BR Physics II Fluids - #22

[adsf123]

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This may be a tough one to answer without the diagram but.

What will be observed when a more viscous liquid of the same mass density is substituted for the less viscous liquid in the system below:

Basically there is a reservoir connected to a horizontal tube. At the right edge of the horizontal tube is a column that is half filled with fluid and open to the environment.

A. A greater fluid velocity at point B, but an unchanged fluid height in Column 1.
B. A greater fluid velocity at point B, and a greater flow height in Column 1.
C. A lower fluid velocity at point B, but an unchanged fluid height in Column 1.
D. A lower fluid velocity at point B and a greater fluid height in column 1.

Answer is C. I thought by Bernoulli's equation that pressure would be greater against the inner walls forcing the fluid higher up column one due to the lower flow speed. The answer talks about the pressure in the reservoir being the same because of an equal fluid mass density and the pressure of the atmosphere being the same, so the height of the fluid in column 1 remains the same.

 

[SSerenity]

This is a strange question....

So, imagine you put your finger on point B, to prevent the flow. The height in the column should remain the same. Therefore the height is only dependent on the pressure difference across the pipe. This pressure difference is described by the mass in the reservoir (pgh). Im pretty sure atmospheric pressure can be ignored, since both are exposed to the outside.

Now the moment you release your finger, the height of your column would drop because the height in your reservoir would also drop. The velocity exiting would be lower than with a less viscous liquid too. But if we imagine this reservoir is magic and always maintains the same height... As soon as the liquid starts the flow, the pressure against the walls HAS to decrease. So, the more viscous fluid moves slower, so the pressure against the walls is higher, hence the column height must be higher.

I think this question only works if we consider the column height BEFORE the liquid flows AND consider the exiting velocity AFTER the fluid flows, but separately.

This seems like a pretty insanely misleading question, unless my reasoning is wrong. Does anyone else agree here?

 

[monkeyvokes]

This is a strange question....

So, imagine you put your finger on point B, to prevent the flow. The height in the column should remain the same. Therefore the height is only dependent on the pressure difference across the pipe. This pressure difference is described by the mass in the reservoir (pgh). Im pretty sure atmospheric pressure can be ignored, since both are exposed to the outside.

Now the moment you release your finger, the height of your column would drop because the height in your reservoir would also drop. The velocity exiting would be lower than with a less viscous liquid too. But if we imagine this reservoir is magic and always maintains the same height... As soon as the liquid starts the flow, the pressure against the walls HAS to decrease. So, the more viscous fluid moves slower, so the pressure against the walls is higher, hence the column height must be higher.

I think this question only works if we consider the column height BEFORE the liquid flows AND consider the exiting velocity AFTER the fluid flows, but separately.

This seems like a pretty insanely misleading question, unless my reasoning is wrong. Does anyone else agree here?

Yeah I agree, especially if that fluid is really viscous

 

[sps27]

This is a strange question....

So, imagine you put your finger on point B, to prevent the flow. The height in the column should remain the same. Therefore the height is only dependent on the pressure difference across the pipe. This pressure difference is described by the mass in the reservoir (pgh). Im pretty sure atmospheric pressure can be ignored, since both are exposed to the outside.

Now the moment you release your finger, the height of your column would drop because the height in your reservoir would also drop. The velocity exiting would be lower than with a less viscous liquid too. But if we imagine this reservoir is magic and always maintains the same height... As soon as the liquid starts the flow, the pressure against the walls HAS to decrease. So, the more viscous fluid moves slower, so the pressure against the walls is higher, hence the column height must be higher.

I think this question only works if we consider the column height BEFORE the liquid flows AND consider the exiting velocity AFTER the fluid flows, but separately.

This seems like a pretty insanely misleading question, unless my reasoning is wrong. Does anyone else agree here?

Perhaps you are over analyzing this. Visocity will lead to less velocity, we already know that. So you can eliminate A and B here. And the question states same mass density of fluid which has to do with column height. So it will remain the same for a more viscous fluid. So I would go with a C for the answer.

 

[adsf123]

But isn't their some venturi effect?

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

 

[adsf123]

Double post... sorry

 

[monkeyvokes]

But isn't their some venturi effect?

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

yeah seems like there would be, although lower fluid height isnt a choice

 

[adsf123]

higher fluid height... increased viscosity = lower velocity = increased pressure = higher fluid height in column 1... at least that was my reasoning...

i've given up on this question though haha... seems very unlikely something of the sort that can be interpreted so many different ways won't be on the actual test

 

[monkeyvokes]

higher fluid height... increased viscosity = lower velocity = increased pressure = higher fluid height in column 1... at least that was my reasoning...

i've given up on this question though haha... seems very unlikely something of the sort that can be interpreted so many different ways won't be on the actual test

ohh derrff that makes sense