Why can a column of water only be 10 meters high or so?

Discussion in 'MCAT Study Question Q&A' started by Gauss44, 09.21.14.

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1. Gauss44 2+ Year Member

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Why can a column of water only be 10 meters high or so? Or why can you only pump water up for a distance of 33 feet or so?

3. Cawolf 2+ Year Member

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We know that the column can rise until it meets atmospheric pressure. This because the water column is being drawn up by a vacuum, so the pressure on top of the column is essentially zero. The column is above atmospheric pressure, this is basically "supporting" the column. When the water column gets to a height that it exerts a pressure greater than the pressure below it, it will not rise anymore. This concept applies to mercury thermometers as well.

I will provide a quantitative explanation as well.

P = Po (pressure = atmospheric pressure)

P = ρgh and P0 = 101,325 N/m^2

P = Po

ρgh = P0

(1000 kg/m^3)(9.8 N/kg)(h) = P0

h = P0/(9800 N/m^3) = (101,325 N/m^2)/(9800 N/m^3) = 10.34 m

4. Gauss44 2+ Year Member

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Thanks, so what does this and does this NOT apply to?

It seems to me that part of the system would have to be exposed to atmospheric pressure for this to be the case. This topic has always confused me. I think I am thinking of the wrong scenarios or something. (However, I do fine with manometer questions.)

5. Cawolf 2+ Year Member

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That's sort of a vague question. The general principle applies broadly but this specific example is when a column of fluid has a vacuum above it and atmospheric pressure below it. Like a mercury thermometer.

6. Gauss44 2+ Year Member

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So theoretically, you could install a pump at the bottom of the column of water to increase the pressure up the column, so that it could rise above 10 meters or so, maybe to 20 meters?

7. Cawolf 2+ Year Member

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Yes, if you increased the pressure below the column, then the fluid column would lift higher.