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

russ_watters
Mentor
If normal air pressure was 100 times higher, my lungs would have to push my chest with 100 times higher force. And this is the "only" difference. I hope this is correct?
No. This is getting frustrating because you are not listening: pressure DIFFERENCE.

I'm sorry but it is frustrating for me too because I simply do not understand.

Please explain to me like I was a child why the enourmously higher pressure at 5km down the sea doesn't even matter for the fishes and it what way it does.

Because if it's only about pressure difference why even point out the high pressure that surrounds the deep water fishes?

I can for sure understand that if the pressure on either side of for instance a curtain is the same, the curtain will remain still (untill I poke it). So yes, the pressure difference here is zero.

Best regards, Roger

I'm sorry but it is frustrating for me too because I simply do not understand.

Please explain to me like I was a child why the enourmously higher pressure at 5km down the sea doesn't even matter for the fishes and it what way it does.

Because if it's only about pressure difference why even point out the high pressure that surrounds the deep water fishes?

I can for sure understand that if the pressure on either side of for instance a curtain is the same, the curtain will remain still (untill I poke it). So yes, the pressure difference here is zero.

Best regards, Roger
The fish, like the curtain, remains still. the pressure doesn't crush the fish because the inside of the fish is also at high pressure. Pressure, nevertheless, is very important because of subtle physiological effects. Notably, the amount of air dissolved in your blood varies with pressure. Just a few extra kg/cm2 will dissolve enough nitrogen in your blood to get you drunk. That's why scuba divers breath helium (plus oxygen, obviously) instead of nitrogen.

russ_watters
Mentor
I'm sorry but it is frustrating for me too because I simply do not understand.

Please explain to me like I was a child why the enourmously higher pressure at 5km down the sea doesn't even matter for the fishes and it what way it does.
[snip]
I can for sure understand that if the pressure on either side of for instance a curtain is the same, the curtain will remain still (untill I poke it). So yes, the pressure difference here is zero.
It seems like you understand it fine, you just keep forgetting or just don't want to believe it. I don't know what to tell you other than to keep rereading the thread and keep saying it to yourself until you stop forgetting and/or start believing it.

Or go for a swim and test it. Or try to suck air through a really tall straw.

Ultimately there isn't all that much we can do if you just choose not to accept it. We can teach you, but learning is completely up to you.
Because if it's only about pressure difference why even point out the high pressure that surrounds the deep water fishes?
You are the one who pointed it out!

It seems like you understand it fine, you just keep forgetting or just don't want to believe it. I don't know what to tell you other than to keep rereading the thread and keep saying it to yourself until you stop forgetting and/or start believing it.

Or go for a swim and test it. Or try to suck air through a really tall straw.

Ultimately there isn't all that much we can do if you just choose not to accept it. We can teach you, but learning is completely up to you.

You are the one who pointed it out!
I'm getting to become a believer now.

If I'm wrong at this one, you might as well close the thread

Imagine two me's. One has adapted to 1atm the other to 500 atm. We both have lungs. The one deep below water can somehow extract oxygen with his lungs too. We both need to differentially change the pressure to breath. A childish estimate for us both would be some +/-0,01atm (which equals +/-7,6mm Hg, a unit I have come to like due to Kashishi above). This change of pressure is thus the same for us both in spite of totally different ambient pressure.

So far, so good (I think).

Looking at the actual breathing it is getting harder.

$$p=n_mRT=nkT≈nE_k$$

were the last rough equality struck me not until yesterday.

Anyway, while n=N/V both N and V may change. Taking a "fast" breath I recon N is constant while the sucking is created due to increasement of volume and after a short while assimilated to 1atm (i.e zero difference) due to particle flow and thus pressure equalisation. Hope I'm right here.

I don't know why this pressure business is so hard to understand but I think it's because it's a scalar and thus omnidirectional.

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'm so stupid!

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).

Best regards, Roger

russ_watters
Mentor
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.

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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).

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

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

Best regards, 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?

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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.

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.

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

Or?

Best regards, Roger

Staff Emeritus
2019 Award
Why is the skin any different than any other part of its body?

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?

Best regards, Roger

Staff Emeritus
2019 Award
I ask again. Why is the skin different from any other part of its body?

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

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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
Mentor
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

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??

Thx

Sent from my iPhone using Physics Forums

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??