This is a simplified answer I’m seeking.
A bucket of water and a pipe with a 10 psi less pressure than that of the surrounding atmosphere in otherwords a vacuum.
The only place that has an opening is the end that almost goes to the bottom of the water bucket.
Will the pipe fill with about 10 pounds of water above the bucket’s water line?
How do you figure the amount of water that will draw into the pipe if the above is wrong?
A vacuum is not usually 10psi less the atmospheric, more like 14psi at sea level.
It would make a difference if the water totally fills the pipe or if you start with a airspace. If there is no airspace you can get negative pressure by lifting the pipe - not negative gage pressure but pressure below vacuum.
Here is a converter.
10 pound/square inch = 276.799 048 425 inch of water [4 °C]
Since nothing is said about the diameter of the pipe, I believe the answer to #3 is no.
Nope. The number of pounds lifted depends on the diameter of the pipe.
If you stick the pipe in the bucket and then pull vacuum, the water column will be high enough to exert 10psi downward force at the point it passes the surface of the liquid in the bucket; about 21 feet above the bucket’s water line.
For a 1" pipe (area = 5.1 cm) the water lifted would mass about 3.4 kg. For a 2" pipe, it’d mass 13.6 kg.
It will not fill with 10 lb of water unless the cross section of the pipe is 1 sq. inch, and the end of the pipe is just at the surface when it has sucked that much water from the pipe.
Water density is 62.43 LPCF (lbm/foot^3). So to lift water a foot up a pipe it takes 62.43 LPCF/144sqinches/foot =.434 psi.
10 psi/.43 psi/foot = 23.06 feet of lift. This is measured from the surface of the water in the bucket, not the end of the pipe.
On edit, this is only correct if the pressure in the pipe is maintained, which is not the OPs case. Never mind, carry on.
I agree. It depends on the cross section of the pipe. You can’t get water higher in a pipe than atmospheric pressure allows. Remember, it is the atmospheric pressure pushing the water up the pipe, not the vaccuum sucking it.
IIRC, atmospheric pressure is about 32 ft of water. Even if the conditions in the pipe remained a perfect vaccuum, you would’t be able to get more than 32’ of water in the pipe.
I once saw a kid’s science show on TV where the Mr. Professor-type guy had his kid assistant drink from from a glass with longer and longer straws. When he got the kid using a straw from a stepladder (with the glass on the ground) he couldn’t get the liquid up to his mouth.
Then, with a hose and a vacuum pump, they tried to pump water from a bucket on the ground up to a second-story window but couldn’t get the water any higher than 20’.
At least, that’s my recollection but my memory could be faulty.
Yes, in practical terms you can seldom do better than 28 feet or thereabouts.
With mercury the column is much smaller, of course, because the liquid is much denser. (The only variables are atmospheric pressure and liquid density, assuming both the pipe and the liquid itself are vapour-tight.) This means that if you fill, say, a three-foot closed pipe with mercury and then invert it so the open end is in a bath of mercury and the closed end is at the top, you’ll have nearly a foot of pretty good vacuum at the top of the tube. We can improve on it these days as the vacuum’s subject to some imperfections (I suppose there might be a little dissolved gas in the mercury, and there will be small amounts of mercury vapour as well) but the Torricellian vacuum took a fair bit of beating for many years.
Of course you can pump water a lot higher if you push it instead of suck it.
Taking a bit from everybody is a help here. I’m not concerned about more than raising water more than a couple feet with a limited vaccum. A continous running device for providing a vacuum will not be used. It’s a home experiment I was wondering about trying.
Thanks for the help.
And the acceleration due to gravity. You could lift liquids six times higher in a moon base (with Earth-normal air pressure inside the base, of course).
As I pointed out above this is not true, it is possible to life the water higher then the atmospheric pressure allows by creating a negative absolute pressure in the pipe - yes it is possible. If the pipe is filled with water when lifted the water’s surface tension will not allow a vacuum space form till that is overcome. IIRC the total distance water can be lifted using this method is more around 65 feet as opposed to the apx 30 ft.
Could you speak further on this subject, because it sounds like something I have not been taught yet. I’m a Chem Eng student. Pressure is always wrt to something. Absolute pressure is wrt vacuum. A negative absolute pressure would seem to imply that it is possible to have less than nothing.
When we use an equation and get a negative absolute pressure answer, we were told that the result is meaningless, and what would happen is that the water would boil.
You are saying that the intermolecular energy/bonds of water allows the water to “hold together” where an ideal fluid would boil/(the model breaks)?
Here’s a start:
And one last question:
The “no head space” is because water is incompressible (cannot stretch) right? If you started with an arbitrarily close to zero amount of air you would get a liquid/vapour equilibrium that would prevent you from ever getting less than 611Pa? (0.0886 psi)?
The show you are thinking of was Mr. Wizard’s World, on Nickelodeon in the 1980’s. I watched the same show.
This is my understanding. if there is ‘no head space’ the water needs a nucleation site to start it’s transition from liquid to vapor. If there is no such site the water will stay liquid. Superheated water is similar in this way, which is liquid water above 212F at 1 atm but won’t boil because of no nucleation site.
You can have very negative absolute pressures, for example in the middle of a piece of steel that is under great tension.
Liquids have tensile strengths as high as hundreds of PSI. They have very small tear strengths, so imperfections like floating particles and tiny bubbles can initiate a crack or tear that lets the liquid fall apart, but on a small scale where you have regions without any imperfections you can have big strengths - this is the mechanism behind the powerful cavitations in ultrasonic cleaning.
That is a start, but it suffers from having exactly zero references attached to it. As an alternative, here’s Negative Pressures and Cavitation in Liquid Helium from Physics Today and The Physics of . . . Negative Pressure from Discover Magazine. Nature seems to have a number of interesting references to this phenomenon, but I’m too lazy to register to find out for sure, so I offer a Google summary.
Is your use of pressure different from stress? Because for steel, positive stress (measured in units of pressure) is tension, and negative stress is compression. Negative stress in a structural sense just means direction.