Am I understanding the concept of suction correctly?

sophiecentaur

Oh yes, I see - very much hermitacally sealed. I have been thinking how much you could expect windows to bow inwards or outwards. You could expect a pressure range of, say 20% between extremes of high and low. That would represent 20kPa of pressure variation at sea level (+/- 10kPa around an average value). If the gap in the glass were 4mm then you could imagine each sheet could move by 0.2mm (Boyle's Law and assuming the sheets were frictionless pistons) If they were very flexible and fixed around the outside, you would get about twice this movement at the centre (0.4mm). The radius of curvature of such a mirror would be, I estimate, about 600m - giving a focal length of 300m. As it is, glass is fairly rigid so the flexing would be quite a bit less (half / quarter??) than that. I wonder just how much this amount of curvature would actually be visible. It's hard to tell. Perhaps distortion of images of nearby buildings would,in fact, be visible. We're quite sensitive to that sort of thing.
I'd expect a similar effect due to a +/-10% variation in temperature (60^oC in 300K) - also significantly reduced by the rigidity of the glass.

I think my back of fag packet sums are reasonable.

Nugatory

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

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

But based on your previous description of quantifying "suction":

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

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

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.

"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.



http://en.wikipedia.org/wiki/Transpiration_pull

sophiecentaur

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.

Who made that argument?

sophiecentaur

OK, calm down. I meant all the other prats who don't know Science. And there are a lot of them.

A.T.

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

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.