What-If 0143: "Europa Water Siphon"

[ratatoskr]

Please excuse my ignorance of physics, but in the video in note #5, isn't the vacuum setup actually communicating vessels rather than siphon?

[veryslightlygeeky]

Mills and Boon used to publish science and engineering!

Mills and Boon are well known for publishing romantic fiction. Yet, on Eric Laithwaite's paper on coil guns we see a reference to his book on Linear Electric Motors, published by Mills & Boon Technical in 1972.

Thank you, Randall. I did not know that.

A quick google search will reveal scans of M&B's ads from the 1970s, in New Scientist and IEEE Electronics and Power.

I have a ghastly feeling that this fact could lead to innuendo relating to coils, guns and such. Just don't bother, please.

[ealloc]

liquid cohesion can allow a siphon system to exhibit flow in excess of the barometric limits under which siphoning primarily operates.

So in summary, you agree that atmospheric pressure is not required to make siphons work. A degassed water siphon can work up to 5m in vacuum. Therefore siphons don't work "because of" atmospheric pressure, but because of a more general principle.

I agree with you all that both atmospheric pressure and cohesion contribute to the siphon height, and it's not really fair to frame it as "competing theories" as those papers and wikipeia do.

Here's how I would like to define the "platonic siphon": It involves balancing forces transmitted through a substance in a tube which are unbalanced by gravity acting on the substance, causing flow. Those forces can be compressive or tensile. The siphon breaks down if tensile forces overome molecular cohesion, but that is a substance-dependent detail that's not at the core idea of a siphon. Atmospheric pressure just adds extra compressive force that prevents/delays breakdown of molecular cohesion. All those videos on youtube of chains pulling themselves out of beakers work under the same principle.

[mikejr388]

If air pressure was the reason a siphon worked, water would flow uphill. In reality the air pressure is higher at the low point. Fluids flow from a high pressure to a lower pressure. Gravity and cohesion is the reason a siphon works, IMHO. If you are thinking surface area, one can siphon water from a regular drinking glass into a five gallon bucket.

Love xkcd comic.
SUDO, get me a sandwich.

[Soupspoon]

If air pressure was the reason a siphon worked, water would flow uphill. In reality the air pressure is higher at the low point. Fluids flow from a high pressure to a lower pressure.

I've got a perpetual motion machine, here, would you like to invest in it? I guarantee that once the money starts coming in, it won't stop...

SUDO, get me a sandwich.

*goes to get a sandw...*

Goddarnit!

[mikejr388]

[sevenperforce]

If air pressure was the reason a siphon worked, water would flow uphill. In reality the air pressure is higher at the low point. Fluids flow from a high pressure to a lower pressure. Gravity and cohesion is the reason a siphon works, IMHO.

Gravity, yes. Cohesion...no.

Gravity is the reason that a siphon works; air pressure is the primary driver which keeps a vacuum from forming and allows the flow to remain continuous; cohesion contributes only minutely to this.

True, the air pressure pushing up on the water at the lower end of the hose is greater than the air pressure pushing up on the water at the higher end of the hose, and for a hose shorter than about 10 meters, either air pressure is greater than the weight of the water. However, the weight of the water isn't contending with the entire air pressure; it is only contending with the pressure drop. If the weight of the water in the "downstream" side of the hose is greater than the atmospheric pressure drop, it will flow.

We can do a numerical analysis if it's still murky. Let's say you've got a hose with a cross-sectional area of 1 cm² that goes up one meter and goes down two meters. The volume of the shorter side is thus 100 mL while the volume of the longer side is 200 mL. The weight of the water in the long side is about 2 N, while the weight of the water in the short side is about 1 N.

Meanwhile, the pressure at the bottom end (assuming sea level) is 101.325 kPa; times the cross-sectional area in question, this comes to an upward force of 10.133 N. On the bottom end, which is 1 m higher, the atmospheric pressure is 101.313 kPa, for an upward force of 10.131 N. So although the total atmospheric-pressure force on each end is greater than the corresponding weight of the water on that segment, the difference between the water weight on the long end and the water weight on the short end (1 N) is much greater than the difference between the atmospheric-pressure-force between the two ends (0.002 N).

Because water is mostly incompressible, it transmits this pressure all the way up, and so the nonzero total forces lead to flow.

[Coyne]

And if it turns out Europa's water has some weird alien pathogen in it, you might accidentally kill all your customers.

Hey, let the buyer beware.

[ijuin]

Yes, but dead customers don't come back to buy more. Unless of course they are undead customers.

[Coyne]

Yes, but dead customers don't come back to buy more. Unless of course they are undead customers.

So long as the bottom line for this quarter looks good who cares?

[ijuin]

Well, that's a problem for NEXT quarter.

[mathmannix]

Hope you all like salty alien water!

Why on earth would it be salty?

[cyanyoshi]

Hope you all like salty alien water!

Why on earth would it be salty?

Liquid water is really good at dissolving stuff. If there is an ocean under all that ice, then it's reasonable to expect that it has gathered all kinds of minerals from meteorites and the surrounding rock, and it would therefore be salty like Earth's ocean. It is speculated that Europa's slightly dirty appearance could be the result of sea salt.