Water has a tendency to seek its own level. The physics behind water’s tendency to flow until all of it is at the same level is related to potential energy, the energy stored in forces such as those of gravity. The higher water is, the more gravitational potential energy it will have. Water, like everything else, accelerates in whatever direction reduces its total potential energy as quickly as possible. In the case of water that’s not level in an open container, the higher water accelerates toward the lower water so that the higher water reduces its potential energy as quickly as possible. Open water always accelerates so as to level itself.
The first time you use a siphon, it may seem like magic, however it’s simply scientific principles of at work. Exactly what principles are at work, though, is still a matter of debate. Even though siphons have been used for thousands of years, modern scientists are still arguing about exactly which forces make siphons work. A basic siphon consists of a tube or a pipe in a larger container that goes up over a hump (the edge of the container) to empty out into a container at a lower level. The word siphon is used to refer to a wide variety of devices that involve the flow of liquids through tubes. Siphons are unique devices that allow water to defy gravity, so water can be raised higher than the supply elevation before eventually falling to the delivery elevation. A siphon uses the differences in elevation and atmospheric pressure to make water flow. If the both ends of the pipe are submerged and air is removed at the high point, that primes the pipe, and the atmospheric pressure on the supply water surface will move water up the pipe to the high point, as long as this point is set at an appropriate elevation, and gravity will move the water from there down to the delivery point.
In a siphon, where the pipe goes first upward from the higher container and then downward to the lower container, the weight of water in descending portion of the pipe actually decreases the pressure inside the rising portion of the pipe. As a result of this extra pressure drop, water in the high container can reduce its total potential energy by accelerating toward and then through the pipe. Water begins flowing through the pipe, even though it has to go upward for a short time during that passage. Surprisingly enough, its total potential energy is decreasing the whole time, even as it rises, because the pressure potential energy drops quickly enough in the pipe to more than make up for the rise in gravitational potential energy.
The traditional theory for how siphons function 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. When liquid is sucked through the tube over the hump and begins to empty into the other container, a decrease in atmospheric pressure is caused at the highest point in the tube (where it passes over the hump). This decrease results in the atmospheric pressure on the surface of the liquid pushing liquid up into the tube toward the area of lower pressure.
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. 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.
While the atmospheric pressure theory seems to make sense, some scientists noted that it requires the presence of air. When tested in a vacuum, a siphon still worked, so it seemed that some other force must also be at work. The atmospheric pressure with gravity theory obviously cannot explain why siphons work in vacuum. Recently, scientists who have studied siphons have theorized that the key force is gravity. When liquid is sucked up the tube and over the hump, the force of gravity continues to pull the liquid through the tube. This newer theory relies upon liquid cohesion, which means a continuous chain of cohesive bonds must exist in the liquid. Cohesion makes all the drops of water stick together and because the molecules are chained together, they behave like a chain. Scientists refer to this as the chain model, because you can think of the water like a chain being pulled through the tube instead of a liquid. When you begin to pull the chain through the tube and over the hump, gravity will take over and continue to pull the entire length of the chain through the tube.
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. These problems can only be satisfactorily explained by understanding Bernoulli’s equation, P + pgh + (1/2)pv2 = constant, which applies to fluids without cohesion forces. Bernoulli’s equation contains both a gravity and a pressure term, so asking whether one of the two drives the operation of a siphon may not be a meaningful question. Moreover, additional forces (such as surface tension) may play some role in a real siphon, although they are not explicitly included in Bernoulli’s equation. If enough fluid flows past the bend of the siphon, continuous operation of the device is established. Once a steady-state flow is reached, a combined mechanism in which both gravity and pressure contribute to siphon operation, without the need of forming a fluid chain.