Demos: 2C-01 Siphon


A siphon is constructed using clear plastic tubing and two beakers. The flow is started and the speed with which the water exits the end of the tubing is shown to be a function of the height h. If h is made smaller and smaller, the flow eventually stops when h = 0.

Bernoulli’s equation applied to the siphon shows that



where v is the speed of the liquid in the tube. It is also shown from Bernoulli’s equation that if y10.3 m, the siphon will no longer operate.

Directions: Place one end of the tube in the upper beaker and suck water into the tube in such a way that when you quickly bring the other end of the tube into the empty beaker, a column of water will be continuous in the tube. (One method is to almost fill the tube with water, then pinch the tube at the end to hold the water in place while you quickly insert the other end into the empty beaker.) Move the end of the tube upward and show that the flow diminishes. When the end of the tube becomes level with the water in the upper beaker, the flow will stop.

Suggestions for Presentation: Students have probably seen or operated siphons, but may not know the necessary conditions for successful operation. Ask if it matters how large h is. Is there a limitation on y? If you were to drop an object from height h, its terminal speed would be the same as that given above. Does this mean that the water is in free fall? What are the practical consequences of the limitation on y?

Applications: Real siphon systems.

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Last Updated: May 9, 2016 11:44 AM