In this experiment, we’ll build a self-propagating water pump that sucks out water without any need for electrical power or other constant energy inputs.
- Folding straws (4-8, depending on the size of the desired pump; a flexible pipe may also be used)
- Scotch tape or masking tape
- Two cups
The experiment can be observed in the following video:
Take Note! Make sure to seal the connections between the straws properly. If there are any leaks, air will come into the system and the experiment won’t work.
A siphon works by having two “arms” full of water at different heights. And as we all know from everyday life, water tends to flow from a high place to a lower place because of gravity, in the same way solid objects fall. So, as long as we have a height difference on either side of the siphon, it will keep working.
The amazing thing about the siphon is that while generally the water goes from a higher plane to a lower plane, it first gets sucked upwards on the short arm of the U-shape.
To understand this phenomenon we need to understand the pressures applied to the water in the siphon:
As the animation shows, the water column in the long (right-hand) arm of the siphon is longer than the water column in the short arm, so the pressure at the end of the long arm of the siphon is higher than the pressure on the short end of the siphon. Air pressure applies to all the water equally and so balances out.
Since the water pressure on the long side of the siphon is greater than on the short side, when we open the openings, water starts flowing down the long side. This can be compared with scales beginning to tilt towards the heavier side.
As soon as water starts coming out of the long side of the siphon, pressure decreases, because water has flown out. This negative pressure causes water on the horizontal part of the siphon to be drawn (or, more accurately, pushed) into the long arm, which, in turn, pushes water from the short arm.
This is like how a syringe sucks water out of a cup when we pull its plunger; it isn’t the syringe or horizontal arm that is sucking the water. The water is pushed in by the surrounding air pressure, causing them to enter places where pressure is lower. Overall, a sequence of pressure differences makes water flow out of the higher cup, up the short arm, across the horizontal arm and down the long arm into the lower cup.
What powers this “pump”? Well, earth’s gravity helps siphons function, and without it, water would not have weight or exert any pressure and therefore not have any flow. Gravity also causes external air pressure, which also participates in siphon function.
Siphons were used in ancient Egypt as early as 1500 BC as a way to empty water vessels. But only in the 17th century (over 3,000 years later!) did the scientist Blaise Pascal figure out the principles that drive siphons. Siphons are used to this day to empty vessels that are hard to tip over, such as gasoline tanks in cars or large beer barrels.
Many interesting experiments can be done using the pressure differences of water and air. Water can be drawn into an upside-down cup using a candle, a balloon can be inflated in a bottle, and a cup of water can be flipped over without spilling. Also, check out the self-emptying cup experiment, which uses the same siphon principle and is easier to perform than this one.
Dr. Avi Saig
Davidson Institute of Science Education
Weizmann Institute of Science
Article translated from Hebrew by Aviv J. Sharon, M.Sc. student at the Weizmann Institute of Science.
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