Ask A Scientist© Physics Archive

name         Leroy
status       educator
age          20s

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?

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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