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Does air keep weight

  1. May 25, 2009 #1
    So I am talking about any gas. I anm going to doubt that the air really has some weight. What is the simplest expeiment that could prove that the air does have weight? May be baloons on a balance, when one of them is blown up, the weight on that pan decreases. I suppose that the gas molcules have no weight. The air has weight only due to the dust paricles and the water droplets/? solved in it which give the impession that the air has weight.

    What expeimental setup would give purely the weight of the gas molecules excluding any of the entities solved in air?
     
  2. jcsd
  3. May 25, 2009 #2

    mgb_phys

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    Take a metal can with an airtight lid.
    Fill with boiling water, screw on lid, watch can collapse as steam inside condenses.
     
  4. May 25, 2009 #3
    Weight of air on earth

    Exactally how much weight is felt per unit suare of area on earth? Is it same on every direction or is more in vertical direction?
     
  5. May 25, 2009 #4

    russ_watters

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    Re: Weight of air on earth

    Air pressure at sea level is approximately 14.7 psi and yes, air pressure acts the same in all directions from a point.
     
  6. May 25, 2009 #5

    russ_watters

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    A similar experiment with a plastic bottle partially full of water and heated in a microwave would work too.
     
  7. May 25, 2009 #6
    Air (80% nitrogen, 20% oxygen) has a mass of about 1.29 grams per liter at sea level and 20 degrees C. Water is about 1000 grams per liter. One way to weigh air is to get a 1 liter (or larger) lightweight metal tank, pump all the air out, weigh it accurately, let the air back in, and weigh it again. You should see a 1.2 or 1.3 gram difference.
     
  8. May 25, 2009 #7
    Re: Weight of air on earth

    What's the source of this pressure then?
     
  9. May 25, 2009 #8
    Bob_s,
    Air isn't only 80% nitrogen and 20% oxygen in my view. It contains so many other suspensions. I was talking about the pure pure air, with no other particles mixed in.

    mgb and russ_waters,
    I think the experiment shows the pressure of air when the can inside is free of air. But it wouldn't tell the air weight quanitatively, isn't it so? Am I confusing the air pressure with the air weight?
     
  10. May 25, 2009 #9
    Re: Weight of air on earth

    If the pressure is same in all directions and is not reinforced in the vertical direction by the gravity, is it sound to say that air does not have some particular weight? Or it is not affected by the grvity.
     
  11. May 25, 2009 #10

    mgb_phys

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    Re: Weight of air on earth

    Air is a fluid so the weight pushes down but it can flow around corners and so the force is felt the same everywhere.
    Imagine a large box full of metal ball bearing with a hole in the side, even though the weight is only downwards there would still be a force pushing the balls out of the side.
     
  12. May 25, 2009 #11

    mgb_phys

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    It shows you that air has weight.
    to measure the actual weight you need more sensitive equipement, air weighs around 1.2Kg/m^3 so to get a reasonable difference in weight you need a large volume container that doesn't collapse when you pump the air out of it.

    Alternatively you can rent a cylinder of compressed air and weigh it full and empty.
     
    Last edited: May 25, 2009
  13. May 25, 2009 #12

    russ_watters

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    Air pressure, 14 psi, can be thought of as the weight of a 1 square inch column of air.
     
  14. May 25, 2009 #13

    russ_watters

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    Re: Weight of air on earth

    It is the weight of the air.
     
  15. May 25, 2009 #14

    russ_watters

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    Identical threads merged...
     
  16. May 25, 2009 #15
    Re: Weight of air on earth

    Thank you for the Ball bearing example. It helps understand why air should have pressure in all directions and not solely in the downward direction. As now I can understand there would be more pressure on the ball bearings to flow out of the hole if the bearings height were more. It definitely explains for the horizontal force.

    Howevere if we consider the case when one is inside a room with 7 feet height. The room may be close except very small holes, with windows and doors closed. So would there be some difference in the pressure or weight due to the air? Apparently now there is not much weight on a person. Rest of the air weight is being supported by the roof. Pressure should reduce.

    Your example once seemed to solve my problem but it has caused some more confusing now.
     
  17. May 25, 2009 #16

    russ_watters

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    No, the air flows into the holes and fills the room, equalizing the pressure. A house's roof could not possibly support the weight of the air above it. A 10'x10' section of roof would have to support more than 200,000 pounds!
     
  18. May 25, 2009 #17

    mgb_phys

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    Compared to the weight of air ( 10tons/m^2) the walls act like elastic balloons.
    The weight of the air outside pushes in on the walls, roof and windows this presses on the air inside and means you have the same pressure on both sides.
     
  19. May 26, 2009 #18

    rcgldr

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    Go to a scuba shop. Weigh a tank, then let them add 80 cubic feet of sea level pressure air into the tank. Weigh the tank again and it's is now 6 pounds heavier because of the 80 cubic feet of air forced into the tank.
     
  20. May 26, 2009 #19
    Dear mgb_phys,
    Sorry I can't digest the idea that roof and walls act like elastic baloons. Weight as per unit square? means pressure? Hope it doesn't say in m^3


    Jeff Reid,
    I am so sorry that I have to repeat again. The air I would weigh would alsocontain water vapours, dust,carbon particles or many other suspensions. My question have been about the exactly pure air, just gaseous molecules, no other contaminations. Hope you understand my point.
     
  21. May 26, 2009 #20

    mgb_phys

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    mabs239 these are very good questions.
    First the weight of air at sea level (ie the air pressure ) is 10tons/square metre.
    It isn't so much that air is heavy (it only weighs 1/800 as much as water) but there is about 10,000 m of it above your head.
    The density of air (how much a box of air would weigh) is only about 1.2Kg/m^3 but if you stack 10,000of these boxes on top of each other you get atmospheric pressure.

    So if you use bathroom scales why doesn't the air press down on the top surface of the scales (roughly 0.1m^2 area) to make you weigh 1ton more?
    This is because air is a fluid, it pushes down inside/underneath the scales and pushes the top of the scale up with equal force. (remember the ball bearings)
    If you took some scales and pumped all the air out of the inside then the top would be pushed down by an extra ton of force. And they would be crushed - remember a vacuum doesn't suck things in - it's the weight of the air outside pushing them down.

    So next obvious question is why is there air pressure inside a room? If there is only 1m of air above my head rather than 10,000m why is there the same air pressure?

    Again back to the ball bearings! The air is pressing down on the roof (and walls and windows) with 10tons/m^2, unless you live in a nuclear bunker the walls and roof have no strength on this scale and so press down on the air inside which pushes back until it is the same pressure as outside.
    It's a bit like a bicycle pump - if you push down on the pump with your arm (pretend that is the outside air pressure) then the piston (the roof) moves down until the force of the air inside the pump is pushing back - this happens when the pressure inside the pump is the same as the pressure you push down with.
     
  22. May 26, 2009 #21

    rcgldr

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    "Pure air" would still have mass, and at the density associated with 14.7 psi of pressure at about 70 degrees Farenheight, "pure air" would still weigh about 2.06 pound per cubic yard, or about 6.1 pounds per 80 cubic feet. Note that helium tanks also weigh more when "filled" than when "empty".
     
  23. May 28, 2009 #22

    Dale

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    Actually, a good SCUBA shop will go to great pains to filter out all of the impurities. Any impurities left can make the tank corrode more and thus reduce the life of the tank. SCUBA air tends to be quite dry and clean, but if you are that concerned about it you could also probably order a medical-grade air cylinder and do a similar experiment (but much more expensively).
     
  24. May 28, 2009 #23

    HallsofIvy

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    Where did you get that idea? If you blow a balloon full of air and then drop it it will fall down. If you have an empty balloon on oneside of a balance scale and a ballon filled with air on the other, the side with the filled balloon will go down. That's because the air in the balloon, with greater pressure, will have greater weight than the surrounding air.

    If you were to do that experiment with helium rather than air, the balloon would go up but that's because air has more weight than helium. But you could show that even helium has weight by weighing a balloon full of helium inside a vacuum.

     
  25. May 30, 2009 #24
    I don't understand how a few kg's of air filled inside a room can support tons of weight that would be placed on the roof. Would the air support(upside pressure of air from below) increase accordingly if the wight on the roof is increased by actually placing some real load (like 100 kg iron weight)? If no , why not?

    It was said that the air would fill in the room through holes to equalize the inside and outside pressures. What if the air is stopped from entering inside?

    Perhaps the ideas presented here are direct generalization of the ones derived for water. I am looking the problem from the perspective that this universe is a combination of four "elements", fire and air have the tendency to go up, water and clay(earth?) have tendency to come down.
     
  26. May 30, 2009 #25

    Dale

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    Why not? This happens all the time in car tires, a couple of pounds of air can support several tons of steel. What matters is the pressure and the area, not the mass.
     
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