h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I feel kind of ridiculous asking this question, but what is air pressure exactly? Most definitions seem to imply that it's the actual physical weight of the air, and that the standard 14.7 psi means that a column of air with a 1 square inch base that extends to the outside of the atmosphere would actually weigh 14.7 pounds. If that were true, however, theoretically the air pressure would drop if you went inside because most of the air is on top of the building. Obviously air, being a gas, doesn't really "rest" on anything, but then doesn't it not make sense to measure it in pounds? (Okay, maybe I'm the only one who thinks that.) But since that is obviously false, I considered the possibility that air pressure is really the force created by collisions of molecules against an object's surface. But if THAT is true, the square inch thing sort of falls apart for me: if you took two vessels with equal capacity but different surface areas and filled them with the same amount of gas, the air pressure should be equal- but then you have the same mass distributed over different areas, so the pounds per square inch really doesn't make sense anymore. . . or am I just thinking about this too hard?

Well, regardless of whether the above makes sense or not, what I really want to know is whether an electronic balance would register negative something if one placed it in a vacuum.

I suppose all I really want to find out is the rationale behind measuring pressure in pounds per square inch.

Can someone please tell me whether I'm blithering or not?

Not blithering but a bit confused.

Air pressure is effectively the weight of the column of air on top of you. But because when air is compressed it pushes back, the air pressure doesn't get less when you go indoors. The weight of all that air on top is pushed in through all the doors and windows, so you feel it indoors as well.

Air is in fact a load of tiny particles called molecules. [These come in two main types, oxygen (about 20%) and nitrogen (about 80%), but you don't need to worry about that. It would be same if they were all identical.] These molecules are bouncing around at very high speeds, constantly on the move. When a molecule hits your hand, it bounces off, but it gives a little push to your hand. This is the pressure of the air. If all the pressure were on one side of you, the total of all these pushes would slam you up against the wall, but of course there is air on the other side of you as well, which pushes you in the opposite direction, so you don't feel anything.

Air molecules collide with each other as well, so slow ones get speeded up and fast ones get slowed down until they are all travelling at roughly the same speed on average. As you go higher up in the atmosphere, the air starts to thin out. The molecules can travel further before they collide with another molecule.

If you take an "empty" bottle, one with no liquid in it, it is full of air, at normal atmospheric pressure. You put a cork in it. The pressure inside is the same as the pressure outside. The pressure inside is not the weight of the air inside, but the weight of air above the bottle all the way to the top of the atmosphere. It's like putting compressing a spring using a giant weight, then putting the spring in the bottle without letting it expand. The energy in the spring is not just the weight of the spring, it is also the stored energy of the giant weight. In the case of air, the energy of the wieght of the atmosphere is stored in the motion of the molecules. Since the molecules move around at random, all the surfaces inside get an equal share of the pressure.

But if the bottle is big enough, you'll be able to notice a very tiny difference between the pressure at the top and the pressure at the bottom. A tall thin bottle about a mile high will have noticeably less pressure at the top than the bottom. Turn that bottle on its side, and the pressure will even out, with everywhere at the same pressure. This is because there is enough air in the bottle for its weight to make a difference.

Hope that makes some sense.

>>whether an electronic balance would register negative something if one placed it in a vacuum.

A balance does not measure the weight of the air, because the under side of the balance is being pushed upwards by the same air pressure that is pushing downwards on the upper side. So it wouldn't be any different if the whole thing was in a vacuum.

You could devise an instrument which had one part in a vacuum and another in air, and this would measure the weight of the air. Such an instrument exists - it is called a barometer and is used for measuring air pressure. If you took it into space, it would show zero air pressure.

Really, I have taken a few chemistry classes, so I'm quite aware of molecules, thank you. Perhaps I should have been more concise in my original question. (In retrospect it made me appear slightly more idiotic than I intended.)
A better question would perhaps be, whose bright idea was it to measure pressure in pounds? It doesn't seem helpful to me to say that the column of air previously mentioned would weigh 14.7 pounds, even if it's true, because most of the time one is not dealing with blocks of air sitting on things. It seems rather more productive, at least to my mind, to speak in terms of the proximity of molecules, or the force of their collisions. Okay, obviously force is measured in pounds but it seems somewhat misleading, especially when one considers Avogadro's Hypothesis about the molar volume of a gas.
And that still leaves the question about the balance in a vacuum: with no molecules to collide with the plate, wouldn't it register -14.7 pounds? I know that if you blow on the top of one it goes all wonky.

Yes, but it would be much more exciting to see it with a balance.
I had been aware of barometers also, by the way.

Maybe I should just go drink some coffee or something instead.

Force isn't measured in pounds. It's measured in Newtons. The SI unit of pressure is Newton per square metre (or Pascal).

B

Well, by actual scientists, yes, but I mean like "15 foot pounds of torque" or that sort of thing. Laypeople measure force in pounds occasionally.

I wasn't trying to talk down to you, I was just trying to make sure you understood. If there was anything in there you already knew, then that's great!

Air pressure is not measured in pounds, it is measured in Pascals, which are Newtons per square metre. Since standard air pressure is very close to 100,000 pascals, they invented a new unit called the bar which is exactly 100,000 pascals. This is divided into millibars. This is the unit that is normally used in weather reports etc.

Sorry, simulpost there.

Barometers normally measure pressure in bars and millibars.

But it is true that when pumping tyres, most people use pounds per square inch. The measurement in pounds predates the discovery of molecules.

In some other corners the atmospheric pressure is most often measured in mmHg. And in gas stations they go for atm (standard atmosphere) when checking tyres.

Actually, in chemistry we mostly used atmospheres, one atmosphere being standard pressure, which at the time I thought was rather an unnecessary simplification but now seems more appropriate considering how many alternative units there seem to be.

So 1 atm = 1 bar then?

And what's that in Planck units? (You don't have to answer that)

http://en.wikipedia.org/wiki/Atmosphere_(unit)

It refuses to take the last ) into the hyperlink.

"I feel kind of ridiculous asking this question"

Remember, there are no stupid questions - only stupid people.

"but what is air pressure exactly?"

It's the pressure (force per unit area) exerted by the air in the atmosphere.

"Most definitions seem to imply that it's the actual physical weight of the air"

They imply that because it's very very close to the truth.

"and that the standard 14.7 psi means that a column of air with a 1 square inch base that extends to the outside of the atmosphere would actually weigh 14.7 pounds."

Yup. Spot on.

"If that were true, however, theoretically the air pressure would drop if you went inside because most of the air is on top of the building."

Ah, no. An analogy - if you're scuba diving, you experience the pressure of the water around you - it makes your ears pop. But the pressure at a given depth acts equally in all directions at that depth - it doesn't matter if your ear is pointing up or down or north or south, you feel the same pressure. And this applies even if you go inside a wreck - you don't suddenly get popping ears.

Consider this, also: if the air pressure inside a building WAS significantly lower than the air pressure outside, what would happen when you opened the door?.....


...


All the air outside would rush inside and equalise the pressure. Difference of pressure is the driving force for flow. Difference of pressure is literally what makes the wind blow.

Another way of visualising what pressure actually IS is to consider the ideal gas equation:

PV = nRT.

Rearrange that and you get:

P = nRT/V

Now, in that equation:

P = pressure
n = number of molecules
R = the ideal gas constant (i.e. a fudge factor)
T = temperature
V = volume

So... for a given volume of gas - say a litre - at a given temperature - say 20 C - what is the pressure?

R is a constant. V is a constant. T is a constant. Call those lot together k for konstant.

P = kn.

The pressure is directly proportional to the number of molecules present. In a very real sense, the pressure IS the number of molecules present. More molecules = more pressure.

"Obviously air, being a gas, doesn't really "rest" on anything, but then doesn't it not make sense to measure it in pounds?"

Pressure is measured in psi (pounds per square inch). I.e. a weight acting over an area. The area thing is important.

It's also worthwhile mentioning that chemical engineers (such as me) make a distinction between psia and psig - which is to say psiAbsolute - number of pounds per square inch in total, and psiGauge, which is number of psi above atmospheric pressure.

"I considered the possibility that air pressure is really the force created by collisions of molecules against an object's surface."

Absolutely correct. Well done.

"But if THAT is true, the square inch thing sort of falls apart for me: if you took two vessels with equal capacity but different surface areas and filled them with the same amount of gas, the air pressure should be equal- but then you have the same mass distributed over different areas, so the pounds per square inch really doesn't make sense anymore. . . or am I just thinking about this too hard?"

Not at all. You're on a VERY good route, just keep thinking about it.

For instance... consider this: you talk about two different vessels with different surface areas but the same capacity.

Turn that around - consider, for a fixed surface area, what is the MAXIMUM capacity you can achieve? And then have a think about why bubbles might be the shape they are... (or gas storage tanks at oil refineries, for that matter)

"what I really want to know is whether an electronic balance would register negative something if one placed it in a vacuum."

Depends where/how you calibrate it.

Let me rephrase your question: if I put a sealed container full of air at atmospheric pressure on an electronic balance, and then pumped the air out, would the balance reading go down?

Answer: yes, it would. Air has weight.

"I suppose all I really want to find out is the rationale behind measuring pressure in pounds per square inch."

The rationale is that a FORCE is being applied across a given AREA.

Other things being equal (i.e. temperature and volume), the amount of force, per unit area, is a direct measure of how much gas is present.

Hope that helps.

SoRB

"So 1 atm = 1 bar then?"

No. One atmosphere is 1.01325 bar, as standard, although ACTUAL atmospheric pressure varies with the weather. For instance, where I am today atmospheric pressure is 1.01900 or so, predicted to rise to 1.02200 by Friday.

SoRB

Newton is a unit of force (the SI unit), pound is also a unit of force though. Probably the easiest and most direct measure of "pressure" is to measure the force exerted on a given area. Hence the use of "pounds per square inch". The confusion appears to stems more from the use of "pound" as a unit of force (rather than weight), more than anything to do with pressure, I think.

If you say the pressure in a can is 100 psig, and the top of the can is 2 square inches, then the force on the top of the can is the equivalent of a 200 pound weight.

out of curiosity, I looked up where bar comes from, it's the cgs (centimeter-gram-second) unit of pressure. 100 000 pascals = 1 bar.

""
" and that the standard 14.7 psi means that a
column of air with a 1 square inch base that
extends to the outside of the atmosphere would
actually weigh 14.7 pounds.
"

Yup. Spot on.

""

Is that true? The weight of the air is [mass of the air]*[acceleration due to gravity]. 14.7 pounds-force = 65.38 Newtons, implying that there is a mass of air of 65.38 / 9.8 = 6.67 kg for every square inch of surface area --> 1 sq-in = 6.45 sq cm ==> mass of about 10 000 kg per sq-metre of the earth's surface area. Density of the air is reckoned at about 1.3 kg per cubic metre (according to one website). This implies the atmosphere is about 7.7 km deep? I saw quotes that 99% of the air is in 30km of atmospheric depth? About an order of magnitude difference? Does the density of air increase significantly at any particular altitude? Or is my maths wrong?

By "weight of air" do (they) mean the *measured* weight of air, per square inch... If you had a closed piston, with the volume of the piston in vacuum, and the piston being pushed out by a balance of somekind (ie, a spring), then if the end of the piston was 1sq-inch, then you would measure 14.6 pounds-force on the end of the piston, because you'd measure 14.6 psi * 1 si = 14.6 p. But that's not really the same as weight, is it? Or is it? How do you measure the mass of a volume of gas?

If you put a section of gas in a box, could you weigh the box? (ie, and then subtract the weight of the box, etc..) Does the added downward velocity due to gravity increase the pressure experienced on the bottom of the box in order to allow you to measure the mass of the gas? Is the problem related to bouyancy? Probably... ???

PS. Apparently pound is strictly mass, and for a force the unit is "pound-force"... I had to learn this when I moved to a lab in the US!