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Am I understanding the concept of suction correctly?

by spacediver
Tags: concept, correctly, suction
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Nugatory
#19
Oct31-12, 12:54 PM
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Quote Quote by spacediver View Post
I'm not sure that the term "negative pressure" will adequately serve this purpose.
It works better than you'd expect at first, because a pressure drop is the difference between two pressures, so doesn't care about the absolute number assigned to either pressure. I can think of a 4 psi drop as 14 psi outside, 9 psi inside, or as 0 psi outside, -4 psi inside, and I get pretty much the same flow behavior.
AJ Bentley
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Oct31-12, 02:10 PM
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Quote Quote by Nugatory View Post
It works better than you'd expect at first, because a pressure drop is the difference between two pressures, so doesn't care about the absolute number assigned to either pressure. I can think of a 4 psi drop as 14 psi outside, 9 psi inside, or as 0 psi outside, -4 psi inside, and I get pretty much the same flow behavior.
The reason I have a 'down' on the idea of suction is because of a much admired Chemistry master at my school (many, many decades ago).
He would hand someone a test tube full of water with a bung and tube in the top and instruct them to suck the water out.
It's impossible of course - "There's no such thing as a 'suck'" he would say.

He was quite right - the idea is both unnecessary and fallacious.
spacediver
#21
Oct31-12, 03:12 PM
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Quote Quote by Nugatory View Post
It works better than you'd expect at first, because a pressure drop is the difference between two pressures, so doesn't care about the absolute number assigned to either pressure. I can think of a 4 psi drop as 14 psi outside, 9 psi inside, or as 0 psi outside, -4 psi inside, and I get pretty much the same flow behavior.
But based on your previous description of quantifying "suction":

Quote Quote by Nugatory View Post
The standard measure is volume of air moved per unit time at a given pressure drop, something like "500 cubic feet per minute at 3 psi" for example.
it seems that the volume of fluid moved per unit time is partially independent of the pressure drop. And if this is the case, then pressure drop alone isn't enough to quantify the phenomenon in question - we would presumably also need information about the area and length of the channel connecting the two volumes.
Nugatory
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Oct31-12, 03:23 PM
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Quote Quote by spacediver View Post
it seems that the volume of fluid moved per unit time is partially independent of the pressure drop. And if this is the case, then pressure drop alone isn't enough to quantify the phenomenon in question - we would presumably also need information about the area and length of the channel connecting the two volumes.
Yes, you need both the flow and the pressure drop... I was just observing that the "negative pressure" concept doesn't have to interfere with calculating the pressure drop.
spacediver
#23
Oct31-12, 05:47 PM
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This thread has inspired me to learn about fluid dynamics - gonna start with khanacademy :)

As for "suction", I suppose it's better to speak of rate of flow, given a host of variables, pressure differential being one of them.
A.T.
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Nov12-12, 04:06 AM
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Quote Quote by AJ Bentley View Post
You can talk about negative pressure if you like - but suction is a lay term more properly applied to house-wifery than physics.
"Negative pressure" can mean different things:
http://en.wikipedia.org/wiki/Pressur...tive_pressures
- When dealing in relative (gauge) pressures. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of −21 kPa (i.e., 21 kPa below an atmospheric pressure of 101 kPa).

- When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable since the particles will move closer together until repulsive forces balance attractive forces. Negative pressure exists in the transpiration pull of plants, and is used to suction water even higher than the ten meters that it rises in a pure vacuum.
For the latter case the terms "pull" or even "suction" seem appropriate.

Quote Quote by AJ Bentley View Post
The force that moves fluid particles around comes from mutual repulsion and collisions. There isn't a 'suction force'. It's just woolly thinking.
Try to explain how trees get water 100m high, using only repulsion and collisions.



http://en.wikipedia.org/wiki/Transpiration_pull
sophiecentaur
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Nov12-12, 05:08 AM
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Quote Quote by A.T. View Post
"Negative pressure" can mean different things:
http://en.wikipedia.org/wiki/Pressur...tive_pressures

For the latter case the terms "pull" or even "suction" seem appropriate.


Try to explain how trees get water 100m high, using only repulsion and collisions.
That's a very good little movie and it shows just how wrong people are (pretty well always) when they use the term 'Suction'. Yes, they are mostly wrong - because in almost every case, they are NOT talking about what goes on in trees.
The argument that there must be suction in a drinking straw because of how trees work is a nonsense. It uses one, very specialised situation to expain, wrongly, another.

The 'liquid state' is by far the most difficult to explain, of the three states, and it is the least common. The particles in liquids behave in a very weird way, bonded together in a very promiscuous way, yet mobile - a bit like the outer electrons in metals. Only two elements are liquid at room temperature and I wouldn't mind betting that proportion would apply, whatever temperature your room happens to be. With mixtures, the description 'liquid' can span a wide temperature range and gives rise to a lot of 'schoolboy' confusion as a result of the dreaded 'classification' thing that we all are made to think is so important.
A.T.
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Nov12-12, 06:31 AM
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Quote Quote by sophiecentaur View Post
The argument that there must be suction in a drinking straw because of how trees work is a nonsense.
Who made that argument?
sophiecentaur
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Nov12-12, 07:08 AM
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OK, calm down. I meant all the other prats who don't know Science. And there are a lot of them.
A.T.
#28
Nov12-12, 07:44 AM
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Quote Quote by sophiecentaur View Post
I meant all the other prats who don't know Science.
They deserve a differentiated explanation. Claiming there is no "suction" in general, because fluid particles interact only through repulsion is wrong too. AJ Bentley's teacher got away with it decades ago, because kids didn't have internet then.
sophiecentaur
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Nov12-12, 08:24 AM
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The problem with the 'larger' differentiated explanation can be that, if it isn't a really full explanation (and the receiver of the explanation must be prepared to hear it all the way through) the exceptions can be taken as the rule. Details count. (Hence the frequent questions about Wormhole transportation and the like)
There's a guy in Devon who reckons he's done a siphon demo which exceeds the 10m limit. Fair enough - not impossible, as trees show us - but he claims that the reason it works is because he has a continuous U of pipe and the bit 'hanging down' is pulling the bit that's going up.
I'd like a good explanation of the effect and also to know why it's not been shown to work more often. The video from A.T. above points out the need for the plant tubing to be completely clear of air and bubble-forming nuclei for trees to do the trick.


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