Physics Education
A practical example of a siphon at work
To cite this article: Stephen W Hughes 2010 Phys. Educ. 45 162
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www.iop.org/journals/physed
A practical example of a siphon at work
Stephen W Hughes
Department of Physics, Queensland University of Technology, Gardens Point Campus, Brisbane, Queensland 4001, Australia
E-mail:[email protected]
Abstract
In this article, some classroom experiments are described for correcting the common misconception that the operation of a siphon depends on
atmospheric pressure. One experiment makes use of a chain model of a siphon and another demonstrates that flow rate is dependent on the height difference between the inflow and outflow of a siphon and not atmospheric pressure. A real-life example of the use of a siphon to refill a lake in South Australia is described, demonstrating that the siphon is not only of academic interest but has practical applications.
S
Online supplementary data available from
stacks.iop.org/physed/45/162/mmediaIntroduction
This article describes a real-life environmental application of a siphon that may be of use to science teachers. Siphons are often used to empty containers of liquid that would otherwise be difficult or impossible to empty, for example, a large glass fish tank. In a siphon, the water falling down one side of the tube pulls up water on the other side (figure 1). The column of water acts like a chain with the water molecules pulling on each other via hydrogen bonds. A useful analogy is that of a chain, as shown in figure 2. If a chain is arranged so that the drop on one side is greater than the rise on the other, the chain is pulled over the top as demonstrated in a supplementary video (available at stacks.iop.org/physed/45/162/mmedia). A curved tube of water is in effect a chain of trillions of water molecules in parallel linked by hydrogen bonds.
A very common misconception is that siphons work through atmospheric pressure pushing water through the tube of the siphon. An extensive check
long ‘chain’ of water molecules linked by hydrogen bonds
h B
H-bonds A
Figure 1. Schematic diagram of a siphon. A water siphon is in effect a chain comprising trillions of water molecules linked by hydrogen bonds.
O H H
∂+
∂+
∂+
∂+
∂-
∂-
∂-
∂-
of online and offline dictionaries did not reveal a single dictionary that correctly referred to gravity being the operative force in a siphon. The author checked the entire collection of dictionaries in the Queensland University of Technology library (listed in the appendix). The Oxford English Dictionary (OED) definition of the siphon is a good example of this misconception: ‘A pipe or
A practical example of a siphon at work
chain
PVC tube
bench
Figure 2. The chain analogy to a siphon. The weight of the chain on the downside is greater than that on the upside and the frictional force between the chain and PVC tube, therefore the chain ‘flows’ down to the ground.
tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe.’ Over 25 online dictionaries were checked (see appendix) and not a single definition referred to gravity as the operative force in a siphon. In contrast to dictionaries, encyclopaedias tend to have a correct explanation of a siphon, e.g. Encyclopaedia Britannica,EncartaandWikipedia.
The ubiquitous siphon misconception in dictionaries is curious in view of a number of non- scientists the author has spoken to who understand that siphons work on gravity and not air pressure.
This misconception is a myth that needs busting in the style of the Discovery channel (http://dsc.
discovery.com) MythBusters programme.
Misconceptions about the siphon appear to be enduring, for example in 1971, Potter and Barnes [1] published a paper in this journal specifically targeting misconceptions about the modus operandi of the siphon. Evidence for siphons working in a vacuum was presented by Nokes in 1948 [2]. In spite of this, recent publications such as Ganci and Yegorenkov (2008) [3] demonstrate that misconceptions still persist 60 years later! This particular article has been published in the hope of assisting in the removal of this misconception from the mainstream dictionaries.
To the un-initiated, the siphon appears to have a ‘magical’ quality as, contrary to expectations,
it involves water travelling ‘uphill’. From physical considerations the siphon is readily understandable from energy conservation. A key aspect of the siphon is that the outflow of the siphon tube must be lower than the inflow. When this is the case, the amount of energy required to lift the water over the top of the siphon is more than compensated by the gravitational energy released at the bottom of the tube. Water flow out of the bottom of the siphon is dependent on the height difference between the inflow and outflow of the siphon—the greater the difference in height the greater the difference in pressure and therefore flow.
Another seeming ubiquitous misconception is that the maximum height of a siphon is dependent on atmospheric pressure. The maximum height of a water siphon actually depends on the tensile strength of water, i.e. the maximum weight that hydrogen bonds are able to support. Once again, the chain analogy can be put to good use. If the height of a ‘chain siphon’ were to be continually increased we would reach a point where one of the links close to the top would break due to the weight of the chain below. Atmospheric pressure does have some influence on the operation of a siphon in that it compresses the water in the tube increasing the maximum operating height of a siphon.
The basic operation of a siphon
Neglecting the effects of friction and turbulence, the velocity (v) of the water emerging from the bottom of the siphon is given by:
v =
2gh
where g is the acceleration due to gravity (≈9.8 m s−2) and h is the difference in height between the inflow and outflow of the siphon.
This expression is usually derived from Bernoulli’s equation [4]. A more intuitive way of arriving at the same expression is as follows.
Imagine the static situation of a finger placed over the lower end of a siphon so that the siphon is ready to go. As the finger is removed, the velocity of the water increases from zero to some final value. We can think of the water on the upside and downside of the siphon as ‘balancing’ each other.
The excess weight is therefore produced by the column of water of heighthas shown in figure1.
March 2010 PH Y S I C SED U C A T I O N 163
In principle, this mass of water accelerates in exactly the same way as a ball dropped from a heighth. The water molecules can be considered to be a collection of tiny balls stuck together by hydrogen bonds. As the water molecules in the downside of the siphon at the same level as the inflow accelerate under the influence of gravity they pull the chain of water molecules behind them without requiring any net amount of energy due to the balancing mentioned above. In practice, of course, there would be some friction losses so a slightly greater length of water is required on the downside to counterbalance a given length of water on the upside.
Continuing our derivation of the above equation, we use the basic equations of motion
s=ut+12at2
wheresis distance travelled in a timet,u is the initial velocity anda is acceleration. In this case the initial velocityu =0,a =g(the acceleration due to gravity) ands=h. So the above expression becomes
h =12gt2.
We now rearrange this to find the time (t) it takes a ball to fall through a heighth:
t =
2h g .
The question now is, at what velocity (v) is the ball/water moving after it has fallen through a distancehin a timet? This is
v=at=g
2h
g =
2gh
i.e. the same expression as given by the Bernoulli derivations.
Classroom experiments
The chain analogy described above can be easily implemented in the classroom (figure 3). A thin metal chain can be placed on a bench with some of the chain draped over a tube on the bench, for example a horizontal glass cylinder. If conditions are right, the chain will fall to the ground pulling the rest of the chain off the bench. (This experiment is shown in a supplementary video clip available at
Figure 3. A chain set-up in a laboratory to illustrate the operation of a siphon. The chain starts and ends in a glass beaker to strengthen the analogy to a water siphon. A glass measuring cylinder was used as this happened to be available in the laboratory and enables smooth flow of the chain. The base of the measuring cylinder prevents the chain from slipping off.
stacks.iop.org/physed/45/162/mmedia). Students can readily see that it is the weight of the downside of the chain that pulls the chain off the bench and up over the tube and down to the ground. In this case it is obvious that atmospheric pressure is not pushing the chain up over the cylinder and it is also fairly easy to imagine that this experiment would work in an airless environment, i.e. a vacuum, such as on the Moon.
Another simple experiment can be used to clearly demonstrate that the flow of water out of the bottom of a siphon depends on the difference in height between the inflow and outflow, and therefore cannot be dependent on atmospheric pressure as the air pressure at the surface of the reservoir and outflow is effectively constant throughout the experiment. Since the length of the siphon tube is constant, so the resistance to flow must also be constant. In fact, as the end of the siphon is lowered, atmospheric pressure at the outflow increases which will slightly retard the flow!
A practical example of a siphon at work
as the siophon is raised and lowered the flow rate changes
Figure 4. Schematic diagram of an experiment to illustrate that the flow of water out of a siphon is dependent on the difference in height between the water level in the upper reservoir and the bottom of the siphon tube and not on atmospheric pressure.
A schematic diagram of the experimental set- up is shown in figure 4 and a photograph in figure 5. All that is required is two buckets and a flexible tube. One end of the siphon tube is placed in the upper bucket and the other end held over the lower bucket. The outflow of the siphon is held just above the water surface of the lower container and gradually raised. A supplementary video clip (available at stacks.iop.org/physed/45/162/mmedia) shows that the flow rate reduces as the outflow is raised and stops just before the reservoir level is reached.
This experiment could be made quantitative by collecting the water in a measuring cylinder for a certain amount of time to measure flow rate. Alternatively water could be collected in a container that is weighed to determine the volume of water collected.
Practical example: the refilling of Lake Bonney in South Australia
The refilling of Lake Bonney, in the Riverland region of South Australia, is an excellent example of a siphon in operation, demonstrating that the physics of the siphon is not just of academic interest, but can be used to save substantial amounts of money and help protect the
Figure 5. Demonstrating that the flow of water in a siphon is dependent on gravity and not atmospheric pressure.
environment. In December 2008/January 2009, a siphon was used to transfer 10 Gl of water from the Murray River into Lake Bonney (figure6) in the Riverland region of the state of South Australia, Australia. The supplementary material section contains a video of the Lake Bonney siphon and a document containing statistics relating to Lake Bonney, a discussion of the options considered for refilling the lake and flow rate calculations etc that may be of use for teaching purposes.
Discussion
It would be useful if someone could perform a demonstration of a siphon working in a vacuum.
A siphon could also be constructed using two immiscible fluids of different density. In this case it would be very easy to demonstrate that the pressure at the outflow of the siphon is greater than at the inflow using a pressure transducer and therefore that fluid pressure cannot be the operative force in a siphon. It also follows from this that atmospheric pressure also cannot be the driving force in a siphon since air is also a fluid.
March 2010 PH Y S I C SED U C A T I O N 165
Figure 6. Eighteen 200 mm internal diameter polypipe tubes being used to siphon water from the River Murray to Lake Bonney in South Australia. The difference in height between the upper and lower pools is about 1.3 m.
It is hoped that this article may assist in correcting the common misconception that the operation of a siphon is dependent on atmospheric pressure. In view of the extensive search made of online and offline dictionaries, it is possible that every English dictionary in the entire world needs to be corrected. Foreign language dictionaries were not checked in this study but it is likely that many dictionaries in other languages also contain misconceptions about the siphon1. In many cases only a very small change is required to a dictionary entry. For example, the online edition (2009) of the Oxford English Dictionary quoted in the introduction could be modified to read: ‘A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of gravity, which pulls the liquid up the shorter leg and over the bend in the pipe’. Similarly, definitions in other dictionaries could be easily modified.
Acknowledgments
I would like to thank Tim Scott of Taylorville, South Australia for first telling me about the refilling of Lake Bonney, Chaz Noakes of the Central Irrigation Trust, Barmera, South Australia, Ian Penno, former owner of Barmera Irrigation, and Peter Symons, a local expert on Lake Bonney, for providing useful information about the refilling
1 The author would be interested to hear from readers about definitions of the siphon in non-English dictionaries.
of Lake Bonney. Thanks also to David Klokman, QUT, for taking and editing the laboratory video clips.
Appendix
Hard-copy dictionaries checked for the definition of a siphon:
(1) Oxford Advanced Learners Dictionary, 7th edition.
(2) Australian Concise Oxford Dictionary.
(3) The Macquarie Dictionary, 2nd revision.
(4) The Oxford English Dictionary (full version).
(5) Collins Compact Dictionary.
(6) The Australian Oxford Dictionary.
(7) Collins COBUILD Dictionary.
(8) Collins Australian Dictionary, 7th Australian edition.
Online copy dictionaries checked for the definition of a siphon:
(1) The Free Dictionarywww.thefreedictionary.
com.
(2) http://dictionary.com.
(3) http://www.onelook.com/?w=siphon&ls=a (26 dictionaries are accessible from this site).
Received 4 September 2009, in final form 9 November 2009 doi:10.1088/0031-9120/45/2/006
References
[1] Potter A and Barnes F H 1971 The siphonPhys.
Educ.6 362–6
[2] Nokes M C 1948 The siphonSch. Sci. Rev.29 233 [3] Ganci S and Yegorenkov V 2008 Historical and
pedagogical aspects of a humble instrumentEur.
J. Phys.29 421–30
[4] Gianino C 2007 A strange fountainPhys. Educ.
42 488–91
www.samdbnrm.sa.gov.au/Portals/7/
Potential%20impacts%20from
%20extended%20closure%20June%2008%
20Final.pdf
Stephen W Hughes holds a PhD from King’s College, London, and is currently a senior lecturer in physics at Queensland University of Technology (QUT), Australia. He has previously held appointments in bioengineering and medical physics at King’s College Hospital and Guy’s and St Thomas’
Hospital, London. His research interests include medical physics, astronomy and physics education.
