31098a - QUT Digital Repository: http:/eprints.qut.edu.au/...

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QUT Digital Repository: http://eprints.qut.edu.au/ Hughes, Stephen W. (2010) A practical example of a siphon at work. Physics Education, 45(2). pp. 162-166. © Copyright 2010 Institute of Physics Publishing Ltd.
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A practical example of a siphon at work 1 A practical example of a siphon at work Dr. Stephen W. Hughes Department of Physical and Chemical Sciences, Queensland University of Technology, Gardens Point Campus, Brisbane, Queensland 4001, Australia Phone: +617 3285 3087 Email: [email protected] Abstract In this paper, 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. Introduction 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. 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 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 the misconception: “ 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 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.
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31098a - QUT Digital Repository: http:/eprints.qut.edu.au/...

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