BR Phys. Ch. 7 #28

[dougkaye]

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Their explanation is really messing me up and the explanation in the text for Bernoulli-type problems is very sparse. They say in the key that hydrostatic pressure is greater at the output end of the the siphon. I thought that flow was always from greater pressure to lower pressure, but now they're saying the opposite: "In order to have water flow from the intake end to the output end, there must be a a net pressure difference, where the gauge pressure is greater at the output end. This occurs when the output end is at a lower height, this is at a point of greater hydrostatic pressure." Obviously I'm missing something, as they seem to be contradicting themselves in terms of which direction fluids flow with regards to pressure.

Any ideas welcome. In the meanwhile I'm looking for a physics textbook.

 

[attixx]

yo doug,

Did you ever get any clues about this problem? I don't have anything like this in my TBR fluids section so I can't take a good look at it, but it's been buggin me ever since I read this.

 

[BerkReviewTeach]

A siphon functions according to Poiseuille's law, because you're getting a fluid to flow. Bernoulli's principle is applied for pressures against the wall (like when a bus drives by you really fast and you get sucked into the stream--which has a reduced pressure because it's in motion and you are the wall).

I have to agree though, that wording is tripping me up too. I read it twice and while I get their concept, something just doesn't seem worded right. A typical siphon works because the two ends are at different heights. The weight of the water above pushes on the water below, so the water will flow out from the lower end. You are correct that water (any fluid for that matter) will flow from a higher pressure to a lower pressure. I think the way to look at it is that the two ends differ in pressure by a value that depends on the height (P = rho x g x h), and the deltaP is proportional to the flow rate (Q).

 

[BerkReviewTeach]

yo doug,

Did you ever get any clues about this problem? I don't have anything like this in my TBR fluids section so I can't take a good look at it, but it's been buggin me ever since I read this.

It's the first question on the Toilet tank passage.

 

[mocharoca]

I came across this same question while studying and was wondering if anyone has an answer for it?

 

[MrNeuro]

yeah the wording for the answer and the wording for the answer choice makes no sense anyone figure what they're trying to say?

i thought syphons work according to Bernoullis (however you spell his name)

where

pgh(top) = pgh(bottom) + 1/2 p v^2

with a greater pressure at the top and a lower pressure at the bottom due to a velocity...

 

[milski]

Pressure at the top of the reservoir = atmospheric pressure, so nothing happens there.
Pressure of the water in the syphon = atmospheric pressure + mgh for the water.
Pressure on the outside of the syphon = atmospheric pressure

The difference between the last two makes the water move from inside to the outside of the syphon, since the pressure of the water on the inside is higher than the pressure of the air on the outside. As soon as some of the water goes out, the pressure at the botom of the reservoir becomes lower than what's right on top of it and that makes the water settle further down.

Bernoulli has nothing to do with it since you need to have some flow to apply it. For a fluid which is not moving there are no pressure changes so you cannot use it in that case.

 

[ayocaptain628]

Pressure at the top of the reservoir = atmospheric pressure, so nothing happens there.
Pressure of the water in the syphon = atmospheric pressure + mgh for the water.
Pressure on the outside of the syphon = atmospheric pressure

The difference between the last two makes the water move from inside to the outside of the syphon, since the pressure of the water on the inside is higher than the pressure of the air on the outside. As soon as some of the water goes out, the pressure at the botom of the reservoir becomes lower than what's right on top of it and that makes the water settle further down.

Bernoulli has nothing to do with it since you need to have some flow to apply it. For a fluid which is not moving there are no pressure changes so you cannot use it in that case.

quoted for future reference, idk if there is a feature that lets me see posts i've liked in the past