page_title: Siphon
section_title: Theory
hierarchical_section_title: Siphon / Theory
context_page_description: A siphon is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in an inverted "U" shape, which causes a liquid to flow upward, above the surface of a reservoir, with no pump, but powered by the fall of the liquid as it flows down the tube under the pull of gravity, then discharging at a level lower than the surface of the reservoir from which it came.
  There are two leading theories about how siphons cause liquid to flow uphill, against gravity, without being pumped, and powered only by gravity. The traditional theory for centuries was that gravity pulling the liquid down on the exit side of the siphon resulted in reduced pressure at the top of the siphon. Then atmospheric pressure was able to push the liquid from the upper reservoir, up into the reduced pressure at the top of the siphon, like in a barometer or drinking straw, and then over. However, it has been demonstrated that siphons can operate in a vacuum and to heights exceeding the barometric height of the liquid.
context_section_description: A practical siphon, operating at typical atmospheric pressures and tube heights, works because gravity pulling down on the taller column of liquid leaves reduced pressure at the top of the siphon (formally, hydrostatic pressure when the liquid is not moving). This reduced pressure at the top means gravity pulling down on the shorter column of liquid is not sufficient to keep the liquid stationary against the atmospheric pressure pushing it up into the reduced-pressure zone at the top of the siphon. So the liquid flows from the higher-pressure area of the upper reservoir up to the lower-pressure zone at the top of the siphon, over the top, and then, with the help of gravity and a taller column of liquid, down to the higher-pressure zone at the exit.
  The chain model is a useful but not completely accurate conceptual model of a siphon. The chain model helps to understand how a siphon can cause liquid to flow uphill, powered only by the downward force of gravity. A siphon can sometimes be thought of like a chain hanging over a pulley, with one end of the chain piled on a higher surface than the other. Since the length of chain on the shorter side is lighter than the length of chain on the taller side, the heavier chain on the taller side will move down and pull up the chain on the lighter side. Similar to a siphon, the chain model is obviously just powered by gravity acting on the heavier side, and there is clearly no violation of conservation of energy, because the chain is ultimately just moving from a higher to a lower location, as the liquid does in a siphon.
  There are a number of problems with the chain model of a siphon, and understanding these differences helps to explain the actual workings of siphons. First, unlike in the chain model of the siphon, it is not actually the weight on the taller side compared to the shorter side that matters. Rather it is the difference in height from the reservoir surfaces to the top of the siphon, that determines the balance of pressure. For example, if the tube from the upper reservoir to the top of the siphon has a much larger diameter than the taller section of tube from the lower reservoir to the top of the siphon, the shorter upper section of the siphon may have a much larger weight of liquid in it, and yet the lighter volume of liquid in the down tube can pull liquid up the fatter up tube, and the siphon can function normally.
  Another difference is that under most practical circumstances, dissolved gases, vapor pressure, and (sometimes) lack of adhesion with tube walls, conspire to render the tensile strength within the liquid ineffective for siphoning. Thus, unlike a chain, which has significant tensile strength, liquids usually have little tensile strength under typical siphon conditions, and therefore the liquid on the rising side cannot be pulled up in the way the chain is pulled up on the rising side.
  An occasional misunderstanding of siphons is that they rely on the tensile strength of the liquid to pull the liquid up and over the rise. While water has been found to have a significant tensile strength in some experiments (such as with the z-tube), and siphons in vacuum rely on such cohesion, common siphons can easily be demonstrated to need no liquid tensile strength at all to function. Furthermore, since common siphons operate at positive pressures throughout the siphon, there is no contribution from liquid tensile strength, because the molecules are actually repelling each other in order to resist the pressure, rather than pulling on each other.
  To demonstrate, the longer lower leg of a common siphon can be plugged at the bottom and filled almost to the crest with liquid as in the figure, leaving the top and the shorter upper leg completely dry and containing only air. When the plug is removed and the liquid in the longer lower leg is allowed to fall, the liquid in the upper reservoir will then typically sweep the air bubble down and out of the tube. The apparatus will then continue to operate as a normal siphon. As there is no contact between the liquid on either side o
caption: Even the falling lighter lower leg from C to D can cause the liquid of the heavier upper leg to flow up and over into the lower reservoir[20]