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Name:  Leroy
Status:  educator
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
I'm trying to verify something. During a lab on the structure of the heart, students used a siphon bulb and a mechanical bulb (only allows fluid to flow in one direction) in an attempt to see which was more efficient at moving a fluid. I demonstrated the ability of using the siphon bulb to begin the flow of water from one container to another as long as the opening of the outlet tube was lower than the water level of the intake tube. The questions of how this worked came up and I explained to students that it was due to atmospheric pressure. I began to doubt my explanation on the way home tonight. I found conflicting information on the internet. Some sites said it was due to atmospheric pressure and others said it was due to gravity. Can you confirm that it's due to atmospheric pressure or is it due to gravity. If it's gravity, can you explain to me so I can explain to the students?


Replies:
Both pressure and gravity play a part in the flow of fluid. Pressure at each end pushes into the tube. Gravity pulls downward on the fluid in the tube. If the effect of the pressures work with gravity, the fluid will flow. If they work against gravity, the fluid may not flow.

The pressure at the surface of the water is air pressure. Gravity, the weight of the water, causes the pressure below the surface to be greater than air pressure. This difference in pressure, caused by gravity, is just enough to push water through the tube back up to the surface level(working against gravity). This relates to the "water seeking its own level" phenomenon. If the tube exit is above the water surface, the pressure cannot push the water high enough to reach the exit. If the tube exit is below the surface, the pressure can push the water beyond the exit.

Kenneth Mellendorf


It is atmospheric pressure, to verify and a slight correction, "I demonstrated the ability of using the siphon bulb to begin the flow of water from one container to another as long as the opening of the outlet tube was lower than the water level of the intake tube." is not exactly true. If you were to increase the distance of your siphon tube while you have a flow going from one tank to the other (lower) tank and you were to try to extend your tube over a wall of (I think its close to 30-37 feet) the siphon would break down. this is due to the suction pressure nears the vaporization pressure and the water starts to "boil" and slowly the water column in each half of the tube would fall out. If you can get a clear tube in the shape of a "U" or clear hosing for that section of tube, and use that as the top section of the siphon you can actually see vapor bubbles form as you go up in height. Michael Baldwin


Hello,

A liquid moves from a higher elevation to a lower one due to its higher potential energy, the energy that it gained (or the work that was done on it) to lift it up against gravity to that higher elevation. Thus the answer to your question is gravity.

Atmospheric pressure between the two points in question are almost identical and cannot be the driving force in your experiment. It does not vary significantly unless one moves miles high. Atmospheric pressure is due to the the column of air extending many miles above, which in value corresponds, approximately, to the pressure exerted by a 10-m-tall column of water.

AK

Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory


Both are required, and the surface tension of water is also important.

If the water in the tube was replaced by a train of tiny railroad cars connected together, everyone's intuition would lead them to the conditions under which the train will run. Gravity favors the operation when there is more mass going down than coming up. But, in a train, the cars are physically connected, and it's easy to see how the cars going down can pull the cars going up.

Imagine water again, filling the tube completely, and imagine that the exit end is not the highest point of the tube. All the water from the high point to the exit wants to fall one way; the rest of the water in the tube wants to fall the other way. If both got their way, there would be a vacuum created at the highest point in the tube. Atmospheric pressure will prevent this, and so in effect it fills the same role as the physical connections between railroad cars in the first example.

Tim Mooney


For a siphon to work, you need both. The gravity is necessary to provide a direction to the liquid flow: the liquid flows from the higher level to the lower level, reducing its overall gravitational potential energy. The atmospheric pressure is necessary to get the liquid to flow uphill to begin with.

In a siphon, the "downhill" side of the tube is longer than the "uphill" side. Once the siphon is going, there is consequently more liquid on the "downhill" side of the tube. Gravity pulls downward on the liquid on both sides, but the overall gravitational force on the downhill side is greater than the overall gravitational furce on the uphill side, just because there is more liquid to pull on. IF THE LIQUID IS NOT ALLOWED TO SEPARATE (form bubbles or the like), then the liquid on the heavier downhill side will pull the liquid on the lighter uphill side after it.

The need for atmospheric pressure comes in at the need to keep the liquid from separating. If atmospheric pressure were absent, there would be no reason for liquid to move up the tube after the liquid that preceeds it. It needs to have some pressure behind it, which is provided by the atmospheric pressure. The liquid preceeding it merely reduces the back-pressure by flowing away. If the atmospheric pressure is greater than the pressure of a column of the liquid with a height equal to the height difference between the top of the siphon and the level of the "uphill" liquid, then the liquid can flow through the siphon.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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