Q:

I ran across an interesting custom-built aquarium online:
http://jasonlpsmith.googlepages.com/myaquarium
In case you don't want to directly link to it, here is a description: it is L-shaped, with a closed top and open upper portion of the lower leg of the L. It is filled from the lower portion, and the top has its air evacuated with a pump, so that the water fills the tall leg.
My question is: since water pressure is commonly calculated by measuring the height of the column of water above it, is the water pressure higher on the tall side than it is on the lower side? Or is there an evening-out action (Gauss' law or something) that is just not normally a factor and so is normally discarded? Is the pressure differential high, and is there an upper limit (presumably 15psi or so)?
Thanks!

- Simon R. (age 20)

Birmingham, UK

- Simon R. (age 20)

Birmingham, UK

A:

Hi Simon,

Great question. It is related to the fact that you can't pump water out of a well using a suction pump more than about 10 meters. This pressure corresponds to the 15 psi you were referring to. So the answer is the pressure at the bottom is uniform and equal to that developed by the short side. If you would poke a hole in the lid of the tall side, air would leak in and and the water level would drop to that of the short side. The overall level of water would raise just a bit and the pressure at the bottom would raise the same fractional bit.

LeeH

Great question. It is related to the fact that you can't pump water out of a well using a suction pump more than about 10 meters. This pressure corresponds to the 15 psi you were referring to. So the answer is the pressure at the bottom is uniform and equal to that developed by the short side. If you would poke a hole in the lid of the tall side, air would leak in and and the water level would drop to that of the short side. The overall level of water would raise just a bit and the pressure at the bottom would raise the same fractional bit.

LeeH

*(published on 06/08/2009)*