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How does a siphon work?

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Quick answer

A siphon moves liquid from a higher container to a lower one over a hill without a pump. Atmospheric pressure pushes liquid up the short side, while gravity pulls the longer, heavier column down the exit side. The pressure difference between the two columns drives continuous flow as long as the tube stays full and the exit is lower.

Step by step

  1. The siphon tube is completely filled with liquid, either by suction or by submerging it, so no air remains inside.
  2. The short end of the tube is placed in the upper container, below the liquid surface.
  3. Atmospheric pressure pushes down on the liquid in the upper container, forcing liquid up into the tube.
  4. The liquid flows over the highest point of the tube and down the longer exit side.
  5. Gravity pulls the longer column of liquid down, creating a lower pressure at the top of the tube that keeps pulling liquid up from the upper container.
  6. The flow continues until the liquid level in the upper container drops below the tube's opening or the exit is no longer lower.

Key things to understand

  1. A siphon is a tube or channel that carries liquid from a higher level to a lower level over an obstacle.
  2. The tube must first be filled with liquid, either by suction or by immersion, so it contains no air pockets.
  3. Atmospheric pressure pushes down on the liquid surface in the upper container, forcing liquid up into the tube.
  4. Gravity pulls the longer column of liquid on the exit side down, creating a pressure difference that drives the flow.
  5. The flow continues as long as the exit is lower than the liquid surface in the upper container and the tube remains full.

Labelled diagram

  1. Upper container

    Liquid surface is higher; atmospheric pressure pushes down on it.

  2. Siphon tube

    Completely filled with liquid; no air bubbles inside.

  3. Highest point

    Liquid rises here due to pressure difference; must be below the barometric limit.

  4. Exit side

    Longer column of liquid falls, pulled by gravity, creating suction.

  5. Lower container

    Liquid collects here; exit must be lower than the upper liquid surface.

How a siphon moves liquid over an obstacle

Real-world examples

  • A flush toilet uses a siphon in its curved trapway to pull water and waste out of the bowl.
  • Draining a fish tank by placing a hose from the tank to a bucket on the floor, using suction to start the flow.
  • Emptying a swimming pool or waterbed by running a hose from the pool to a lower drain, using the pool's water pressure to prime the siphon.
  • In ancient Rome, siphons were used in aqueducts to carry water across valleys, using closed pipes that followed the terrain's contours.

Common mistake

✗ Myth: A siphon works because suction pulls the liquid up the tube.

✓ Fact: Suction is not a force; it is the result of a pressure difference. Atmospheric pressure pushes the liquid up the tube, while gravity pulls the longer column down, and the difference in weight between the two columns drives the flow.

People also ask

What causes a siphon to stop working?
A siphon stops when air enters the tube, breaking the continuous liquid column. This can happen if the upper container empties below the tube's opening, if the tube develops a leak, or if the highest point of the siphon exceeds the barometric limit (about 10 meters for water at sea level). Once the column breaks, the pressure difference disappears and flow ceases.
How does a siphon work without a pump?
A siphon works without a pump because it uses gravity and atmospheric pressure. Once the tube is primed with liquid, the weight of the longer column on the exit side pulls liquid down, creating a lower pressure at the top of the tube. Atmospheric pressure then pushes liquid up from the upper container to replace it, maintaining continuous flow.
Can a siphon flow uphill?
A siphon can make liquid flow uphill temporarily, but only over an obstacle and only if the final exit is lower than the starting liquid surface. The liquid is pushed up by atmospheric pressure, but the overall flow is always from a higher level to a lower level. A siphon cannot make liquid flow to a higher final destination without external energy.
Why does a siphon need to be primed?
Priming fills the tube completely with liquid, removing air. Air is much less dense than liquid, so if air remains, the weight difference between the two columns is too small to overcome friction and push the liquid up the short side. Once primed, the continuous liquid column allows the pressure difference to drive flow.
Can a siphon work in a vacuum?
No, a siphon cannot work in a vacuum because it relies on atmospheric pressure to push the liquid up into the tube. In a vacuum, there is no external pressure to support the rising column, so the liquid would simply fall back down. Gravity alone cannot lift the liquid without the pressure difference created by the atmosphere.
What limits the maximum height a siphon can lift water?
The maximum height is limited by atmospheric pressure. At sea level, atmospheric pressure can support a column of water about 10 meters (34 feet) high. If the siphon's highest point is more than this above the water surface, the pressure is too low to push water up, and the column breaks, stopping the flow.
Does a siphon work with any liquid?
A siphon works with any liquid, but the maximum height depends on the liquid's density. Denser liquids, like mercury, can be siphoned to much lower heights because the same atmospheric pressure supports a shorter column. Less dense liquids, like alcohol, can be lifted slightly higher than water.

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