siphon in a vacuum
Tomoyuki Tanaka
i have 2 questions.
1. the easier question.
the maximum height that water can be drawn in a tube
sealed at the top (a water barometer) is around 34 feet.
does this depend on the shape of the tube?
i get the feeling that if the open end of the tube is
narrower and the closed end is wider (like a wine bottle),
it would be less than 34 feet.
2. the harder question.
according to
The Straight Dope: How does a siphon work?
http://www.straightdope.com/columns/010105.html
Encyclopedia Britannica notes that "a siphon will work
in a vacuum".
is this (as someone noted on Usenet) a meaningless
statement, because any liquid will boil/bubble vigorously
in a vacuum and hence we can't have a siphon?
i have a feeling that some liquid metals (like mercury)
would make a siphon work in a vacuum.
Uncle Al
It's been discussed here ad nauseum. If the liquid has no vaor
presure vs the ambient pressure you have a simple balance limited only
by the tensile strength of the liquid. If the liquid has a vapor
pressure commensurate with the applied presure you are limited to the
weight of the atmosphere in liquid head if the liquid column is flawed
(nucleation centers leading to cavitation). If the liquid is inflawed
you go back to the tensile strength case.
How do you think water makes it to the top of a 500 ft redwood/sequoia
when the atmospheric head of water is only 32 feet?
--
Uncle Al
http://www.mazepath.com/uncleal/
http://www.ultra.net.au/~wisby/uncleal/
(Toxic URLs! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
Edward Green
>It's been discussed here ad nauseum. If the liquid has no vaor
>presure vs the ambient pressure you have a simple balance limited only
>by the tensile strength of the liquid. If the liquid has a vapor
>pressure commensurate with the applied presure you are limited to the
>weight of the atmosphere in liquid head if the liquid column is flawed
>(nucleation centers leading to cavitation). If the liquid is inflawed
>you go back to the tensile strength case.
If I didn't know better, I might think your concept of vapor pressure
was confused. I know this cannot be the case, so I put it that
your prose is misleading. As in "has no vapor pressure vs. the
ambient pressure"? Turgid, man.
Anyway, question is "can siphon work in vacuum", and the answer is a
resounding "Yes, (I hasten to add that the following epithet is not
addressed to the innocent and harmless OP but, for example, the
harmful and blameful oolie-meister who may have promulgated false or
misleading information in the so-called straight dope) fool".
So the liquid is boiling. So what! Put inlet below surface. Boiling
does not mean "flashes to vapor". Obviously if the liquid flashes to
vapor, the siphon cannot work. Because we have no liquid. =8^}
Duh^3!
>How do you think water makes it to the top of a 500 ft redwood/sequoia
>when the atmospheric head of water is only 32 feet?
Also discussed ad nauseum in sci.physics, and with typical sci.physics
fallacy: if effect exists, and would potentially explain observation,
then the hypothesis that indicated effect explains observation is
proved beyond any shadow of a doubt, weanie-head.
(Weanie-head tag optional).
Yea, I have seen this argument many times, that "Ok, smart boy!
You don't think is effect B? But somewhere in world _is_ effect
B. So here is effect B, smarty boy pants!"
Yea have I seen this argument many times, so yea must it be true,
promulgated by wiser heads than mine, Ignorant Layperson Man.
So I realize that the ability of liquids to support tensile stress
under some conditions is prima-facie elegant, eloquent and amply
adequate proof that tree sap is under tension. WQED, FM and AM.
_____________
[I poisonally is of the opinion that water in thin phloem is being very
happily locally supported by walls, not in tension, but in deep state
of satisfaction with its fate. Like damp spot high up on wall. Is
that water less happy, will it evaporate more quickly, because it is
high up on a wall, and is dissatisfied with its gravitational
potential? If a hundred foot brick wall is damp on a wet day, will the
top dry off, because it is above the capillary limit? And is the thin
film of water wedded to the brick pores therefore under ungodly
tension?
Are these rhetorical questions?
I suspect, but cannot prove, that our analysis of thin water films in
the presence of pore or cilia is not properly handled by conventional
capillary theory.
Thus saith Ignorant Layperson Man.]
Edward Green
>On 7 Jan 2001, Tomoyuki Tanaka wrote:
>
>> the maximum height that water can be drawn in a tube
>> sealed at the top (a water barometer) is around 34 feet.
>>
>> does this depend on the shape of the tube?
>
>No! Pressure is the same at each depth no matter how much water is either
>side of any molecule.
Yes true.
>> The Straight Dope: How does a siphon work?
>> http://www.straightdope.com/columns/010105.html
>
>Inertia and capillary action.
Would capillary action apply to a siphon tube significantly larger
than a capillary? Most are, you know.
>> Encyclopedia Britannica notes that "a siphon will work
>> in a vacuum".
>
>Yes. Though it would be difficult to get it started the usual way (by
>sucking not blowing).
Good point. I'll remember that next time I have to start a siphon
while breathing vacuum. :)
>> is this (as someone noted on Usenet) a meaningless
>> statement, because any liquid will boil/bubble vigorously
>> in a vacuum and hence we can't have a siphon?
>
>Why would a liquid boil in a vacuum?
I can't tell if you are being rhetorical or not. A liquid would boil
in vacuum for the same reason it would boil anywhere else... because
its vapor pressure is higher than the ambient pressure... pretty much
by definition in vacuum! (At least at the surface, in the presence of
a gravitational field).
>> i have a feeling that some liquid metals (like mercury)
>> would make a siphon work in a vacuum.
>
>Why is mercury any different to water?
Well, much lower vapor pressure, for one. One has the feeling that
Hg(L) would not boil in vacuum at temperatures near 300K, despite
above remarks, because bubble nucleation would not occur in thin film
of metal actually below P_vapor.
Rich Grise
>
> i have 2 questions.
>
> 1. the easier question.
>
> the maximum height that water can be drawn in a tube
> sealed at the top (a water barometer) is around 34 feet.
>
> does this depend on the shape of the tube?
No.
>
> 2. the harder question.
>
> according to
> The Straight Dope: How does a siphon work?
> http://www.straightdope.com/columns/010105.html
>
> Encyclopedia Britannica notes that "a siphon will work
> in a vacuum".
No, it won't. Even if you have a zero vapor pressure liquid.
It won't siphon because there's nothing to push the source liquid
up the pipe against gravity. You know, the riser side of the pipe.
And, of course, in vacuum, "suction" is meaningless.
Cheers!
Rich
Edward Green
>On 8 Jan 2001, Edward Green wrote:
>
>> Would capillary action apply to a siphon tube significantly larger
>> than a capillary? Most are, you know.
>
>Every tube exerts capillary action to a greater or lesser extent. It
>refers to the affinity (or phobia) of the liquid and the container. Water
>inside a measuring cylinder (of any width) will climb the walls. Mercury
>will be repelled by the walls (such as in a thermometer made of glass).
Yes, thanks... I know what it is. I was suggesting that it might not
be too significant in tubes much beyond, what, a few mm? Siphons are
not confined to small tubes, AFAIK.
I've given the weighty issue a soupcon's more thought, and I would
like to weightily add, I misdoubt there is one way a siphon works:
rather a range of parameters over which siphoning is possible,
with predominance of various effects, like inertia, tensile strength
of liquid, possibly capillarity, and so forth, in various regimes.
Clearly, by reducing ambient pressure, vacuum would overall shift
the operating parameters of any particular arrangement, possibly to no
effect, possibly from "rock stable" to "can be disrupted", possibly to
"unsustainable". So there is no adequate very simple answer to "how
does a siphon work" or "will it work in vacuum".
But assuredly, _some_ arrangements will work in vacuum, while for
others, vacuum will be the last straw.
Edward Green
>> Encyclopedia Britannica notes that "a siphon will work
>> in a vacuum".
>
>No, it won't. Even if you have a zero vapor pressure liquid.
>It won't siphon because there's nothing to push the source liquid
>up the pipe against gravity. You know, the riser side of the pipe.
>And, of course, in vacuum, "suction" is meaningless.
There is nothing to "push the source liquid up the riser side of the
pipe" under air pressure, either, since air pressure is working
equally (disregarding minor pressure gradients in air) at either end
of the pipe. What air pressure does buy for you is an overall
compression, tending to keep the liquid in the pipe in liquid, rather
than vapor phase.
Perhaps that what you meant.
I have taken to thoughtless repeating "inertia"; but I am no longer
sure how much explanatory power that buys us.
True, liquid has inertia, but even if we put the entry of the riser at
the base of high reservoir, by the time a liquid slug in the pipe has
reached the surface level, it has exhausted any possible inertial
boost (my ignorant layman's talk for KE) supplied by its pressure head
-- else we would have a perpetual motion fountain!*
So if the liquid is going over the hump, additional KE for this
contained fountain is being supplied by the down side, which only does
its magic by pulling.
So I remove my "duh^3" estimation. That a siphon might work in vacuum
is really only duh^1.5 or so; it does, or would, AFAICT, really upon
the tensile strength of liquid, which as we know exists, but cannot be
relied upon. E.g. a big dirty rusty pipe siphon might start boiling in
the pipe and break the siphon if you reduced ambient pressure, while a
nice neat in the capillary lab bench siphon might happily drain away.
Duh^1.753!
Ed seizing more opportunities to pronounce upon the bleeding
physically obvious as befits a layman Green
*frowned upon in the better circles.
Edward Green
>On Mon, 8 Jan 2001, Rich Grise wrote:
>
>> No, it won't. Even if you have a zero vapor pressure liquid.
>> It won't siphon because there's nothing to push the source liquid
>> up the pipe against gravity. You know, the riser side of the pipe.
>> And, of course, in vacuum, "suction" is meaningless.
>
>I take your point, but that's why I said: You must blow it from one end,
>rather than suck, to get it going. Once it's going, inertia and other
>forces will keep it moving. Capillary action may also create some
>movement. (It does in trees).
Yes, sah, but please see my other witticism regarding inertia in a
reply to Mr. Grise. Where are the forces coming from which supply the
liquid with the inertia needed to get over the hump?
I don't think we can escape the observation that the liquid is under
tension at the top, under zero ambient pressure. Doesn't mean it
won't work, just means conditions will have to exist under which
liquid may support tension.
As for capillarity, I _think_ what you intuition may be trying to tell
you by invoking this is that liquid can indeed, under some conditions,
support tension, and more readily in capillaries than in gross pipes.
___
[I also have my own kind of crack-potty feeling that the standard
theory of capillaries is not necessarily adequate to describe the
physical state of liquids in certain very thin continuous cavities,
and that surface layer effects may effectively make the liquid quite
happy to reside where, in bulk, it would like to boil away; so that
nucleation may not even be an issue.
In effect I am proposing that the macro thermo model "bulk +
infinitesimal surface layer" breaks down for very thin films (since
after all the surface layer is not infinitesimal), and this may
have some significance in capillarity, particularly large scale
"anomalous" capillarity, like water rising to the tops of tall
trees, a hotbed of like discussion.
But this is partially a digression, partially crack-potty, though I
think I now see how to cloth it in semi-respectable language, above.]
Matthew T. Russotto
}In article <3A599D...@vel.net>, Rich Grise <rich...@vel.net> wrote:
}
}>> Encyclopedia Britannica notes that "a siphon will work
}>> in a vacuum".
}>
}>No, it won't. Even if you have a zero vapor pressure liquid.
}>It won't siphon because there's nothing to push the source liquid
}>up the pipe against gravity. You know, the riser side of the pipe.
}>And, of course, in vacuum, "suction" is meaningless.
}
}There is nothing to "push the source liquid up the riser side of the
}pipe" under air pressure, either, since air pressure is working
}equally (disregarding minor pressure gradients in air) at either end
}of the pipe. What air pressure does buy for you is an overall
}compression, tending to keep the liquid in the pipe in liquid, rather
}than vapor phase.
The problem is STARTING a siphon in vacuum.
--
Matthew T. Russotto russ...@pond.com
"Extremism in defense of liberty is no vice, and moderation in pursuit
of justice is no virtue."
Bryan Reed
>It won't siphon because there's nothing to push the source liquid
>up the pipe against gravity. You know, the riser side of the pipe.
>And, of course, in vacuum, "suction" is meaningless.
>
Water can have negative pressure, you know. And I'm not just talking
about capillary forces, I mean ordinary tension in an actual bulk
material. It's unstable, of course, and you have to avoid nucleation
sites where cavitation can occur, but if you do avoid cavitation, you can
achieve surprisingly large negative pressures. Physics Today had a nice
article on this about 6 months ago.
Have fun,
Bryan
Tomoyuki Tanaka
i'm still not sure about the answer to my "easier" question.
inetia is a factor only AFTER the siphon gets going.
so it's a only minor factor.
capillarity is not a factor. for one thing, capillary tubes
won't enable siphons.
cap-il-lar-i-ty \,kap-e-'lar-et-e^-\ pl -ties (1830)
2: the action by which the surface of a liquid where it is in
contact with a solid (as in a capillary tube) is
elevated or depressed depending on the relative
attraction of the molecules of the liquid for each
other and for those of the solid
--------------------------------------------------------------------
In article <Pine.OSF.4.10.101010...@cassius.its.unimelb.edu.au>,
Richard John Cavell <rjca...@student.unimelb.edu.au> wrote:
>On 8 Jan 2001, Edward Green wrote:
>
>> Would capillary action apply to a siphon tube significantly larger
>> than a capillary? Most are, you know.
>
Tomoyuki Tanaka
i'm still not sure about the answer to my "easier" question.
Mr Uncle Al, do you have an answer?
my comments are mostly at the end.
it would be nice if Uncle Al could provide a longer explanation
in his web site.
tensile strength seems to be a liquid's strength to resist
cavitation or sudden negative pressure. no such sudden
pressure changes are involved in a siphon. so my tentative
conclusion is that tensile strength is not a factor in a
siphone.
likewise, i don't think tensile strength is what enables
water to be drawn up to the top of a 500 ft redwood/sequoia.
i tend to think it's something like capillarity.
Edward Green
> i'm still not sure about the answer to my "easier" question.
> Mr Uncle Al, do you have an answer?
Your intuition may tell you there is more weight bearing down on that
poor neck of fluid, but your intuition is mistaken. Tell your
intuition the extra pressure is just taken up by pressure on the
portion of surface with upward normal component... which must be there
to neck down the fluid mass.
> tensile strength seems to be a liquid's strength to resist
> cavitation or sudden negative pressure. no such sudden
> pressure changes are involved in a siphon. so my tentative
> conclusion is that tensile strength is not a factor in a
> siphone.
>
> likewise, i don't think tensile strength is what enables
> water to be drawn up to the top of a 500 ft redwood/sequoia.
> i tend to think it's something like capillarity.
Conventional explanation of capillarity, like explanation of siphons,
would require water to support a tensile stress about about 34 ft.
of head. Your tentative conclusion is mistaken. And why do you
only accept answers from Uncle Al? Is he God?
(Uncle Al disqualified from answering that one).
Edward Green
> i'm still not sure about the answer to my "easier" question.
>
> inetia is a factor only AFTER the siphon gets going.
> so it's a only minor factor.
Non-sequitur.
> capillarity is not a factor. for one thing, capillary tubes
> won't enable siphons.
Why not?
Edward Green
>The problem is STARTING a siphon in vacuum.
You can't suck, so you blow... apply momentary positive pressure over
the source tank.
Uncle Al
>
> i'm still not sure about the answer to my "easier" question.
> Mr Uncle Al, do you have an answer?
>
> my comments are mostly at the end.
>
> Uncle Al <Uncl...@hate.spam.net> wrote:
> >>
> >> i have 2 questions.
> >>
> >> 1. the easier question.
> >>
> >> the maximum height that water can be drawn in a tube
> >> sealed at the top (a water barometer) is around 34 feet.
Untrue. If the container is rigid and the water column is pure,
flawless, and without vapor voids, you can draw it up to a negative
head equalling the limiting tensile strength of pure water: -300
bars, or about -4400 psi. How do you think water gets to the top of
redwoods/sequoias?
http://www.forestry.uga.edu/warnell/service/library/index.php3?docID=167&docHistory%5B%5D=4
> >> does this depend on the shape of the tube?
No. Though the narrower the bore the less chance there is for a
nucleating center to be present either on the inner surface of the
container or in the fluid itself. See how tube surface area and
volume vary with radius.
> >> i get the feeling that if the open end of the tube is
> >> narrower and the closed end is wider (like a wine bottle),
> >> it would be less than 34 feet.
Shape is irrelevant until you hit capillarity. The curvature of the
meniscus drives capillary rise. Get a physical chemsitry textbook and
stop "feeling."
> >> 2. the harder question.
> >>
> >> according to
> >> The Straight Dope: How does a siphon work?
> >> http://www.straightdope.com/columns/010105.html
A siphon is a balance. The lower end overbalances the fluid level.
> >> Encyclopedia Britannica notes that "a siphon will work
> >> in a vacuum".
Breakage of the fluid column depends on vapor pressure, dissolved
gases, flaws... The world is dirty. A mercury siphon in vacuum is
perfectly OK. Water's vapor pressure near ambient temp is about the
same number in torr as the temp in degrees C.
[snip]
Look up past discussion in http://www.deja.com/
Tomoyuki Tanaka
In article <3A5A231A...@hate.spam.net>,
Uncle Al <Uncl...@hate.spam.net> wrote:
>
>> >> i get the feeling that if the open end of the tube is
>> >> narrower and the closed end is wider (like a wine bottle),
>> >> it would be less than 34 feet.
>
>Shape is irrelevant until you hit capillarity. The curvature of the
>meniscus drives capillary rise. Get a physical chemsitry textbook and
>stop "feeling."
ok. i'll do that. thanks.
the last physical chemistry class i had was almost 20 years ago.
and it didn't cover any of this stuff.
--------------------------------------------------------------------
tensile strength seems to be a liquid's strength to resist
cavitation or [great] negative pressure. no such negative
pressures are involved in a [typical] siphon. so my tentative
conclusion is that tensile strength is not a factor in a
[typical] siphon.
but tensile strength (of water) would come into play
(because great negative pressure or pulling forces would exist)
if
i wanted to have a siphon in low pressures (e.g. vacuum)
or
i wanted to siphon up water like 10 meters.
Tomoyuki Tanaka
>>
>> inetia is a factor only AFTER the siphon gets going.
>> so it's a only minor factor.
>
>Non-sequitur.
this above was in response to an earlier comment.
>
>> capillarity is not a factor. for one thing, capillary tubes
>> won't enable siphons.
>
>Why not?
and also, read the description below. depending on the
material of the tube. water would resist going into the tube.
cap-il-lar-i-ty \,kap-e-'lar-et-e^-\ pl -ties (1830)
2: the action by which the surface of a liquid where it is in
contact with a solid (as in a capillary tube) is
elevated or depressed depending on the relative
attraction of the molecules of the liquid for each
other and for those of the solid
> And why do you only accept answers from Uncle Al? Is he God?
he surely sounds like he knows what he's talking about.
Foobar T. Clown
Edward Green wrote:
>
> Tomoyuki Tanaka <tan...@web1.calweb.com> wrote:
>
> > capillary tubes won't enable siphons.
>
> Why not?
An operating siphon must draw liquid into the tube at one end, and
expel it from the other. Capilary action wants to draw the liquid
into *BOTH* ends.
-- Foo!
Patrick Hamlyn
Apply it how? Since we're constrained to be in a vacuum, you have to seal the
container with some sort of close-fitting surface/lid/plunger thing. Now you're
not siphoning, you're squeezing as in toothpaste out of a tube. So you remove
said plunger thing. At this point any siphon you started is probably broken by
the more gross effects of adhesion of liquid to lid as it lifts.
I guess you could make the surface from a very thin sheet of ice which melts as
soon as the siphon gets underway...this is starting to get a little contrived
Uncle Al
Don't be perserveratively stooopid. You immerse the siphon hose
wholly in the liquid to wholly fill it, seal both ends, set up your
siphon, then open both ends. Is that so hard? No piston, no gas
pressure, no exogenous kickstart necessary. KISS.
Uncle Al says, "any production line that requires a PhD to run is
already dead."
Edward Green
Well, this is the kind of non-sequitur I was beating on in an attack
of ill will, and desire to find somebody wrong, somewhere, somehow.
Now, since you are a target I know can defend itself skillfully, I am
at least being sportsmanlike. :)
Because water likes to live in the capillary, it does not follow
that there is any energetic penalty to be paid for moving water
_through_ the capillary. In fact, (pretty ironic irony here),
assuming that water inside the capillary is in an energy trough
and does not want to leave, a process of driving water _through_
the capillary is a kind of inverted energy siphon. Water in, water
out, net delta E, zero.
I'm not actually 100% sure about this. ;)
My intuition tells me capillaries won't make great siphons, and I have
to have a heart to heart with my intuition in the woodshed..
"What you up to boy! What's this nonsense you been spreading, I hear!"
Whoooooop! Gonna give that boy a good whooping.
Edward Green
>e...@panix.com (Edward Green) wrote:
>
>>Matthew T. Russotto <russ...@wanda.vf.pond.com> wrote:
>>
>>>The problem is STARTING a siphon in vacuum.
>>
>>You can't suck, so you blow... apply momentary positive pressure over
>>the source tank.
>
>Apply it how? Since we're constrained to be in a vacuum, you have to seal the
>container with some sort of close-fitting surface/lid/plunger thing. Now you're
>not siphoning, you're squeezing as in toothpaste out of a tube.
Look... I anticipated this somewhat silly objection. We have to
decide if we are discussing physics, or some kind of TV game show
("Can you start a siphon in vacuum using only these tools?").
Under atmosphere we can start a siphon by either decreasing pressure
at the outlet or increasing at the inlet to get fluid started over
the hump. Well, in vacuum, I guess we cannot decrease pressure at the
outlet, so the only option is to increase pressure at the inlet! I
guess we could blow a puff of gas at the surface... Now, I hear some
game show contestant complaining "but you said it was a vacuum".
This is silly. The point is, conditions must be altered _somehow_ from
prior statis to get things started. Say, some differential pressure;
then, we can go back to vacuum.
Another thing we could do is fill the tube below the liquid level, cap
one end and remove it from the liquid, and uncap it when we are in
siphon position. Or do the game show rules prevent us from
manipulating the siphon tube in any way???
>I guess you could make the surface from a very thin sheet of ice which melts as
>soon as the siphon gets underway...this is starting to get a little contrived
Quite.
Rich Grise
>
> On Mon, 8 Jan 2001, Rich Grise wrote:
>
> > It won't siphon because there's nothing to push the source liquid
> > up the pipe against gravity. You know, the riser side of the pipe.
> > And, of course, in vacuum, "suction" is meaningless.
>
> rather than suck, to get it going. Once it's going, inertia and other
> forces will keep it moving. Capillary action may also create some
> movement. (It does in trees).
Yes, this would work, but I wouldn't be very comfortable calling the
resulting system a "siphon."
And just for curosity's sake, which "other forces" do you mean?
Cheers!
Rich
Rich Grise
>
> In article <3A599D...@vel.net>, Rich Grise <rich...@vel.net> wrote:
>
> >> Encyclopedia Britannica notes that "a siphon will work
> >> in a vacuum".
> >
> >No, it won't. Even if you have a zero vapor pressure liquid.
> >It won't siphon because there's nothing to push the source liquid
> >up the pipe against gravity. You know, the riser side of the pipe.
> >And, of course, in vacuum, "suction" is meaningless.
>
> There is nothing to "push the source liquid up the riser side of the
> pipe" under air pressure, either, since air pressure is working
> equally (disregarding minor pressure gradients in air) at either end
> of the pipe.
There's a pressure differential caused by the "down" side of the
pipe.
> So if the liquid is going over the hump, additional KE for this
> contained fountain is being supplied by the down side, which only does
> its magic by pulling.
It "pulls" only in the sense that the pressure on the output side
is lower than the pressure on the input side.
Imagine using a Hoover Vacuum cleaner in a vacuum. There's nothing
to "pull."
Hmmm... maybe the weight of the liquid itself would provide enough
pressure to keep it going. It'd be a fascinating experiment. Anybody
got a vacuum chamber in their basement?
Cheers!
Rich
Rich Grise
set up an experiment to see if it would actually work! Because
the negative pressure on the down side of the tube could
concievable "suck" liquid out of the source tank, with the
liquid itself providing the necessary pressure. Does anybody
know of a liquid with infinitesimal vapor pressure at ordinary
temperatures? It'd be a cool-as-a-rat science fair project,
either way (and I'm starting to believe it might actually
work.)
Here's another one: Can you siphon molasses?
Cheers!
Rich
Virgil
wrote:
> In fact, (pretty ironic irony here),
> assuming that water inside the capillary is in an energy trough
> and does not want to leave, a process of driving water _through_
> the capillary is a kind of inverted energy siphon. Water in, water
> out, net delta E, zero.
>
> I'm not actually 100% sure about this. ;)
You shouldn't be.
Whether you have a net delta E of zero depends on where within the
gravity field the water enters and where it leaves. If it leaves at a
higher point than it enters, there is an energy deficit. If it leaves at
a lower point, there is an energy surplus.
Richard John Cavell
> capillarity is not a factor. for one thing, capillary tubes
> won't enable siphons.
They can under some circumstances. Imagine that I had a swimming pool
full of water, and I hold a dry T-shirt just above its surface. The
T-shirt won't get wet. But if I stick a straw under the water and push it
up from under the surface to touch the T-shirt, the shirt will end up
soaked.
The straw might not be able to be very long at all in a vacuum, but
capillary action does exist. You can see it in your thermometer.
-------------------------------------------------------------
Richard Cavell - richar...@mail.com
Newsgroups - Please keep any discussion on the group, and copy your
replies to me via email. (Server problems). Sending me bulk email
guarantees a nasty response.
Judge Thomas Penfield Jackson on Bill Gates: "He has a Napoleonic concept
of himself and his company, an arrogance that derives from power"
-------------------------------------------------------------
Tomoyuki Tanaka
ok, so i was wrong about trees. tensile strength of water
is what draws up water to tall treetops.
see
http://www.britannica.com/bcom/eb/article/4/0,5716,120804+7,00.html
http://www.britannica.com/bcom/eb/article/1/0,5716,25071+1,00.html
since water, gasoline, and other liquids we typically siphon
are full of vapor bubbles, the tensile strengths of those
liquids are irrelevant.
if we had to siphon up water 12+ meters, then (and only then)
we would have to rely on its tensile strength.
(and even then, it'd be much easier to achieve this
by increasing the ambient pressure rather than by
removing vapor bubbles to rely on water's tensile
strength.)
--------------------------------------------------------------------
my earlier comment was:
tensile strength is a liquid's strength to resist cavitation
(formation of vacuum bubbles) under negative pressure.
no such negative pressures are involved in a typical siphon
so my tentative conclusion is that tensile strength is not
a factor in a typical siphon.
but tensile strength (of water) would come into play
(because great negative pressure or pulling forces would exist)
if
i wanted to have a siphon in low pressures (e.g. vacuum)
or
i wanted to siphon up water like 10 meters (?).
--------------------------------------------------------------------
all of this must be obvious to Uncle Al, so my tentative
conclusion #2 is that Uncle Al was simply having fun by
testing the gullibility of Cecil Adams, Mr Know-it-all.
Tomoyuki Tanaka
(i just realized sci.misc was moderated.)
--------------------------------------------------------------------
====================================================================
Re: siphon in a vacuum
From: Uncle Al <Uncl...@hate.spam.net>
Date: Sun, 07 Jan 2001 16:05:55 GMT
Newsgroups: sci.physics, alt.tanaka-tomoyuki, sci.misc, rec.puzzles
Tomoyuki Tanaka wrote:
>
> i have 2 questions.
>
> 1. the easier question.
>
> the maximum height that water can be drawn in a tube
> sealed at the top (a water barometer) is around 34 feet.
>
> does this depend on the shape of the tube?
>
> i get the feeling that if the open end of the tube is
> narrower and the closed end is wider (like a wine bottle),
> it would be less than 34 feet.
>
> 2. the harder question.
>
> according to
> The Straight Dope: How does a siphon work?
> http://www.straightdope.com/columns/010105.html
>
> Encyclopedia Britannica notes that "a siphon will work
> in a vacuum".
>
> is this (as someone noted on Usenet) a meaningless
> statement, because any liquid will boil/bubble vigorously
> in a vacuum and hence we can't have a siphon?
>
> i have a feeling that some liquid metals (like mercury)
> would make a siphon work in a vacuum.
>
> see also: http://www.deja.com/=dnc/getdoc.xp?AN=702733338
It's been discussed here ad nauseum.
If the liquid has no vapor presure vs the ambient pressure you
have a simple balance limited only by the tensile strength of
the liquid.
If the liquid has a vapor pressure commensurate with the
applied presure you are limited to the weight of the atmosphere
in liquid head if the liquid column is flawed (nucleation
centers leading to cavitation).
If the liquid is inflawed you go back to the tensile strength
case.
How do you think water makes it to the top of a 500 ft
redwood/sequoia when the atmospheric head of water is only 32
feet?
--
Edward Green
You missed the point. I wasn't talking about gravitational
potential... I was talking about the affinity of water for the inside
of capillaries.
The question was implicitly, having grokked the incredibly complex
question in early 21st century gravimetric physics, "how does a siphon
work", how are our august conclusions modified by using a capillary
tube as the siphon.
Edward Green
>On 8 Jan 2001, Tomoyuki Tanaka wrote:
>
>> capillarity is not a factor. for one thing, capillary tubes
>> won't enable siphons.
>
>They can under some circumstances. Imagine that I had a swimming pool
>full of water, and I hold a dry T-shirt just above its surface. The
>T-shirt won't get wet. But if I stick a straw under the water and push it
>up from under the surface to touch the T-shirt, the shirt will end up
>soaked.
Now, here is a thought experiment (I really don't know the answer):
Drop an end of that t-shirt into the pool, and drape the other end
over the side of the pool (above ground pool). The shirt will get
soaked. Will water run down the side of the pool from the wet shirt?
Edward Green
...
I think we can say that a siphon may only work in vacuum if the
working fluid can support tension.
Uncle Al
>
> Ladies and gentlemen, I believe I stand corrected. Now I want to
> set up an experiment to see if it would actually work! Because
> the negative pressure on the down side of the tube could
> concievable "suck" liquid out of the source tank, with the
> liquid itself providing the necessary pressure. Does anybody
> know of a liquid with infinitesimal vapor pressure at ordinary
> temperatures? It'd be a cool-as-a-rat science fair project,
> either way (and I'm starting to believe it might actually
> work.)
>
> Here's another one: Can you siphon molasses?
dissolve Polyox ultrahigh molecular weight poly(ethylene oxide) in
water. The thick mess will siphon itself without need for a tube.
Get it started pouring ove the edge, gets its level below the
container meniscus, then right the container. Polyacrylamide will do
the same, without the tube.
Don't confuse kinetics with thermodynamics.
Uncle Al
>
> ok, so i was wrong about trees. tensile strength of water
> is what draws up water to tall treetops.
> see
> http://www.britannica.com/bcom/eb/article/4/0,5716,120804+7,00.html
> http://www.britannica.com/bcom/eb/article/1/0,5716,25071+1,00.html
>
> since water, gasoline, and other liquids we typically siphon
> are full of vapor bubbles, the tensile strengths of those
> liquids are irrelevant.
>
> if we had to siphon up water 12+ meters, then (and only then)
> we would have to rely on its tensile strength.
>
> (and even then, it'd be much easier to achieve this
> by increasing the ambient pressure rather than by
> removing vapor bubbles to rely on water's tensile
> strength.)
Better. In fact, good enough.
Mark Sproson
Tomoyuki Tanaka <tan...@web1.calweb.com> wrote in message
news:3a583f9b$1...@news3.calweb.com...
>
> i have 2 questions.
>
> 1. the easier question.
>
> the maximum height that water can be drawn in a tube
> sealed at the top (a water barometer) is around 34 feet.
>
> does this depend on the shape of the tube?
>
> i get the feeling that if the open end of the tube is
> narrower and the closed end is wider (like a wine bottle),
> it would be less than 34 feet.
My instinct says you're right. The upward force is proportional to the
volume of the vacuum at the top. The downward force is proportional to the
volume of water in the tube. If the tube isn't uniform, then we won't get
the 34 feet thing...
>
> 2. the harder question.
>
> according to
> The Straight Dope: How does a siphon work?
> http://www.straightdope.com/columns/010105.html
>
> Encyclopedia Britannica notes that "a siphon will work
> in a vacuum".
>
> is this (as someone noted on Usenet) a meaningless
> statement, because any liquid will boil/bubble vigorously
> in a vacuum and hence we can't have a siphon?
>
> i have a feeling that some liquid metals (like mercury)
> would make a siphon work in a vacuum.
A siphon would work in a vacuum.
Imagine two containers of water, with different levels. Connect them
together with a tube. The water levels will even up, thereby equalising the
*liquid* pressure in the two containers. Nothing to do with air pressure.
Nothing to do with capilliary action (which acts at the meniscus, and when
the connecting tube is full of water there is no meniscus). It doesn't
matter whether the tube stays below or extends above the level of the water
in the tanks - except that if it extended more than 34 feet above, I guess
you'd get a vacuum at the top and no water would flow over the 'top' of the
tube.
Well, I sounded confident, didn't I? Go on, knock me down someone.
Mark
Foobar T. Clown
Patrick Hamlyn wrote:
>
> e...@panix.com (Edward Green) wrote:
> >
> >You can't suck, so you blow... apply momentary positive pressure over
> >the source tank.
>
> Apply it how? Since we're constrained to be in a vacuum, you have to
> seal the container with some sort of close-fitting surface/lid/plunger
> thing. [...] then you remove said plunger thing. At this point any
> siphon you started is probably broken by the more gross effects of
> adhesion of liquid to lid as it lifts.
You don't have to use the lid to push on the liquid! Use the lid to
contain some air and let the air do the work, and then once the liquid
begins to flow, open the valve and let the air back out.
-- Foo!
Foobar T. Clown
>
> Because water likes to live in the capillary, it does not follow
> that there is any energetic penalty to be paid for moving water
> _through_ the capillary.
And he wrote this in another:
>
> Drop an end of that t-shirt into the pool, and drape the other end
> over the side of the pool (above ground pool). The shirt will get
> soaked. Will water run down the side of the pool from the wet shirt?
I yield. You are right. I've done that with a rag over the side of
a bucket, and yes, the water will drip off the free end of the rag.
-- Foo!
Tomoyuki Tanaka
(removed sci.misc)
when Uncle Al wrote in the post attached at the end:
>
>Better. In fact, good enough.
i assume it was Prof Uncle Al telling me that my explanation
(of why tensil strength is irrelevant in a typical siphon)
got a passing grade.
(btw, does Uncle Al have a PhD in something?)
--------------------------------------------------------------------
capillarity is irrelevant in a typical siphon.
explanation #1.
(as someone noted already)
the liquid would want to get sucked into (or repelled
from) both ends, so these forces cancel out.
explanation #2.
some liquid & tube material combinations result in
attraction, and other combinations result in repulsion.
as long as the tubes are broad enough (= not like a
capillary), siphoning works just as well for all
combinations. this proves that capillarity is irrelevant.
maybe you can do siphoning with capillary tubes.
for a certain reason i don't want to go into here, i've been
doing something like the t-shirt experiment (described below)
every night, and i've noticed it works alot like a siphon.
--------------------------------------------------------------------
In article <93f0rl$irp$2...@news.panix.com>, Edward Green <e...@panix.com> wrote:
>Richard John Cavell <rjca...@student.unimelb.edu.au> wrote:
>
>>> capillarity is not a factor. for one thing, capillary tubes
>>> won't enable siphons.
>[...]
>
>Now, here is a thought experiment (I really don't know the answer):
>
>Drop an end of that t-shirt into the pool, and drape the other end
>over the side of the pool (above ground pool). The shirt will get
>soaked. Will water run down the side of the pool from the wet shirt?
yes, but just like with siphons and capillary tubes, the
water can be drawn upwards only certain distance from the
water level.
--------------------------------------------------------------------
In article <3A5B32B0...@hate.spam.net>,
Uncle Al
>
> (removed sci.misc)
>
> when Uncle Al wrote in the post attached at the end:
> >
> >Better. In fact, good enough.
>
> i assume it was Prof Uncle Al telling me that my explanation
> (of why tensil strength is irrelevant in a typical siphon)
> got a passing grade.
>
> (btw, does Uncle Al have a PhD in something?)
Not in this newsgroup. One would be remiss to confuse certification
with ability - that, or an MBA.
Edward Green
>> (btw, does Uncle Al have a PhD in something?)
>
>Not in this newsgroup. One would be remiss to confuse certification
>with ability - that, or an MBA.
I, who have no standing, salute you.
Edward Green
God bless. I wasn't actually sure what would happen myself... though
I possess the experimental apparatus nearby. Substitute bucket for
pool. Clever. ;)
Patrick Hamlyn
>dissolve Polyox ultrahigh molecular weight poly(ethylene oxide) in
>water. The thick mess will siphon itself without need for a tube.
>Get it started pouring ove the edge, gets its level below the
>container meniscus, then right the container. Polyacrylamide will do
>the same, without the tube.
I've heard that helium at a smidgeon above absolute zero behaves in the same
way. Never seen it though, my method of making things cold (crushed ice plus
salt) was a few degrees short :>
Patrick Hamlyn
>Imagine two containers of water, with different levels. Connect them
>together with a tube. The water levels will even up, thereby equalising the
>*liquid* pressure in the two containers. Nothing to do with air pressure.
>Nothing to do with capilliary action (which acts at the meniscus, and when
>the connecting tube is full of water there is no meniscus). It doesn't
>matter whether the tube stays below or extends above the level of the water
>in the tanks - except that if it extended more than 34 feet above, I guess
>you'd get a vacuum at the top and no water would flow over the 'top' of the
>tube.
>
>Well, I sounded confident, didn't I? Go on, knock me down someone.
Well... what has 34 feet got to do with anything? This is a vacuum. If your
water is pure (ie free of cavity-forming nucleation points, dissolved gases etc)
then your pipe can theoretically be 300 times as high as 34 feet - this being
the tensile strength of an ideal column of water so I'm led to believe.
Patrick Hamlyn
>The question was implicitly, having grokked the incredibly complex
>question in early 21st century gravimetric physics, "how does a siphon
>work", how are our august conclusions modified by using a capillary
>tube as the siphon.
The obvious major factor will be friction - to get water to move through a
capillary tube at any kind of observable speed required enormous pressure due to
the very high friction.
If you're talking about a siphon operating at abysmally low speed then I don't
believe anything else would prevent a capillary tube being a siphon.
Edward Green
Throughput can be improved somewhat by ganging many capillaries
together. But I think you are correct.
ort da sport
bwr...@u.washington.edu (Bryan Reed) wrote:
> In article <3A599D...@vel.net>, Rich Grise <rich...@vel.net>
wrote:
> >
> >No, it won't. Even if you have a zero vapor pressure liquid.
> >It won't siphon because there's nothing to push the source liquid
> >up the pipe against gravity. You know, the riser side of the pipe.
> >And, of course, in vacuum, "suction" is meaningless.
> >
>
> Water can have negative pressure, you know. And I'm not just talking
> about capillary forces, I mean ordinary tension in an actual bulk
> material. It's unstable, of course, and you have to avoid nucleation
> sites where cavitation can occur, but if you do avoid cavitation, you
can
> achieve surprisingly large negative pressures. Physics Today had a
nice
> article on this about 6 months ago.
>
> Have fun,
>
> Bryan
>
Bryan,
do you mean due to temp gradients?
how bout the ancient dishcloth pumping almost all the forgotten
dishwater over a sink edge overnight by continual wicking.
must be some juicy formulas for that function.
have fun,
ort
--
pardon my small letters, i am severely disabled
http://www.fh-niederrhein.de/~physik07/knobelecke/k_dorton.htm
Žrt da sport http://www.fh-niederrhein.de/~physik07/index.html
Sent via Deja.com
http://www.deja.com/