Saturday, May 15, 2010
Vacuous dictionaries
A scientific trivium making the rounds this week:
Australian physicist finds long-standing "error" in dictionaries.
In a narrow strict sense, Hughes is right.
But as much as I enjoy seeing the experts knocked off their horses, I have to conclude that the dictionaries are perfectly correct in practical usage, which is the main job of a dictionary.
Here's the familiar siphon. Gravity is certainly involved; the liquid level in the destination must be lower than the liquid in the source. As long as the source has higher potential energy than the destination, you'll get flow, even though part of the hose may go higher than either level.
But atmospheric pressure is more salient to the task. As Polistra shows in #1, you have to create a pressure lower than ambient at the far end of the hose, in order to fill the hose completely with liquid. And then you have to keep air strictly out of the hose during the transfer; otherwise the adhesive forces between the liquid molecules will break, and the flow will stop.
In any practical siphon, your manipulation of air pressure is more important than your manipulation of gravity.
= = = = =
Initially I wasn't going to write about this until I saw this more recent news item which coincidentally proves the point from a different angle. BP is using a "siphon tube" to pull oil from the busted wellhead in the Gulf. Gravity has nothing to do with this process; they're certainly not trying to transfer the oil into an even deeper undersea well! And yet we all understand what they mean by a siphon: a pumping action that depends on keeping air out of the tube. ¶ 3:02 PM
Australian physicist finds long-standing "error" in dictionaries.
Dr. Stephen Hughes of the University of Technology in Brisbane said he was researching an article for science teachers when he discovered an error in the definition of "siphon."
He said the definition falsely claims siphons work due to atmospheric pressure when it is actually gravity that performs the function.
In a narrow strict sense, Hughes is right.
But as much as I enjoy seeing the experts knocked off their horses, I have to conclude that the dictionaries are perfectly correct in practical usage, which is the main job of a dictionary.
Here's the familiar siphon. Gravity is certainly involved; the liquid level in the destination must be lower than the liquid in the source. As long as the source has higher potential energy than the destination, you'll get flow, even though part of the hose may go higher than either level.
But atmospheric pressure is more salient to the task. As Polistra shows in #1, you have to create a pressure lower than ambient at the far end of the hose, in order to fill the hose completely with liquid. And then you have to keep air strictly out of the hose during the transfer; otherwise the adhesive forces between the liquid molecules will break, and the flow will stop.
In any practical siphon, your manipulation of air pressure is more important than your manipulation of gravity.
= = = = =
Initially I wasn't going to write about this until I saw this more recent news item which coincidentally proves the point from a different angle. BP is using a "siphon tube" to pull oil from the busted wellhead in the Gulf. Gravity has nothing to do with this process; they're certainly not trying to transfer the oil into an even deeper undersea well! And yet we all understand what they mean by a siphon: a pumping action that depends on keeping air out of the tube. ¶ 3:02 PM
