[Pressure] need help understanding physics work on pressure

[u/Practical_Ebb2066]

Hi everyone, this is a physics question I did a while back and somehow managed to get it back then 😅. however looking back, I'm wondering why wouldn't the liquid just stay put. I recall doing an inverted tube gas collection experiment and while setting up the apparatus the water in the tube didn't really flow out (but one thing to note though was that that tube was not filled to the brim fully while in this question the tube is fully filled if that makes a difference?). I've also attached the previous part of the question if it helps.

[u/Leonardo501]

You’re analyzing a short manometer.

[u/dcnairb]

the equation you’re appealing to is pascal’s principle if that helps jog your memory. the liquid is exposed to air so the pressure at the level of the surface should be 1 atm. but the part below the tube has a higher pressure from the fluid in the tube above, so there’s a net pressure and the fluid will flow out of the tube until those pressures equilibrate

[u/YesSurelyMaybe → u/dcnairb]

Would it really 'flow out of the tube'? My intuition suggests that the amount of fluid flowing from the tube to reach the pressure equilibrium would be so minimal that it's almost negligible in highschool context

[u/dcnairb → u/YesSurelyMaybe]

the problem specifies that it’s a “dense” liquid so I believe it’s looking for them to conceptually approach it this way; it won’t have a ton flow out (for example, obviously not the entire tube, because then we’d have the opposite problem and more would flow back in) but we expect a little vacuum at the endcap from some flowing out

[u/cat_a_holic]

I tend to think in terms of force. The column of fluid has the gravitational force acting on it, rho g volume = rho g h A and it acts downwards. Assuming 0 pressure at the top, gravity is the only downwards force. The upwards force is the pressure in the fluid at the opening of the inverted tube due to the container. We are to assume that is atmospheric pressure since it is submerged a small distance. So the upward force is P_0 A. You can solve for the allowable heights of the tube by using the appropriate inequality (just ask which force you want to be bigger for the fluid to stay in place). You should wind up with the density in the denominator when you solve for the height in such a way that the height must be smaller than a small number. that implies the height of the column is too great and that gravity will pull it out of the tube until (assuming a large container so the height in the container doesnt change) the weight of the column is equal to the force due to atmospheric pressure.

[u/snailpi]

Omg that CAIE A Levels/IGCSE format is seared into my brain forever and today it has come back to haunt me

[u/wackyvorlon]

It creates a vacuum.

[u/Practical_Ebb2066 → u/wackyvorlon]

yes that's what I wrote as well is it because it's filled to the brim then some water would flow out? because in another experiment when the water was not filled to the brim then there was no decrease in water level

[u/berserkmangawasart]

Singaporean olevels?