Comments on: Oh, Snap! Physics Prof Finds 99-Year-Old Mistake in the Dictionary

By: Bill

Bill — Wed, 19 May 2010 21:19:14 +0000

Gravity IS responsible for a siphon’s action. Atmospheric pressure works on both ends of a siphon and, in fact, is a little higher at the lower end. Atmospheric pressure’s primary function is to keep the water from boiling at normal temperatures. Look at a rope over a pulley. If you have more rope over one side of a pulley, gravity will pull the shorter end through, atmospheric pressure plays no part. A siphon works the same way. A siphon’s upper end is limited by how high the water is drawn upward. Too high and the water will separate, a vapor lock forms and the siphon fails.

By: MickT

MickT — Wed, 12 May 2010 02:30:06 +0000

The lower leg of the siphon is indeed emptied by gravity. And if there is no leakage for compensation, there is a resulting reduction in pressure transferred to the upper leg, whereupon atmospheric pressure PUSHES the liquid up the upper leg of the siphon. Gravity has no direct effect on the liquid in the upper leg of the siphon. Like my old high school physics teacher said, “There is no suck”.

By: romar

romar — Tue, 11 May 2010 12:17:42 +0000

I agree with neal. His explanation is the best, because it incorporates both principles (gravity and air pressure) that makes the siphon work. And for that reason the original explanation and the new one are flawed.

By: Tel

Tel — Tue, 11 May 2010 12:16:15 +0000

“… the water in the longer downward arm pulling the water up the shorter arm …”

With this fantastic breakthrough discovery of high tensile water, all sorts of possibilities open up. I’m expecting ropes made of water very soon, and after that maybe fishing line made of water, even umbrellas made of water. Hopefully Dr Hughes can demonstrate a siphon such that the shorter leg lifts water by a distance of 15 meters (NB: atmospheric pressure can only lift approx 10m of water) and the whole world will be very impressed.

By: neal

neal — Tue, 11 May 2010 01:36:49 +0000

I wouldn’t be too quick to say that original definition is wrong, though I suppose it might be a bit incomplete. Air pressure pushes the liquid up the siphon on the high side. It does this because liquid falling out of the low side of the siphon reduces the pressure inside the siphon, leading to the great difference in pressure. If the upper chamber (source of liquid) is sealed, the siphon will stop working when the air pressure there drops low enough.

The statement “… with the water in the longer downward arm pulling the water up the shorter arm” I find very misleading as well, because it implies there are tension forces in the liquid, and I’m pretty sure that they cannot be strong enough to “pull” the liquid up.

By: MutantJedi

MutantJedi — Tue, 11 May 2010 00:55:48 +0000

Seriously atmospheric pressure common sense? The air pressure difference of a couple feet could hardly account the brisk flow of water draining my fish tank. Moreover, wouldn’t this be calling for flow from a lower pressure area to a higher pressure area (a couple feet up would have an immeasurably lower air pressure)?

By: Mr. Owl

Mr. Owl — Mon, 10 May 2010 23:07:16 +0000

Funny…That’s how I always thought Siphoning worked until just recently, it seemed like common sense to me to be the result of Atmospheric Pressure….I’m glad to see I’m not alone

By: joe

joe — Mon, 10 May 2010 22:40:18 +0000

Wikipedia seems to have a pretty good definition for ‘siphon’:
“the resulting flow is driven by hydrostatic pressure exerted due to the force of gravity”

However, I can’t be sure if Wikipedia always had this definition or if someone changed it as a result of Dr. Hughes discovery?