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Atmospheric pressure

  1. Nov 27, 2009 #1
    Hello all, i would like to know if atmospheric pressure exists on the surface of water in a completely sealed water tank.

    So, does atmoshperic pressure exist even on surfaces that are not in contact with the atmosphere, meaning in closed areas above.

    Thank you.
     
  2. jcsd
  3. Nov 27, 2009 #2

    Astronuc

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    Staff: Mentor

    If air volume in the tank has the same density (and temperature) as the atmosphere at sea-level, then it would have the same pressure. Normally water storage tanks are vented to the atmosphere, so as to preclude compression or decompression of the gas above the water surface.

    In a sealed water tank, the pressure in the gas above the water will increase or decrease with the level of the water. In the volume of gas, the pressure is taken as constant throughout the gas - assuming it's a relatively small volume or dρ/dz ~ 0. In water the pressure will increase with depth because the mass of water above a given elevation (depth) increases with depth.
     
  4. Nov 27, 2009 #3
    Hello 133t V.I think it depends on the method of sealing the tank.If you sealed it ,for example,by pushing in a cork the air inside would get compressed slightly and the pressure would rise.Once sealed the pressure inside is not subject to pressure variations from the outside.You could use a vacuum pump to remove the air before sealing and then the pressure inside would drop to a low value and would be due to the saturated water vapour above the water surface.
     
  5. Nov 27, 2009 #4
    I see.

    So if we have a tank of height H, and we fill it with water with height h and then we close it tightly so it won't have contact with the atmosphere and assuming that a point A exists in the bottom of this sealed tank. What will be the pressure (under normal conditions) at that point A at the bottom ?

    Is it P(A) =Patm + Density(W)*g*h
    or simply P(A)=Density(W)*g*h ?
     
  6. Nov 27, 2009 #5

    Astronuc

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    Staff: Mentor

    Assuming the air volume in the tank has contact (e.g., is vented) and the tank is filled to height h, then the vent is closed and no more water added, then the air pressure is still 1 atm, and the pressure at A at depth h is just

    Patm + ρgh, were ρ is the density of water.

    If however, the valve is closed during the filling process, the air would be compressed and one would have to calculate the air pressure by the ratio of the volume at Patm to the volume once the tank is filled. In other words, if the volume of air decreases by a factor of 2, the pressure increases by a factor of 2 (Boyle's law). This of course assumes the gas is ideal, i.e., pV = k = nRT, or p1V1 = p2V2, the water vapor is neglible, and the gas does not condense, nor does it dissolve in the water.
     
  7. Nov 27, 2009 #6
    Have you ever seen or used a mercury (column) barometer or blood pressure meter? Have you noticed the empty volume of air above the mercury? It's actually a fairly good vacuum. The sea level height of the mercury is 760 mm. Mercury has a density 13.6 times water. How high would the column be if the barometer used water instead?
    Bob S
     
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