# pressure through a seal

#### Halcyon32

2+ Year Member

In this diagram, the 2 ends of the pipe are sealed i.e. the bottom of container A and the end near D are sealed. Why is is that the pressure at C is still relative to the water level at 10 cm? In other words aren't the pipe and the container two separate entities with pressures individual from each other since there is a seal between them and no fluid is flowing between them?

#### popopopop

7+ Year Member
Hmmm, pressure at 10cm is atmospheric pressure. Pressure at C, because it's sealed, would be P = pgh. What's the answer to this if you have it lol.

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#### Halcyon32

2+ Year Member

here's the question that got me confused. the answer is B. I'm not sure if I'm allowed to post the passage since its prep book material and I don't wanna get in trouble but it states that "the top of the pipe at end A is sealed" so only a negligible amount of vapor pressure is on top of the liquid and no atmospheric pressure since its sealed I think. However, upon reading the passage and question more closely, I think that they're saying that the pipe is the entire structure, including what I referred to as the container in my initial post. so the two sealed ends are the parts on top of point A and to the right of D and the part under A must be opened. If that's the case, it makes sense to me why the pressure is relative from the top of the container where it's marked as 10cm but its just the way the diagram is shown (with that line on the bottom part of the container seemingly separating the top and bottom part) and the passage is worded that confused me.

#### popopopop

7+ Year Member
Ahhh, your post said 10cm, not A. No, your first post about the pipe being closed at the bottom of A and the other side D looks correct. If you use Bernoulli's, you set up point A as P + pgh1, then point C as simply pgh2 without the P since there's no atmosphere. You can ignore the kinetic portion since velocity is zero.

P + pgh1 = pgh2
-p and g cancels out since they are the same, leaves you with...

1 + 7 = 4

You can see the static pressure on point A is doubled that of point C.

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#### Halcyon32

2+ Year Member
"B is correct. Point C is twice the depth as point A, so the pressure is twice as great. (P = pgy) This question is really just asking "Do you measure y from the top, or from the bottom?" Of course, you measure y from the top." that's what the answer key says.
They're saying, from what I can tell: P=pgy, at h=7cm, y=3cm lets give p an arbitrary value of 1 since its not given so, P=1(10)(3)=30 now at h=4cm y=6cm so P=1(10)(6)=60 so its doubled. I don't see why we would use bernoulli's (or maybe what i just did is bernoulli's). is the P in your "P + pgh1 = pgh2" supposed to represent atmospheric pressure, because I think atmospheric pressure is 0 since the top is sealed as well. Going off this, I get the feeling that they're actually saying the part under A isn't sealed but rather what they mean by the 2 sealed ends is the part above A and right of D.

#### popopopop

7+ Year Member
If it's sealed at top then I'm wrong. It's good to know the difference between absolute, gauge, and static pressure I guess. It's good to talk this out, but I'm stopping here on fluids lol.

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#### Halcyon32

2+ Year Member
Yeah the passage explicitly says in the first line that the top is sealed and the vapor pressure on top of the fluid is negligible. And yeah talking it out always helps. Thank you for your help!