incompressible (assumption I'm making), \rho is constant and cannot decrease. g is a constant and cannot decrease. So it's possible that the height decreased. But that's not one of the answer choices.

(Note: If the fluid were not incompressible, a possible answer would be that \rho, the density, decreases. Since volume is constant, this would mean the mass of the fluid decreased. This is not an answer choice.)

(2) P decreases. Notice that the question says "same horizontal pipe", which means the volume doesn't change to compensate. So the pressure in a fixed volume is being decreased, which means it can be reasonably be assumed (too lazy to go through the math of the relationship between pressure/volume and temperature for fluids, I'm assuming it's similar to gases) that the temperature decreases. That's why I would say it's A.

 

[Patassa]

(2) P decreases. Notice that the question says "same horizontal pipe", which means the volume doesn't change to compensate. So the pressure in a fixed volume is being decreased, which means it can be reasonably be assumed (too lazy to go through the math of the relationship between pressure/volume and temperature for fluids, I'm assuming it's similar to gases) that the temperature decreases. That's why I would say it's A.

n/m

 

[grburst]

No, it's bernoulli's. Velocity goes up, pressure goes down. You can't treat liquid like a gas, that's why we use bernoulli's and not ideal gas laws to do fluid flow.

What is Bernoulli's?

Ideal gas laws AFAIK don't involve flow at all (even for gases), so I don't think the equations are comparable. I was referring to the relationship between pressure, volume and temperature for incompressible fluids (which, I think, would require a different relationship than just Bernoulli's to understand -- I'm not sure what that is).

 

[Patassa]

What is Bernoulli's?

Ideal gas doesn't involve flow at all (even for gases), so I don't think the equations are comparable. I was referring to the relationship between pressure, volume and temperature for incompressible fluids (which, I think, would require a different relationship than just Bernoulli's to understand -- I'm not sure what that is).

Sorry, I got mixed up, I thought you were saying (2) as in the second question.

 

[grburst]

I think you mixed up the questions, the second question doesn't say "the same horizontal pipe", A decrease in which of the following fluid characteristics would increase the pressure at any given point in a moving ideal fluid?

We're dealing with fluid flow, not a fixed vessel.

My explanation was all about the first question, 569. I didn't discuss 570. My "(2)" was referring to the second possibility that would allow for the velocity to increase while keeping Bernoulli's equation true.

 

[reising1]

From Bernoulli's equation, you have

(1/2)(\rho)v^2 + \rho*g*h + P = constant

where \rho = density, v = velocity, g = acceleration due to gravity, h = height and P = pressure.

If the first term increases, one or both of the other two terms must decrease in order to satisfy the relationship. How can this happen?

(1) \rho*g*h decreases. This means either \rho, g, or h decreases. Since the fluid is incompressible (assumption I'm making), \rho is constant and cannot decrease. g is a constant and cannot decrease. So it's possible that the height decreased. But that's not one of the answer choices.

(Note: If the fluid were not incompressible, a possible answer would be that \rho, the density, decreases. Since volume is constant, this would mean the mass of the fluid decreased. This is not an answer choice.)

(2) P decreases. Notice that the question says "same horizontal pipe", which means the volume doesn't change to compensate. So the pressure in a fixed volume is being decreased, which means it can be reasonably be assumed (too lazy to go through the math of the relationship between pressure/volume and temperature for fluids, I'm assuming it's similar to gases) that the temperature decreases. That's why I would say it's A.

This is a great explanation so far, but unfortunately, the most important part is left out 🙁

I still don't understand why temperature has anything to do with it. Can you explain further?

 

[grburst]

This is a great explanation so far, but unfortunately, the most important part is left out 🙁

I still don't understand why temperature has anything to do with it. Can you explain further?

I asked some physics experts. It turns out that the question as posed is bad, because there's no general statement you can make about fluids relating pressure to temperature in a closed volume. It depends on the fluid and its properties.

 

[reising1]

Yay, this means I don't have to worry about it. Okay, thanks!!