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Thread: Atmospheric Pressure

  1. #1 Atmospheric Pressure 
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    I'm a little confused on how it works. From what I understand, it's just the gravitation force upon us from the air. So...

    Why aren't we crushed by atmospheric pressure?

    Why doesn't having a roof above me effect atmospheric pressure?

    Why does atmospheric pressure affect all sides of something, and not just the top? Or does it only affect the top?

    As you increase in altitude, atmospheric pressure drops because there's less air above and around you, right?

    Thanks.[/tex]


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  3. #2 Re: Atmospheric Pressure 
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    Quote Originally Posted by brushman
    I'm a little confused on how it works. From what I understand, it's just the gravitation force upon us from the air. So...

    Why aren't we crushed by atmospheric pressure?
    Because, with minor variations, it's environment we've evolved to live in ...
    if you went to the ocean floor without a deep-sea submersible, you would be crushed - and yet there are creatures living there for which that pressure is normal; bring one of those to the surface without a pressurised container, and it would be like one of us going into space without a ship or suit ...

    There are people living at very high altitudes in the Andes and Himalayas, but if I tried it, I'd be gasping for breath and keeling over ...

    as I understand it, it's not the gravitation force upon us from the air, but gravitation from the Earth causing the mass of air to remain at the surface - under normal circumstances, the kinetic energy of the molecules which make up our atmosphere is not enough to accelerate the molecules to "escape velocity" - what we experience as air (or barometric) pressure is the mass of the column of air above us ...


    Quote Originally Posted by brushman
    Why doesn't having a roof above me effect atmospheric pressure?
    It does, but you might not notice the variation in most circumstances -
    eg: if the roof is part of an airtight structure (similar to a modern passenger aircraft, submarine, or decompression or biohazard chamber), then the air pressure within can be substantially different to the outside environment ...

    eg: even in a normal house, during summer if you have ducted evaporative cooling (or airconditioning) and don't have sufficient venting, you would notice an outrush of air when you open a door or window - in winter, you might notice a blast of cooler air entering the house when someone opens the door ...

    if it's only a roof (no walls, etc), the difference is much reduced due to continuous lateral exchange of moving air, but it's not zero ...

    Quote Originally Posted by brushman
    Why does atmospheric pressure affect all sides of something, and not just the top? Or does it only affect the top?
    Why it affects all sides is due to the (kinetic?) property of gases, and I'll field a more complete explanation to someone better acquainted with such things ...


    Quote Originally Posted by brushman
    As you increase in altitude, atmospheric pressure drops because there's less air above and around you, right?
    Sounds right


    Nature abhors perfection; cats abhor a vacuum.

    "I don't know; I'm making it up as I go ..." Dr H Jones (Jr).
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  4. #3 Re: Atmospheric Pressure 
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    sorry to break in cran but allow me to elaborate a bit.

    Quote Originally Posted by brushman
    I'm a little confused on how it works. From what I understand, it's just the gravitation force upon us from the air. So...

    Why aren't we crushed by atmospheric pressure?
    We are not crushed because the pressure inside our bodies pushing outward is balanced with atmospheric pressure pushing inward. We can tolerate a certain amount of variation in pressure outside as long as pressure does not change too fast so our inside pressure can adjust (we get ever so slightly bigger or smaller). Ever ridden in an airplane? Notice how your ears pop and click to balance the pressure in your inner ear vs your outer ear? Works the same on a fast elevator. Go to the bottom of a deep pool and your inner ear will complain loudly about the sudden change in pressure outside vs. inside. Divers have to stop every 15 feet or so to allow the pressure to adjust. Go too deep and you won't be crushed because the pressure adjusts, but your blood chemistry will be all messed up due to the high pressure and you will die.

    Why doesn't having a roof above me effect atmospheric pressure?
    Because there are leaks in the roof and the walls and the floor so the pressure balances. Go in a vaccum chamber and it will keep the pressure out.

    Why does atmospheric pressure affect all sides of something, and not just the top? Or does it only affect the top?
    All sides. This is because mass, when influenced by a force including gravity will go into motion unless an equal but oposite force holds it back. Thus each small particle of air is pushed back and forth until the forces balance on all sides.

    Wind is the result of slight imbalances in atmospheric pressure.

    As you increase in altitude, atmospheric pressure drops because there's less air above and around you, right?
    Right

    Thanks.[/tex]
    You're wlecome.
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  5. #4 Re: Atmospheric Pressure 
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    Quote Originally Posted by brushman
    I'm a little confused on how it works. From what I understand, it's just the gravitation force upon us from the air. So...

    Why aren't we crushed by atmospheric pressure?
    Because it only about 15 psi and because there is internal pressure in our bodies as well.

    Quote Originally Posted by brushman
    Why doesn't having a roof above me effect atmospheric pressure?
    Because air is a fluid and uour house is not hermetically seald.

    Quote Originally Posted by brushman
    Why does atmospheric pressure affect all sides of something, and not just the top? Or does it only affect the top?
    Because air is a fluid. See Pascal's principle. http://en.wikipedia.org/wiki/Pascal's_law

    Quote Originally Posted by brushman
    As you increase in altitude, atmospheric pressure drops because there's less air above and around you, right?
    Because there is less above you. All that counts is the 'head".
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  6. #5  
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    "Pressure is the mass of the column of air above us" might be a bit misleading. It's a handy way to reckon air pressure. But of course an umbrella changes nothing.
    A pong by any other name is still a pong. -williampinn
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  7. #6  
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    it keeps the rain off my glasses, Pong ...

    cypress, no need to apologise - it's not a monopoly ...

    I'm not much on human physiology, but I was led to understand that much of our bodies are slightly over-pressurised with respect to the atmosphere ... and that's why our lungs don't deflate (collapse) unless punctured ... ?
    Nature abhors perfection; cats abhor a vacuum.

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  8. #7 Re: Atmospheric Pressure 
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    Quote Originally Posted by cypress
    Why doesn't having a roof above me effect atmospheric pressure?
    Because there are leaks in the roof and the walls and the floor so the pressure balances. Go in a vaccum chamber and it will keep the pressure out.
    This is not strictly accurate. Because the pressure inside the chamber was 101kPa or so when it was sealed, it would stay at this pressure after being sealed. At this point, the pressure is coming from the air molecules being kept squashed together by the inside of the chamber. The force behind the pressure is random kinetic energy of the molecules making up the air, rather than its weight acting on you. The only role of gravity in the atmosphere is in preventing the atmosphere from leaving the planet altogether: gravity does not cause the pressure, it only maintains it.

    If you were to heat the vacuum chamber up, the pressure inside would increase proportionally to the heating.
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
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  9. #8 Re: Atmospheric Pressure 
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    Quote Originally Posted by drowsy turtle
    The only role of gravity in the atmosphere is in preventing the atmosphere from leaving the planet altogether: gravity does not cause the pressure, it only maintains it.
    This perspective is a bit in the eye of the beholder.

    You can calculate the pressure at a point in the atmosphere in terms of the "head" of the fluid above that point. That is entirely due to gravity. All you need to do the calculation is a density profile and the gravitational acceleration.

    In the case of water, which can be accurately approximated as incompressible the pressure is just where is dentsity is gravitational acceleration and is height (or depth). The same basic principle applies to air pressure, but the density is not a constant because air is fairly compressible. So, with this perspective, gravity is the main driver for presssure

    On the other hand air does behave nearly as an ideal gas and the ideal gas law provides a pretty good model where is pressure is volume is the number of moles of gas in the volume and is temperature. Density is implicit in so, all that you need to determine pressure is density, effective molecular weight and temperature. Gravity plays the role here in determining density, by applying a force that is conter-balanced by the forces that are induced thermally. In essence gravity keeps the gasses from escaping because the speed of thermal motion is less than escape velocity, which is determined by gravity.

    In either case gravity is required. If there were no gravity there woould be no pressure, and indeed no atmosphere. If gravity were greatly reduced then density would decrease, specific volume would increase and pressure would decrease.

    So, bottom line, gravity is an essential element in the determinatin of air pressure. The word "cause" is not really applicable. There are several factors that determine pressure, and gravity is as important as any of them.
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