What is normal air pressure in N/cm^2?

[russ_watters]

Even your simple straw-example gets me puzzled. A tall straw would mean a "high" weight of air that I need to suck before I can get me any air. But what has this to do with pressure? The straw has a tiny area and if the pressure is constant regarding my inhale performance, the force on the (weight of the) air inside the straw will be low, thus making it hard to breath. Because in the same time I could take a M5 nut and breath through it's hole without problem.

Sorry, I worded that wrong. What I meant to say was suck water through a really tall straw. It is a good way to demonstrate just how weak our lungs are/how little pressure difference they can deal with.

The opposite side of that coin is a tall snorkel under water. Since the air inside the snorkel is at atmospheric pressure, the pressure in your lungs will be a little lower than the pressure outside your lungs (from the water pressing in). You don't have to go very deep for it to become impossible to breathe through a snorkel.

 

[dauto]

Finally I just want to state the most interesting part of what I've learned (along with the pressure difference part):

1) Water pressure increases 1 atm per 10m (thank you SteamKing)
2) Neither pressure nor density is linear above some 5km (temperature varies too).
2) The atmosphere is as high as 100km which is a full magnitude higher than I thought and linearly calculated
3) Scuba divers suffer from drunkness after only a few atm and thus breaths Helium instead of Nitrogen (thank you dauto).

Roger

On point 2). Atmospheric pressure and density aren't linear below 5km either. An exponential decay is a much better description.

 

[rogerk8]

On point 2). Atmospheric pressure and density aren't linear below 5km either. An exponential decay is a much better description.

Sorry, but viewing this picture https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Pressure_and_thickness tells me that you are wrong.

Both pressure, density and even temperature (from some 280K@sealevel to 220K@10km) seams quite linear to me. Point 2) height should even be changed to 10km.

Considering the common expression for pressure in a "closed" system above and the two me's I have come to the conlusion that the differential pressure for our lungs are around +/-0,1atm and thus a full magnitude higher than my preliminary guess.

The way I have calculated this is:

p\propto 1/V

Esimating our lungs volume to be some 5L, the rest breathing being some +/-0,5L and the normal pressure being 1 atm we roughly get +/-0,1atm.

Now, my deep water me experience 500atm and we both need the same amount of molecules. This wile the much higher pressure ensures higher molecular density (due to V and for simplicity, T being the same). This in turn means that the deep water me needs 500 times lower relative pressure for the same amount of molecules. And we wind up with the same differential breathing pressure. Hope this is right.

One last puzzling but extremely trivial example. This is however an example where pressure is not omnidirectional which I think is important to point out.

Consider a one ft plastic tube of say 1cm^2 cross sectional area, A. We thus have a pea-tube for shooting peas. Pressing our exhale into the tube and omitting the leakage around the pea, the (omnidirectional) pressure is suddenly turned into a force (of direction) by p*A.

Rediculous example, yet interesting somehow

Roger
PS
What is actually the use for

p=nkT≈nE_k[J/m^3]

when it comes to plasma physics?

What does p help us understand/enable?

 

[dauto]

Sorry, but viewing this picture https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Pressure_and_thickness tells me that you are wrong.

Both pressure, density and even temperature (from some 280K@sealevel to 220K@10km) seams quite linear to me. Point 2) height should even be changed to 10km.

Sorry, but actually solving the equations should tell you that I'm right. Do not rely on simple visual inspection of a sketch.

 

[rogerk8]

Does not the skin of the fish deep down in water suffer from the high pressure?

Let's say that the fish lives down at 5km with 500atm of pressure.

This pressure is both outside of its body and inside of its body so it summarizes to zero.

But how about its skin?

It has got to be affected by some force like p*A?

Or?

Roger

 

[Vanadium 50]

Why is the skin any different than any other part of its body?

 

[rogerk8]

The thought is that pressure from the outside of its skin is pressing equally much from the inside of its skin.

And putting one finger of my left hand against one finger of my right hand with increasing force/pressure makes me think that that guys skin is rough.

Don't you think so too?

Roger

 

[Vanadium 50]

I ask again. Why is the skin different from any other part of its body?

 

[rogerk8]

I totally get it now.

Consider a lidless and bottomless box of fragile ginger bread (that we say is water resistant). Let's place this fragile frame at a table (1atm) first. Being causious it will stand unharmed.

Now, moving it slowly down into water it will remain unharmed regardless of depth and therefore pressure!

It is not until we move it at a certain speed (especially sideways) according to

p=1/2\rho v^2
that we generate an additional pressure component which will destroy the frame.

In other words, ambient pressure has no effect whatsoever (it just states that there is some fluid available). It's the density of the fluid and speed of the object, in this case, that has effect. More generally, "felt" pressure is indeed differential.

Roger and out

 

[lendav_rott]

We're simply used to 1 atm, hence we don't feel it. If you can, go to few hundred meters below the ground level, you'll start to feel something's different :D

 

[russ_watters]

We're simply used to 1 atm, hence we don't feel it. If you can, go to few hundred meters below the ground level, you'll start to feel something's different :D

Not correct. You need to reread the thread.

 

[B737]

Hello I'm a pilot
I was discussin with a friend about the center of gravity of an aircraft n we didn't agree
This is my question :
We all know that the center of gravity moves forward and aft the question is
Does the center of gravity moves up and down??
Lookin forward for a reply

ThxSent from my iPhone using Physics Forums

 

[dauto]

Hello I'm a pilot
I was discussin with a friend about the center of gravity of an aircraft n we didn't agree
This is my question :
We all know that the center of gravity moves forward and aft the question is
Does the center of gravity moves up and down??
Lookin forward for a reply

Thx


Sent from my iPhone using Physics Forums

You should start a new thread and make the question a bit more clear.