[chris (OP)]

Clearly gravity plays a part here, because the (e.g.) water is flowing from a higher to a lower place, but isn't air pressure also involved?

Chris

[lyner]

Pressure is what it's all about (caused by Gravity, of course).
The air pressure at the top is higher than the air pressure at the bottom; that's enough to lift the water 'over the top'. As long as no air is admitted into the inverted 'u' at the top then water will always be pushed up by pressure on the surface in the upper container.
The higher the water column (drop), the faster will water be pushed out at the bottom but it is Atmospheric pressure which pushes the water 'up' in the first place.

[Bored chemist]

It's the air pressure that holds the water up in the "up" side of the pipe, but it's the difference in water levels that provides the energy that moves the water. If the "up" pipe is too long it won't work, but if it's short enough you can work a syphon in an arbitrarily low air pressure (down to the vapour pressure of the liquid) and still get lots of flow.

[DoctorBeaver]

I thought it was gravity pulling the water down in the down side and, as nature abhors a vacuum, water from the upper container is sucked into the pipe.

[lyner]

Yes - gravity pulls the water down. BUT air pressure pushes water up INTO the vertical run above  the upper surface of water.

The statement "nature abhors a vacuum" is a very old idea and has been explained in much fuller terms, subsequently. Remember - there is no such word as SUCK in Science. Air molecules are not attracted to each other so they can't pull each other. Put some air into empty space and it will just spread out and out. It's always pressure difference that makes fluids flow.
If you have a hole in the top of the U, air will be pushed in and the syphon stops because there is higher pressure on the outside of the top of the U than on the inside.

[chris (OP)]

Sophiecentair, why would the air pressure be higher at the top than at the bottom? If it's "higher" then there is less atmosphere above the water and therefore the pressure would actually be lower, wouldn't it?

C

[lyner]

Touchee.
I wrote that badly. The air pressure on the upper surface is higher than the air pressure at the top of the U. That is enough to keep the top of the U full of water. (There is a limit of about 10m to the height to which atmospheric pressure will push the water up and over. In practice, this limit is quite a bit less than 10m)

The pressure difference between top and bottom of the down pipe keeps the  water flowing out of the bottom. The greater the 'drop' the faster the flow of water. (Think of old fashioned toilet cisterns put near the ceiling.)

Is that better, Chris?

[DoctorBeaver]

So nature doesn't suck  []

[Andrew K Fletcher]

Pascal demonstrated that the siphon worked by atmospheric pressure, not by horror vacui, by means of the apparatus shown at the left. The two beakers of mercury are connected by a three-way tube as shown, with the upper branch open to the atmosphere. As the large container is filled with water, pressure on the free surfaces of the mercury in the beakers pushes mercury into the tubes. When the state shown is reached, the beakers are connected by a mercury column, and the siphon starts, emptying the upper beaker and filling the lower. The mercury has been open to the atmosphere all this time, so if there were any horror vacui, it could have flowed in at will to soothe itself.
http://mysite.du.edu/~jcalvert/tech/fluids/hydstat.htm

[lyner]

Great demo.
Really clever to arrive at that experiment by thinking about it- and being  the first to figure it out.

[Andrew K Fletcher]

Using a small amount of salt or sugar, the following experiments provide some interesting observations with changes in presure and flow in both closed loop tubing and open U tubing.

The closed loop of tubing when soft latex tube is used shows the downward flowing side with salt added bulges due to the increased pressure, while the return flow salt free side shows the latex tube necks inwards due to the reduced pressure and tension applied to the water inside as each moecule pulls on the next molecule in the bead of water.

This experiment, while simple has implications for our own circulatory system.

[lyner]

The 'C' effect is noticeable when a lock gate between fresh water and sea water is opened. The gates open easily when the forces are equal but fresh water instantly starts to pour out into the sea as soon as they are open because its level is higher. It looks bizarre if you are used to canal locks.

[Andrew K Fletcher]

Great example with the locks, Another example is aquifers in desert regions close to the coast. Fresh water can be pulled up so long as the level does not fall too far. When it does, the salty ocean water floods in and contaminates the once drinkable water. The point being that some of these aquifers have maintained the relatively salt free water for hundreds if not thousands of years. Some of these well have been labled as fossil water due to their unknown age.

When you turn the C: experiment upside down that’s when the density flow becomes really interesting.

The 'C' effect is noticeable when a lock gate between fresh water and sea water is opened. The gates open easily when the forces are equal but fresh water instantly starts to pour out into the sea as soon as they are open because its level is higher. It looks bizarre if you are used to canal locks.

[Bored chemist]

"This experiment, while simple has implications for our own circulatory system."
How?
Blood etc have pretty near constant density.

[Andrew K Fletcher]

Blood filtered by the kidneys comes out from the kidneys less dense than blood flowing into the kidneys. Salts excreted in the urine prove this to be correct. This means the blood flowing back to the heart in the venous return from the kidneys is less dense than the blood flowing in the arteries. Urine density can be regulated using posture!
Respiration evaporates solute free fluid from a fluid that contains solutes, protein colloids, sugars, all of which are denser than water. Evaporating water from the respiratory tract cannot be achieved without changing the density of said solutes.

Tears, saliva and sweat show how evaporation alters the density of liquids. In dry air sweating produces salt crystals on clothing.

loading the blood by evaporating water from it during expiration will induce a movement of concentrated solutes due to the effects of gravity on said solutes to the point of excretion in the urine via the renal filtration.

http://jap.physiology.org/cgi/content/abstract/60/1/327

[Bored chemist]

The density of blood is about 1060; plasma about 1025.
Urine has a density of about 1002 to 1030.
There's not a lot of difference. But the range of body fluids seems to be about 1002 to 1060
Blood is kept under pressure in the body, the pressure varies between individuals but mine runs about 70mmHg (a bit lower than most).
The difference between a 2 metre (rather more than 6 feet) column of a liquid with a density of 1002 and one of 1060 is about the biggest pressure difference you could hope for in a person. It's about 120 mmH2O or about 8.6 mmHg.
The biggest possible effect you could get is barely clinically significant. People are not full of unusually watery urine on one side and blood on the other. The effect really isn't big.
Also the blood in the body is, in effect, well stirred- any density gradients are small.

[Andrew K Fletcher]

http://hypertextbook.com/facts/2004/MichaelShmukler.shtml
Blood density changes with body posture. Venous blood density is higher when a person is standing than when he is sitting. The following charts show the venous blood densities of 6 subjects as they change body positions during a 10 minute period.

[Bored chemist]

Those graphs show changes over the order of 10 mins. that's consistent with this hypothesis. They stand up, their blood pressure falls (it's measured near the neck). Their body notices this and seeks to correct it. Their kidneys take out water and thei blood density rises. They lie down and back diffusion from the intracellular fluid returns the blood density to near it's original value.
Nothing new.
What's not possible is that it takes 10 min for hydrostatic pressure to change.

[Andrew K Fletcher]

Showing how blood density is not a constant density.
This is not correct. Etc presumably relates to other fluids. What happens to the blood where the lymphatic system adjoins the main circulation and releases the waste from the cells?  Cerebrospinal fluid flows back into the circulation too, what does that do to blood density where it takes place? Lymph for example contains proteins and sugars.
The thoracic duct carries a 1000 millilitres of lymph in 24 hours to the jugular venous return. Are you suggesting that this will not alter the density of the blood at the junction?

It is erroneous to suggest that blood has a constant density. Taking a drink alters the density at the point the fluid is absorbed. The same as removing fluid during respiration increases the density at the point where the fluid evaporates. Therefore eating a heavy meal without sufficient water would cause a dragging effect on the uptake of fluids from the gut and intestines and induce lethargy.

 

"This experiment, while simple has implications for our own circulatory system."
How?
Blood etc have pretty near constant density.

[Bored chemist]

A couple of points, first please learn the meaning of the word "near" as used in that quote.

Secondly, all body fluids have a sufficiently similar density that even the biggest possible effect- the one I described earlier) is less than how much your blood pressure probably changed when you read what I had posted.

The thoracic duct handles about a litre a day; but the heart handles about 5000 times more.  About 20% of that is from the brain, down the jugular return, so any effect the lymphatic return has on density will be entirely negligible because they are diluted about 1000 fold.
It would be challenging to detect that change.