Less than total vacum?

  1. I was watching them pump sewage from a broken line yesterday and got thinking. Is there a maximum possible lift?
    Atmospheric pressure is around 15 psi(?), 30 inHg right? What happens when you try to pull 31 inches of mercury?
    Then taking it to a level only mindless commutes can create. If you took a perfect tube and dropped it from space (near perfect vacum) could you suck the oceans dry? Only went part way down, could you suck the pollution from the Los Angeles bowl?
  2. jcsd
  3. Nugatory

    Staff: Mentor

    A perfect vacuum will only lift a column of water about 30 inches. The pressure at the top of the column (zero in a perfect vacuum) is pushing the column down; the pressure at the bottom (14-odd psi, atmospheric) is pushing the column up. When the column rises enough that its weight is more than what 14 psi can lift, it stops rising any further.
  4. I think you know the answer to your own question based on your use of "inHg".

    When thinking about pressure it is best to never think about sucking. PRESSURES ALWAYS PUSH. The 'sucking' effect comes from reduced pressure on one side, which means the other side pushes more, leading to a net force.

    The column of water (or mercury) will only reach as high as you can push it. The reason the liquid rises up the tube is because gas pressure pushes the liquid into the tube. Basically the pressure at the bottom of the liquid column becomes equal to the pressure of the gas pushing on the liquid. The 'vacuum' at the top does nothing except push less than a higher pressure would.
  5. But if you take a 4 ft test tube and fill it with mecury so no air is left then turn it upside down it will stay to the top or will it? If it does then what little non mercury space has to be a perfect vacum. A mythically strong tube could then be ... a mile high and you still wouldn't get anything else on top of the column.
    Again my musings, the atmoshpere will "push" up 30 inches what happens to the space on top of 30" if it is held up in a sealed container?
  6. If you turn it upside down and place the open end in a container of mercury open to the atmosphere then the mercury in the column will drop down to about 760mm.
    The space above the mercury in the tube is a very good vacuum but consists of mercury vapour....called a Toreccelli (spelling?) vacuum after the Italian scientist who first did this experiment.
  7. :thumbs:

    See I figured there were a few people here smarter than me.

    Does the 760 mm in 4 ft extrapolate out eqaully for mythical miles?
  8. phyzguy

    phyzguy 2,489
    Science Advisor

    A correction. A perfect vacuum will only lift a column of mercury about 30 inches. 760 mm is about 30 inches. Since water is so much less dense than mercury, a perfect vacuum will lift a column of water about 30 feet.
  9. Nugatory

    Staff: Mentor

    Good catch, thanks. I don't even remember if I meant inches of mercury or feet of water, but whichever I meant, I mixed it up with the other.
  10. If you fill a tube with liquid and turn it upside down, like the mercury barometer, the vacuum above the liquid is not perfect. It contains vapour of the liquid so it has a pressure of the saturated vapour at that temperature.
    If you did this with water and warmed up the water the level would drop.
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