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."

Somebody thought it was atmospheric pressure?

[Anonymous Coward]

Seriously? If atmospheric pressure had any influence, it would do the opposite: The pressure at the lower end of the tube is higher than at the other end, so the fluid would flow upward. Obviously this doesn't happen.

Re:Straws don't make good siphons anyways.

[TechyImmigrant]

I'll resist the temptation to carry on with the bad puns and innuendo, but....

u-tube plonked in water in an elevated bucket, one end outside the bucket.

1) You suck on the dry end. Water moves up to the apex of the tube.
        It's atmospheric pressure pushing the water up the tube as your sucking reduces the pressure in the tube.

2) Water keeps moving around the bend, past the apex.
          It's a combination of your sucking and momentum that keeps the water moving.

3) The water reaches a point lower than the surface of the water in the bucket. You stop sucking.
          It's the gravity (or the water seeking a lower energy state in a gravitational field) that keeps the water moving through the tube.

So all things are having an effect, which makes sense. Atmospheric pressure doesn't magically stop happening just because gravity is having a stronger effect.

 

Re:Actually it's both.

[GTRacer]

re: the summary's title: One simple word would have needed all this hand-wringing. "Siphons Work PRIMARILY Due To Gravity [...]"

Also, help me out. Isn't reducing pressure at one end how siphoning is started? I understand gravity's role in moving the column of fluid along, but as pointed out, you need both gravity and pressure, right?

Re:Actually it's both.

[DriveDog]

BUT... boiling is not required. If the pressure is zero, the liquid can break apart, leaving voids filled with... nothing. I'm also not convinced that siphoning at pressure zero can never work. Mercury, for example, has a lot stronger bond between molecules than water. Maybe some liquid could pull through a tube, even without filling it, like a string. Can anyone provide such an example, or a good reason that no substance will work that way?

sorry, you can siphon a gas

[raymorris]

> As some liquid pulls out and follows the force of gravity; a suction is created, and water molecules that are adhering follow the flow this creates.

That fact that you can siphon a gas shows that "molecules adhering" has nothing to do with it. A fun way to see this for yourself is to put some dry ice in water, then siphon off the CO2. The cold CO2 isn't MUCH heavier than air, so the siphon doesn't flow very fast, but it does flow.

Gravity pulls the fluid out of the low side, creating low pressure in the tube. The higher atmospheric pressure then pushes fluid into that low-pressure tube from the upper reservoir.

> After pressure is reduced by 80%; the substance ceases to be a proper liquid -- in essence, it loses the properties of water.

Which doesn't matter. Try the dry ice CO2 experiment to see for yourself.