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Pressure and gravity

  1. Feb 25, 2010 #1
    Hey frndz,

    Can anyone explain me what is atmospheric pressure? How it acts? And is there any relation between Atmospheric pressure and Gravity........:confused:

    What i have in my mind is that Atmospheric pressure is the Weight of gas in the atmosphere...so the pressure at any surface is the weight of air above that surface. In this case if a box open from one end is kept on flat surface in inverted position, then the pressure inside the box must be very very less as compare to atmospheric pressure.:confused:

    I think my understanding of pressure is wrong and i need to correct it.
    Please help.
     
  2. jcsd
  3. Feb 25, 2010 #2

    ZapperZ

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    If you just think about it some more, you can falsify your own idea. Those people in the ISS are also breathing in ~1 atm. of air pressure. Yet, there's very little gravity there! So it can't be the weight of the gas.

    Zz.
     
  4. Feb 25, 2010 #3

    russ_watters

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    Well the first part is right, but the second part is wrong. Atmospheric pressure is the weight of a column of air above a certain area. But when air is in a closed box, it is no longer exposed to the atmosphere so pressure can be anything. However, if the box is not sealed (just an open, inverted box)....
     
  5. Feb 25, 2010 #4
    Max sky,
    there is indeed a relationship between gravity and atmospheric pressure. Gravity attracts matter with mass, in this case gas. The atmosphere being a layer of gas, the stronger the gravity, the higher it's pressure.I
    If you seal gas in a tank, and increase it's mass (by adding more gas) the pressure will increase. Now imagine that the tank is the earth, and gravity is exerting a down force on the gas... It's like water. Although it can't be compressed, water exerts pressure because of gravity, the deeper you go the higher the pressure. Now imagine the atmosphere pressure on Jupiter....
     
  6. Feb 25, 2010 #5
    Thanx for the response....

    But if the the pressure is the weight of air column, then why it is not affected by gravity..?

    also for the box example, consider a room. If an open room is sealed from all sides without pumping or removing air from inside, the pressure inside the room must fall...as the height of the air column is now the height of the room...????

    correct me if i m wrong...
     
  7. Feb 25, 2010 #6
    The pressure inside the room will remain at the same pressure as when you first sealed it.
     
  8. Feb 25, 2010 #7
    Atmospheric pressure is not affected by gravity because gravity is constant value(almost, because is too small difference between gravity force on equator and earth poles, but it is constant value for coordinates what are taken in consideration,for example: your city ) and is affected by other environment conditions who are correlated. In this keys for atm pressure are temperature and air humidity. Because on different temperature air will have different density and can take different quantity of water in it(named air humidity),and that will cause different weight of air, as result of that condition in correlation with constant surface size giving us different value for atmospheric pressure. As we know, pressure is weight/surface, for example: pound/square inch (psi).Usually for calculations if you don't have specific values, you taking 68*F for temperature and 101325 Pa for pressure who are called Normal(Standard) conditions.
     
  9. Feb 25, 2010 #8

    russ_watters

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    I'm not sure what you are getting at - weight is a function of gravity.
    A closed room is not "the atmosphere" so the concept of atmospheric pressure no longer applies as soon as you seal it. A closed room is simply a normal pressurized container.
     
  10. Feb 25, 2010 #9

    rcgldr

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    You've simply enclosed a portion of air that was previously pressurized due to gravitational forces, and is now pressurized mostly by the surfaces of the room. Assuming that you don't move the room after sealing it, then the pressure of the air inside and outside the room is the same. Gravity still has the same effect it did before, the pressure at the top of the room will be slightly less than the pressure at the bottom of the room, and the net pressure differential within the room results in a downforce on the room exactly equal to the weight of the air within the room. The air inside and outside the room would have the same pressure gradient versus altitude.
     
  11. Feb 26, 2010 #10
    But now when you have sealed the room you have created a partition between air inside room and the air outside.
    because of this partition no external force(weight of air column) is now acting on air inside room. hence the pressure inside must be = (the pressure before sealing)-(weight per unit area of air column).
     
  12. Feb 26, 2010 #11

    russ_watters

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    Again, pressure in a sealed container does not work the same as atmospheric pressure.
     
  13. Feb 26, 2010 #12
    The pressure which is the same inside and outside of the room is the result of the weight of the air column.
    Before you seal the room.
    So when you seal the room the pressure of the air is also sealed in.
    The pressure is equal.
    If the room was in outer space and you sealed it, there would be no air inside it.
    If the room was then brought back to sea level there would still be no air inside.
    So if you made a small hole in it's outer wall you would hear the air rush in as the pressure equalised.
     
  14. Feb 26, 2010 #13

    rcgldr

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    It's been replaced by the force from the walls. Before you sealed the box, the pressure of the air created a force per unit area on all the interior surfaces of the box, which co-exists with the interior walls exerting a force per unit area on the air, the Newton 3rd law pair of laws at work here.

    Sealing the box will not change the force per unit area between the air and the interior of the box.

    You can try this simple test. Hold an empty glass, and then place your palm over the top of the glass to seal the glass, slowly so you don't increase the pressure inside.. If the pressure is reduced by the 14.696 psi due to the weight of the air above the glass, you will feel a huge amount of suction in the glass. However this doesn't happen.
     
  15. Feb 26, 2010 #14
    What i understood from your comment is that the reaction force induced in the walls is now responsible for creating the weight effect(weight of air column). But suppose if there is no reaction induce in the wall as in case if the air is filled in a chamber in 0 gravity. what would be the pressure if the chamber is brought back to normal gravity condition....?
     
  16. Feb 27, 2010 #15

    rcgldr

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    What I do know is that a pressure differential is created within the box, lower at the top, higher at the bottom, so that the pressure differential within the box creates a downwards force on the box exactly equal to the weight of the air in the box. A common example is adding 80 cubic feet of ambient pressure air to a scuba tank, which increases it's weight by 6 pounds, the weight of the air added to the tank. Regardless of the scuba tanks orientation, the internal pressure differential is how the air inside the tank exerts it's 6 pounds of weight onto the inner surfaces of the tank.

    Although the chamber is a closed system, gravity is an external force, so I'm not sure of the effect of gravity on the total pressure energy within the chamber.
     
    Last edited: Feb 27, 2010
  17. Feb 27, 2010 #16
    Don't you mean the opposite.
    If the pressure is reduced on the outside due to the weight of air above the glass not acting on the air inside the glass.
    Then you will get a huge amount of blowing.

    Even simpler you can hold your breath by shutting your mouth and holding your nose.
    Your chest and lungs don't expand when you do this.
    If there was an increase in pressure on the inside of your chest wall because of a decrease on the outside this would happen each time you held your breath.
     
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