# Zero Psi with a potential of 2000psi????

Discussion in 'Physics & Math' started by Quantum Quack, Oct 15, 2005.

1. ### a_htRegistered Senior Member

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And when released, this system behaves like the harmonic oscillator. (plus a dampener if there is friction).

3. ### Quantum QuackLife's a tease...Valued Senior Member

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I think this is an excellent insight.....
However I am more interested in how something with zero pressure can restrain a force of 2000psi and remain zero pressure.

Pete has mentioned volume - pressure relationship but again I think this is probably delving deeper than the question asks.

It just stood out to me that even though the restraint remains at zero pressure it has generated a balancing force of 2000 psi in it's restraining of the outward force of 2000psi.
If we assume that the assembly is "still" yet restraining a force of 2000psi.
The vacuum or zero pressure has an attribute that has allowed for a counter pressure of 2000psi to exist.

It is tempting to describe this counter pressure in negative terms. But of course it is not -2000psi but a vacuum generated +2000psi.

So the outer rams have a counter force of 2000psi being applied and this is only because of our pressure of zero psi [ and of course the assemblies sturdy structure.]

So what atributes has our vacuum achieved according to physics now that it is generating a counter force of 2000 psi?

By the way I am not trying to be cute or anything like that. I am merely wanting to "ground" this issue or concern in a way that can be described in terms that current physics is familiar with.

The outer rams have sustainable potential energy of say x value.

That x value was given to them by the pump that pressurised the rams. However the potential energy has only been achieved because the vaccuum chamber provided restraint to expansion. So could our vacuum chamber be considered as having a potential energy relative to the value x?

Also if we go from a pressurised state to a non pressuriced state will the inner rams reduce their separation or stay virtualy still as the 2000psi is reduced?

NB. this thread is about clarifying the issue so that a more precisly worded thread can be generated later.

Last edited: Oct 16, 2005

5. ### Quantum QuackLife's a tease...Valued Senior Member

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I've drawn another diagram that may be a little clearer in it's intent.

<img src=http://www.ozziesnaps.com/vacram3.gif>
Diagram 3

The purple area is a piston attached to two hydraulic rams.

Is it correct to say that if we continue to pump pressure into the rams the potential energy of the assembly will also increase? [with out increasing separation of components]

The only limitation to how much pressure is placed on the system is the structural strength....yes?

but at all times our vacuum remains at zero pressure.

Yet it is acting as the counter force to expansion....
So in physics what attributes does the vacuum have besides zero pressure?

Last edited: Oct 16, 2005

7. ### DaleSpamTANSTAAFLRegistered Senior Member

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QQ, your device will fly apart as drawn. Even with 1 psi it will fly apart. Draw a free body diagram of the purple half. You have two forces pointing to the right from your pressurized pistons and nothing pointing to the left to balance them.
It offers no counter force at all. Pressures can never pull, they can only push. Vacuum has nothing in it with which to push. You are thinking that your vacuum can pull in order to hold your assembly together, but pressure does not work that way.

-Dale

Last edited: Oct 16, 2005
8. ### Quantum QuackLife's a tease...Valued Senior Member

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So you are saying that when you place one of those mobiles with a suction cap on it onto your cars windscreeen the mobile will fall off? That vacuum [ low pressure ] has no value?

ahhh!! actually I think I see what you mean. The mobile is held to the screen by ambient pressure and not the vacuum....ok?

hmmmmm....so if this device is assembled in a space vacuum you are saying that trying to expand the vacuum chamber would offer no resistance?
So in effect the volume of space in the chamber can be increased without effort being applied??

9. ### Quantum QuackLife's a tease...Valued Senior Member

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actually if you look at the arrows you will see that the outer rams force is in both directions. This assembly is floating in space and has no other restraints [ un- anchored]

Both the left and right sides of the assembly have the expanding force of 2000psi applied.

<img src=http://www.ozziesnaps.com/vacram3.gif>

Last edited: Oct 16, 2005
10. ### Quantum QuackLife's a tease...Valued Senior Member

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Dale,
To ask a more precise question:

Can the volume of space in the vacuum chamber be increased with out effort being applied?

Just in case I wasn't clear enough, the vacuum chamber is a sealed chamber and not open to ambient vacuum.

Last edited: Oct 16, 2005
11. ### DaleSpamTANSTAAFLRegistered Senior Member

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Precisely, it works in your car because there is atmospheric pressure pushing it on the outside. It would not work on the outside of a space station. A straw is another device that cannot work in a vacuum for the same reason.
Yes. There is nothing pushing against the expansion.

If your assembly were operating at sea level then there would be 1 atm pressure resisting an increase. But this is due to the outside pushing, not the inside pulling.

-Dale

12. ### Quantum QuackLife's a tease...Valued Senior Member

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excuse me for being a little puzzled by this. I would have assumed this is not the case.

So the volume of space in the sealed chamber can be expanded without resistance.

hmmm....I am going to have to think on this and seek a second opinion as this goes so contrary to what I have come to understand.....

Last edited: Oct 16, 2005
13. ### Quantum QuackLife's a tease...Valued Senior Member

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OK lets see if this makes it any easier or different.
I have drawn the following diagram of a tall frame [ at sea level] that is supporting a long pipe. The pipe is sealed at the top end. Inside the pipe is a solid metal rod that is sealed to the inner surface of the pipe. [slip fit]
A 2000 kg weight is suspended by the solid rod at the bottom which applies a force downwards on the solid rod due to gravity.

<img src=http://www.ozziesnaps.com/vacram4.gif>

Will the 2000 kg weight pull the solid rod out of the pipe if no atmosphere can enter the space at the top of the rod between the rod and pipe?
[assume that the assempbly is strong enough not to collapse under the weight]

14. ### DaleSpamTANSTAAFLRegistered Senior Member

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That seems wise to me.

The best thing to do is to educate yourself so that you can be confident in being your own second opinion. Any introductory physics text should have a relevant chapter. I also like the hyperphysics site for a basic overview. Try: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/ktcon.html#c1

-Dale

15. ### DaleSpamTANSTAAFLRegistered Senior Member

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Yes, this is a different situation because now you have ambient atmospheric pressure. You also have it aligned in such a way that the air pressure pushes up, against gravity, and there is vacuum above (no pushing from vacuum). So there is an unbalanced pneumatic force which can oppose the gravitational force. Again the air pressure on the bottom pushes harder than the vacuum pressure (0) on top, so you have a net upwards force.

Now, the question about wether it stays together or pulls apart depends on the cross sectional area of the piston. Let's call the piston area A. On the bottom surface of the piston is 14.7 psi pushing up and on the top is 0 psi pushing down, both acting over an area A. So 14.7 - 0 = 14.7 psi pressure differential, which results in a net pressure force upwards of (A * 14.7 psi). Opposing this is (g * 2000 kg) which is the gravitational force pulling down. Therefore, if (A * 14.7 psi) < (g * 2000 kg) then it will pull apart, otherwise it will stay together.

-Dale

16. ### PeteIt's not rocket surgeryRegistered Senior Member

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Note that it's not likely to stay just where it is unless the parameters of the setup are chosen carefully. If the weight doesn't fall down, it will get pushed/sucked up - because there's no force involved in reducing the vaccuum volume either.

17. ### DaleSpamTANSTAAFLRegistered Senior Member

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Agreed. If the pressure is greater then it will get pushed up all the way, if the weight is greater it will pull completely apart. It is really unlikely that they will be perfectly balanced so that it would stay where it was put.

-Dale

18. ### Quantum QuackLife's a tease...Valued Senior Member

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Thanks guys.....obviously I had it all wrong....

Just another question that comes to mind about all this:
If we have two equal sized and equal weighted hollow glass spheres. [ sea level]

One of the spheres has atmospheric pressure. the other has been vacated to near vacuum.

Question:
Do the spheres weigh the same?

19. ### PeteIt's not rocket surgeryRegistered Senior Member

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No.

The vacated sphere could be lighter than air if the glass is light enough and the sphere big enough.

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21. ### a_htRegistered Senior Member

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158
If i may ask, with what program did you draw your diagrams? They are very pretty.

22. ### Quantum QuackLife's a tease...Valued Senior Member

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freeware, Serif DrawPLus vsn 04

http://www.freeserifsoftware.com/default.asp

I found the animation ability to be really useful especially when talking Special relativity....Pete may recall all those bloody animated diagrams ...ha...best of luck!!