I've just read that water cannot be lifted higher than thirty feet with a vacuum, why?
Answers:
I could give you the math but this may be easier to understand:
The atmosphere exerts a pressure of 1 atmosphere on anything at sea level. This is equivalent to the pressure exerted by a column of mercury 760mm high or a column of water 10m high pressing on us. However, because we are largely water, and because the air spaces in our bodies are also full of air at a pressure of 1 atm, we don't feel this pressure that surrounds us.
Suppose there's a glass tube 20m long attached to a vacuum at the top and with the bottom of the tube in a lake. Water rises into the tube because it's being pushed up by the pressure of the atmosphere on the surface of the lake. This pressure is 1 atmosphere or 10m of water pressure , so the water will only rise 10m or just over 30ft up the glass tube.
Don't trust everything you hear.
The pressure at the high point of a siphon is below atmospheric pressure. You can work out what it must be using Bernoulli's equation:
p + velocity^2/2 + rho*g*z = constant along the siphon
Set velocity = 0 for your base case (it's not moving yet, you only just took your thumb off the low end).
p is in pascals (100,000 Pa is atmospheric pressure, the pressure at the surface of the inlet reservoir)
g is about 10 (m/s/s)
rho is about 1000 (kg/cubic metre)
This means that when z = 10 metres (thirty feet), the pressure in the pipe goes to zero. Negative pressures simply don't exist. The water boiled due to low pressure before you got that high. Siphons don't work with vapours, only liquids.
At that point at the top it becomes a perfect vacuum and there is no more force to pull it up.
http://www3.sympatico.ca/slavek.krepelka.
How much water?
And is a total vaccuum possible? If it is, I guess it could do better than 30 feet.
Because of the water dispertion caused through the vacuum..
Because the pressure exerted on the water by the atmosphere is such that when you remove that pressure (i.e. a vacuum) the water will rise, in that vacuum, to 32 feet.
I have never heard of this. But I do know that vacuum cleaners are rated by how many inches they can lift in a sealed tube. Most good vacuums are in the range of 120 " of lift and higher.
As for thirty feet, a water pump is nothing but a vacuum for water and some can lift water hundreds of feet out of the ground. The pumps in oil fields can lift oil several miles out of the ground.
Normal atmospheric pressure is approx 14.7 pounds per square inch at the surface of the earth, sea level. If you have a vertical pipe, about 30 feet tall, fill it with water, and apply a pressure of say 15 psi to the source of the water, the level will rise about 30 feet. This is the same as applying a suction of minus 15 psi to the top of the water column. You can try the same experiment with a column of mercury, and will find that a vacuum will cause the column to rise approx 760 mm.this is because mercury is much heavier than water. This is of course the principle of the mercury barometer.
Once the weight of the water equals the vacuum the water will no longer go up.
As long as the vacuum exerts more than the weight of the water it will go up.
Water spouts are a good example.
Water will come up off of a lake and create an upgoing stream. This is caused by an extreme low pressure. Once the weight of the water is more than the low pressure "vacuum" then the water will return back down.
Here are some pictures.
http://www3.telus.net/public/ehmang/wate.
it is restricted by the atmospheric pressure
isn't Tornado a vacuum ? dose it not lift water higher then 30 feet ?
By applying a higher pressure at the bottom you could make it to lift more height. At 2 bar it will lift 60 ft
course it can. ever heard of the hanging gardens of babylon.
Hmm,
Nature abhors a vacuum, but the abhorrence ends after 32 feet. -- Galileo.
So how does a sixty foot tree get it's water up higher than thirty feet?
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