# Finding pressure of gas in u shaped tube with liquid

JoeyBob
Homework Statement:
see attached
Relevant Equations:
P=phg
The answer is suppose to be 0.9432. Initially I thought the pressure inside the glass ball would just be the same as the atmospheric pressure because these equal pressures would cancel each other out, but obviously that's not true.

I can calculate the density of the gas using the equation p1/p2=h2/h1, and get a density of 1659.8361. Now I thought I could calculate the pressure of the gas using P=pgh, where p is density, g gravity, h height of liquid. Using the height of the liquid on the gas side (0.244 m) I get 3968.9 which isn't the right answer. If I take the diffirence in height (0.169 m) I get a pressure of 2749.021. I tried subtracting this from the atmospheric pressure, because maybe the difference in height gives the difference in pressure, but this also gives the wrong answer.

The information about moles is useless because I don't know the temperature of the gas. Otherwise the question would be easy.

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Homework Helper
Gold Member
Maybe you don't need the number of moles or the volume of the ball. What expression do you know that for the pressure at depth ##h## below the level of a fluid?

JoeyBob
What expression do you know that for the pressure at depth h below the level of a fluid?

Im not sure what you mean. I know P=hpg. Intuitively I thought that the liquid was higher on the side of the glass ball because it had a lower pressure than the atmospheric pressure. But I thought that the liquid would rise on the glass ball side until it had the same pressure as atmosphere outside.

Homework Helper
Gold Member
It's more than ##p=\rho gh##. What does your textbook say? Will it be the same regardless of the pressure at the free surface of the fluid?

JoeyBob
It's more than ##p=\rho gh##. What does your textbook say? Will it be the same regardless of the pressure at the free surface of the fluid?

Well pressure can be P=F/A, P(h)=P(knot)-pgh, P(atm)=density*Hgh, PV=nRT

I think those sum up the equations

Homework Helper
Gold Member
P(h)=P(knot)-pgh
Can you explain this one? Are you sure you copied it correctly? Specifically what is P(knot)?
The equation I know is
$$P(h)=P_0+\rho_{\text{fluid}}~g~h$$where
##P(h)## = Pressure as a function of ##h##, the depth below the surface of a fluid.
##h## = the depth below the free surface of the fluid, a positive number.
##P_0## = the pressure at the surface of the fluid.
##\rho_{\text{fluid}}## = the density of the fluid.
##g## = the acceleration of gravity.

It says that the pressure at depth ##h## below the surface of the fluid is equal to whatever pressure is at the surface of the fluid plus the pressure exerted by the weight of the fluid to that depth.

Please remember this equation and most importantly understand and remember what the symbols stand for. Then see if you can use the equation to answer this question.

JoeyBob
JoeyBob
Can you explain this one? Are you sure you copied it correctly? Specifically what is P(knot)?
The equation I know is
$$P(h)=P_0+\rho_{\text{fluid}}~g~h$$where
##P(h)## = Pressure as a function of ##h##, the depth below the surface of a fluid.
##h## = the depth below the free surface of the fluid, a positive number.
##P_0## = the pressure at the surface of the fluid.
##\rho_{\text{fluid}}## = the density of the fluid.
##g## = the acceleration of gravity.

It says that the pressure at depth ##h## below the surface of the fluid is equal to whatever pressure is at the surface of the fluid plus the pressure exerted by the weight of the fluid to that depth.

Please remember this equation and most importantly understand and remember what the symbols stand for. Then see if you can use the equation to answer this question.

Yeah that's the equation, i transcribed it wrong. So if I am understanding this correctly, first I need to find the pressure below the surface using the atmospheric pressure. Then I need to find the pressure on the surface on the glass ball side, using the P(h) from the previous step?

I tried it now and it seems to be correct so I guess I understand the equation now.

On a side note if you don't mind, why do the pressures not equalize? For instance, if the glass ball has a lower pressure, why doesn't the atmospheric pressure push the liquid until the pressures are equal? Or is it because as the liquid is pushed up, its weight also pushes against the atmospheric pressure?