posted on 11 Mar 2014 by guy
last changed 28 Oct 2014

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ages: 7 to 99 yrs
budget: $5.00 to $20.00
prep time: 0 to 5 min
class time: 10 to 30 min

This lesson describes how to construct a simple siphon and gives a general tutorial on how siphons work. Lecture slides with extra figures are attached.

subjects: Physics
keywords: siphon, pressure

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Fig. 1: Diagram of a siphon. Liquid from the left beaker climbs through the tube, up over the rim and down into the beaker on the right.

Fig. 2: Siphon with one fat leg and one thin leg. Even though there is more liquid in the tube coming from the upper reservoir than in the tube leading to the lower reservoir, the siphon will still move water from the upper reservoir into the lower reservoir.

Fig. 3: Siphon with a bubble in the tube. This siphon will pull the bubble to the right and down into the lower reservoir, while pulling more liquid up into the tube from the upper reservoir. Cohesion between the liquid in the two halves of the tube is not necessary for the siphon to work.

Fig. 4: Cherry juice held in a straw by air pressure.

Fig. 5: Water siphon with closed valve. Air is at atmospheric pressure at the surface of the upper reservoir and at the output of the tube on the right. Pressure changes by about 1% atmosphere for every 10 cm of water height, as indicated. Therefore, pressure is higher on the left side of the valve than on the right. When the valve is opened, water will flow through the valve from left to right.

To make a siphon, fill a glass with water and set it on a table. Put another empty glass on the floor. Take a couple of meters of clear vinyl tubing and fill it with water. Stick one end in the glass on the table and the other end in the glass on the floor. If you haven't let too much air get into the tube, it should start siphoning water from the glass on the table up over the rim and down to the glass on the floor (see figure 1 for a schematic). You can even run your siphon all the way up to the ceiling and back down to the floor if you like (provided you have enough tubing). The siphon should still work.

Siphons have been around since at least 1500 B.C., when they were depicted on Egyptian reliefs. They can seem to defy gravity, allowing liquid to flow uphill, and they offer a startling glimpse into the way gravity and air pressure work, but they are often misunderstood.

not the way siphons work

One common (misleading) explanation of siphons proposes that because the liquid in the downhill side of the siphon tube (right side in figure 1) outweighs the liquid in the uphill side of the siphon tube, the liquid in the tube flows to the downhill side. This explanation is incorrect however, since siphons work even when the weight of the water in the uphill side is bigger (see figure 2). This explanation would also suggest that the siphon should be able to work for any arbitrary tube height as long as one side of the siphon tube is longer than the other. However, siphons will not work above a certain tube height, depending on the density of the liquid and the surrounding air pressure (see below).

Another incorrect explanation claims that siphons depend on cohesion (see our lesson on Cohesion and Surface Tension) between liquid molecules to pull liquid through the siphon tube.1 This explanation can be proven false by leaving an air bubble at the top of a siphon tube, and observing that the siphon will pull the bubble through the tube and continue to siphon liquid from the upper reservoir (figure 3).

The true mechanism behind the siphon depends on air pressure to push liquid up into the siphon tube.

air pressure

Pressure is the force per area exerted on a surface. At sea level, there is a constant pressure of about 100,000 Newtons/m2 (= 1 atmosphere) from air molecules bombarding the surface. Below the surface of the sea, pressure is higher, coming both from the air pushing on the surface and also from the weight of the water above. The pressure depends on depth, increasing by about 0.1 atmospheres for every meter below the surface of the sea.

In a closed straw filled with liquid, pressure can be lower than one atmosphere. Stick a straw into a glass of cherry juice (or any other colored liquid) and put your finger over the end of the straw. As you draw the straw out of the glass, liquid will remain in the straw (figure 4). In order for the liquid to stay put, all the forces on the liquid must balance. Air pressure below the opening of the straw pushes up on the liquid with a pressure of one atmosphere. Air at the top of the straw, just under your finger, pushes down on the liquid from above. In addition, gravity also pulls down on the liquid. In order to balance the force of gravity, the air pressure at the top of the straw must be lower than the air pressure outside. Once again, if you're using water, the air pressure in the straw decreases by about 0.1 atmospheres for every meter of water in the straw below it. (See our lesson on Upside-down Water for a related experiment.)

It should be clear from this description that there is a maximum height to the column of water that can be supported by atmospheric pressure. If the column of water in a straw is so big that the pressure from gravity is more than one atmosphere, then air pressure from below cannot hold up the water, even if there were no air at all at the top of the straw. For water, this maximum height is a little over 10 meters ($\approx$ 34 feet). For mercury, which is the densest liquid at room temperature and pressure, the maximum height is about 76 cm. Mercury barometers use this fact to measure fluctuations in atmospheric pressure.

a better explanation of siphons

Figure 5 demonstrates how a siphon works in terms of air pressure differences. In this figure, a valve has been inserted into the siphon tube, which is shut to prevent the water from flowing. At the surface of the upper reservoir exposed to air, the pressure is one atmosphere. In the siphon tube just to the left of the valve, 10 cm above this surface, the pressure is about 0.99 atmospheres; reduced by 10 cm of water pulling down below it. At the output of the siphon tube on the right, also exposed to air, the pressure is again one atmosphere. In the siphon tube just to the right of the valve, 25 cm above the output, the pressure is about 0.975 atmospheres; reduced by 25 cm of water pulling down below it.

When the valve is opened, the higher pressure on the left will force water through the valve to the right. The siphon will begin to transfer water from the upper resevoir on the left to the lower reservoir on the right.

further resources

In light of the maximum height limit of 10 meters for a water siphon, it may seem puzzling how a tree can transport water much higher than 10 meters to its leaves. The Veritasium group provides an entertaining and illuminating explanation of this phenomenon at

  • 1. In a simple siphon, cohesive forces between liquid molecules do not play a significant role. However, this can change when the water volumes are microscopic, as they are in the cells of trees. See the video under "further resources" for more on this topic.

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