Pressure and fluids

  • #1
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Homework Statement


Hi, this morning in the school lab we did experiments about pressure and I've some questions., could you help me? thanks!

If I fill a glass with water the atmospherical pressure press on water on top of glass because it is in contact with air, but does atmospherical pressure press on the water on bottom of glass ?
so to calculate pressure I must include the atmosperical pressure on Stevino's formula?
If mustn't, how can atm. pressure pass through the glass material and why if there is no air on the bottom?

When i pump out air from a bell jar do I progressively reduce the atmospherical pressure inside bell?
what happen if pumped out ALL the air inside (so there is no atmosperical pressure) i continue pumping out?

Ancient greek used to weigh a container full of air then squeeze it and weigh it again. So why if squeezing it the air go out the weight is the same in both ?


Thanks!


Homework Equations


P = p0 + d * g * h


The Attempt at a Solution


In my opinion in the first problem atmospheric pressure must be considerate, in the other two i've no idea.
 

Answers and Replies

  • #2
rock.freak667
Homework Helper
6,230
31

Homework Statement


Hi, this morning in the school lab we did experiments about pressure and I've some questions., could you help me? thanks!

If I fill a glass with water the atmospherical pressure press on water on top of glass because it is in contact with air, but does atmospherical pressure press on the water on bottom of glass ?
so to calculate pressure I must include the atmosperical pressure on Stevino's formula?
If mustn't, how can atm. pressure pass through the glass material and why if there is no air on the bottom?

Hi scientifico, I am not sure what is "Stevino's formula", but I assume that is the equation you posted in the relevant equations section. Yes, you need to include atmospheric pressure since at the bottom of the glass, the fluid is exerting a pressure in addition to the atmosphere exerting a pressure at the top of the liquid (remember how pressure is transmitted equally through a fluid?).

When i pump out air from a bell jar do I progressively reduce the atmospherical pressure inside bell?
what happen if pumped out ALL the air inside (so there is no atmosperical pressure) i continue pumping out?

Yes you reduce the pressure inside the glass. (I think it is best to only use atmospheric pressure when you refer to the outside, just use pressure when you refer to pressures in objects, makes it easier to understand in my opinion).

After some point the pressure inside will be too small and most likely your jar may break ("implode").


Ancient greek used to weigh a container full of air then squeeze it and weigh it again. So why if squeezing it the air go out the weight is the same in both ?

You mean they removed air from a container and saw that the container weighed the same as the container + air?:confused:
 
  • #3
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Yes i meant the one in "Relevant equations", because I don't know how is called in english!

So doesn't exist a kind of "negative pressure"? After I remove all the air from jar we couldn't anymore speak of reduction of atmospherical pressure because there is no air, right? so what is that?

I meant something like you take a plastic bottle with air and you weigh it. you squeeze it in the way there is as minus air as possible inside and you weigh it again discovering they have the same weight.
If more of air was removed by squeezing it, why they have the same weight?


thanks!
 
  • #4
rock.freak667
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Yes i meant the one in "Relevant equations", because I don't know how is called in english!

Ah right then, I never learned that with a name.

So doesn't exist a kind of "negative pressure"? After I remove all the air from jar we couldn't anymore speak of reduction of atmospherical pressure because there is no air, right? so what is that?

Yeah you can think of it as negative pressure. Initially, with the jar the pressure inside the jar is P and is equal to the atmospheric pressure. As you pump air out, P < Patm. At the point where you push all the air out of the jar P= 0.

I meant something like you take a plastic bottle with air and you weigh it. you squeeze it in the way there is as minus air as possible inside and you weigh it again discovering they have the same weight.
If more of air was removed by squeezing it, why they have the same weight?

In theory it should have less weight.

If the volume of the bottle is 250 ml or 250(10-6) m3 and the density of air is 1.1644 kg/m3 at 30C, the mass of air is 291.1(10-6) kg.

The mass of the plastic itself is higher than that so usually if you weigh it, you're mostly weighing the weight of the plastic in both cases. (In the first case, the mass of air is already smaller, in the second case the volume of air is smaller)
 
  • #5
gneill
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Heh. What you want is to remove the air from the container leaving it at the same external volume. Otherwise, if you remove the air by crushing, you're also reducing the container's buoyancy in air, and by the same amount as the weight of the air that you've removed!.
 
  • #6
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Otherwise, if you remove the air by crushing, you're also reducing the container's buoyancy in air, and by the same amount as the weight of the air that you've removed!.
Yes, our phisics teacher said somewhat like that but I didn't understant in that moment! :approve:
The minus an object occupe a volume the minus it receive buoyancy in a fluid, right?

So, when i reach P = 0 in a jar isn't possible pump out somethink? the only think happen is the jar implosion?


Thanks!
 
  • #7
gneill
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When the jar is empty, it's empty. There's nothing left to pump out. Absolute pressure is zero. There are no negative absolute pressures.
 
  • #8
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so the jar should implod in a pressure between 0 and atmosherical pressure (if it support atmosperical pressure), right?
 
  • #9
gneill
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so the jar should implod in a pressure between 0 and atmosherical pressure (if it support atmosperical pressure), right?

Not if it's properly made. Atmospheric pressure is only about 14.7 psi. Your car tires withstand a greater pressure differential, from the inside outwards, and they're made of rubber!
 
  • #10
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why do atmospheric pressure exist? Where is from that force? gravity?
However, continuing pump out in a certain point jar should implode, right?

Our teacher said he could put in the test an exercise about a full of liquid cilinder with a stopper on the bottom where i must calculate somethink considering the mass m of the stopper, in your opinion what can i calculate having the stopper mass? it's about boyounce?


Thanks!
 
  • #11
gneill
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why do atmospheric pressure exist? Where is from that force? gravity?
Right. It's due to the weight of the air above you.

However, continuing pump out in a certain point jar should implode, right?
No. It'll only implode if there's a fault in the glass, or if the bell jar was improperly constructed.

Our teacher said he could put in the test an exercise about a full of liquid cilinder with a stopper on the bottom where i must calculate somethink considering the mass m of the stopper, in your opinion what can i calculate having the stopper mass? it's about boyounce?
Thanks!

It would depend upon the specifics of the geometry of the stopper, and how it was seated in the hole in the bottom of the cylinder (how much of the stopper is in the water versus how much is below the bottom and/or in the air below). He'll probably ask something about how much force it would take to pull the stopper out, or what the different forces are that are acting on the stopper.
 
  • #12
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if gravity attract air molecules, why don't they precipitate on the floor?
 
  • #13
gneill
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if gravity attract air molecules, why don't they precipitate on the floor?

That's what vacuum cleaners are for! :biggrin:

If their temperature were absolute zero, then they might precipitate to the floor (and freeze solid there). But their temperature is not absolute zero, so they bounce around and pretty impressive velocities, causing pressure, expansion,... the usual PV = nRT stuff.
 
  • #14
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so if the velocity between molecules is more strong than terrestrian gravity (9,8) why doesn't atmosphere of Earth disperse?
 
  • #15
gneill
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so if the velocity between molecules is more strong than terrestrian gravity (9,8) why doesn't atmosphere of Earth disperse?

Velocity is not force. Why do balls bounce and not stick to the floor? Why do bouncing balls only go so high and not keep going and leave the Earth?
 
  • #16
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yes but crushing continously between themself they produce a force more strong than terrestrian gravity that can make molecules leave Earth, right?
 
  • #17
gneill
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yes but crushing continously between themself they produce a force more strong than terrestrian gravity that can make molecules leave Earth, right?

Have a http://astro.unl.edu/naap/atmosphere/atmosphere.html" [Broken]. It discusses and has demonstrations concerning atmospheric retention for planets.
 
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