Water Seeks Its Own Level?

Question

It is a common saying that water seeks its own level. This can be demonstrated by pouring water into a U-shaped container and noting that the water-surface levels in each side equalize. Buy why does water seek its own level? The reason has most to do with:

a) atmospheric pressure on both surfaces
b) water pressure depending on depth
c) the density of water

Answer 1

B. is the best answer, although all our good answers (and I gave all a thumb's up).



It is the water pressure varying with depth (which is a function of gravity and density) which keeps everything in equilibrium.



At a particular elevation, if there were more pressure on one side of a U-tube, the two columns would no longer be in equilibrium. There would be a net force downward on the side of the column under greater pressure and water would flow towards the side of the tube under less pressure.



So long as atmospheric pressure is constant on both sides of the tube, water, at all points in the column, will be under the same pressure when the two columns are level.



But atmospheric pressure is NOT the driving force. The process would work equally well in a vacuum. Imagine mercury (which has a low vapor pressure) in a vacuum. It would reach equilibruim just like water. It is the pressure gradient in each tube which cause the fluid to balance.



Sure, you can disrupt the system to cause the fluid on each side of the u-tube not to balance, but you have to act to do so. If you had different atmospheric pressures on each side, then it would upset the equilibrium such that the water column facing the lower pressure would rise by an amount equal to the pressure difference. I.e., h = pressure difference/(rho *g). The same principle applies if the density of the liquid in the two tubes varies, such as the water is at different temperatures, or one is saline.



But assuming that the fluid is at constant density, and faces the same pressure on each side of the tube, then B is the answer.



Source(s)



I didn't answer the previous question on helium balloons (Kirchwey did a great job). But I found a good U-tube (pun intended) video about a US Airforce captain who jumped out of a helium balloon at 30k, or above 99% of the atmosphere.



http://www.youtube.com/watch?v=jvRG7LzKcDA

Answer 2

I'd say this is more a semantic than a physical question. There's an argument for any of the possible answers, partly because of their "looseness".

A, because, given that we're talking about water, not two liquids of differing density, the pressures on both exposed surfaces of the water have to be equal for the column heights to be equal.

B, because at equilibrium, regardless of density variations, the pressures at the bottoms of the water columns must be equal.

C, because as linlyons eloquently points out, two liquids of differing densities, when not allowed to mix, will equilibrate at different column heights. But it's not the density "of water" that's responsible for equal heights, it's the fact of a single, uniform density throughout the U-tube.

Does one condition or set of conditions "cause" the other? Well, yes, but only in a sufficient-but-not-necessary or necessary-but-not-sufficient sense. Uniform density combined with the requirement of equal pressures at the bottom and equal pressures applied to the top surfaces cause equal column heights. But you could also get equal column heights by compensating for nonuniform density by adjusting the surface pressures. Note that the bottom pressures would still be equal (since the columns are in equilibrium). Then is the condition of equal bottom pressures necessary? Yes. Sufficient? No. So in summary, if forced to choose one of the three answers at gunpoint, I'd try to seize the weapon and shoot myself; if I failed, I'd choose C.

Answer 3

a) atmospheric pressure on both surfaces

remember--gauge pressure is the the difference between abosolute pressure and atmospheric pressure or mathematically:

P=Po + pgh (where p is the density of the fluid)

if you added another fluid with p different than water (like oil) in one side of the U-tube, then the levels would not be the same because the levels would adjust in such a way that the atmospheric pressure on both surfaces would would be the same.

Answer 4

Water seeks its own level due to static pressure. Static pressure is the pressure which exerts on the object due to the state means position,You can say uneven surface or flat by the atmosphere.as water is in the liquid form it takes its position easily.

Answer 5

Gravity is the answer!

Answer 6

I diria that to occión a