Siphons Work Due To Gravity, Not Atmospheric Pressure: Now With Peer Review

knwny (2940129) writes "Peeved by the widespread misconception that siphons work because of atmospheric pressure, physics lecturer Dr. Stephen Hughes, [in 2010] wrote a mail to the prestigious Oxford English Dictionary(OED) pointing out the error. To back his claim, Dr.Hughes tested a siphon inside a hypobaric chamber to check if changes in atmospheric pressure had any effect on the siphon and demonstrated that gravity and not atmospheric pressure was the driving principle. [This week, the] paper detailing his experiment was published in Nature. The OED spokesperson responded saying that his suggestions would be taken into account during the next rewrite."

Re:Actually it's both.

[jeffb (2.718)]

He demonstrated no such thing. In fact, he demonstrated that the siphon stops working at sufficiently low atmospheric pressure:

When the pressure was reduced further the siphon broke into two columns - in effect becoming two back-to-back barometers.

You can't pull on one end of a column of liquid and drag the whole column up. Something has to push it from the bottom, unless its own inertia can carry it.

Saying "siphons work due to gravity, not atmospheric pressure" is like saying "fire works due to oxygen, not fuel".

Re:Actually it's both.

[Noah Haders]

i didn't realize there was any confusion about this. obviously it's because of gravity. it's like when you have a long chain suspended from above with both sides hanging down. when the two sides are the same length it is stable, but when one side becomes longer than the other then the weight of gravity pulls the whole chain down. duh?

Re:Intuitive

[petermgreen]

AIUI a siphon needs BOTH gravity and air pressure.

Gravity makes water in the down side of the siphon move downwards. This reduces the pressure at the top of the siphon to less than atmospheric pressure. Which in turn allows atmospheric pressure to push water up the up side of the siphon.

Take away gravity and there is nothing to pull the water down the down side. Take away atmospheric pressure and there is nothing to push the water up the up side. Either way your siphon won't work.