Archimedes approves. ;)
Posted on 05/11/2010 9:03:35 AM PDT by Gomez
Nope. Another way of looking at it is this: If pressure has anything to do with the siphon, why does the siphon work with the same (or negligibly different) pressure at both ends of the tube?
I think the problem with the water “boiling” (changes from liquid to gas) at 0 atmospheres. The expanding gas allows the water to fall back out of the tube.
I thought it was caused by the person sucking on one end.
Exactly. And using that, you can construct a siphon higher than 33 feet that is not driven by atmospheric pressure but by tensile strength and gravity.
Sucks, don’t it.
If you could not pre-fill the "down" tube, getting it started would be a problem. Remember the atmosphere is pushing up on the open end of the down tube (or down on the lower reservoir) just as much as it's pushing down on the upper resovoir. (Yes there is a tiny fractional difference, but it's in the wrong direction, the lower reservoir or the open end of the down tube experiences slightly higher atmospheric pressure, because there is more air above it.)
Now if the upper reservoir were sealed the at the top, then the pressure above it would decrease, compared to the pressure pushing up on the open end of the down tube, and that differential would tend to counter the weight of the water in the down tube, and the siphon would at some point stop.
A siphon won’t work in a vacuum for the most basic reason that liquid’s don’t exist in a vacuum. But if you were to have a liquid it wouldn’t flow anyway. The upper loop of the siphon represents an insurmountable energy barrier, this isn’t quantum mechanics. Liquid’s don’t magically flow to lower areas if they don’t have a path to get there.
Archimedes approves. ;)
Here is a good way to explain it.
You have a glass of iced tea. If you drink it without a straw you need to have the glass higher than your mouth and tip it and its all gravity.
If you use a straw, you can drink it with your mouth higher than the liquid. You use suction from your mouth to lower the pressure between the straw and the atmosphere pressing down on the top of the tea, which allows the tea to “defy gravity”. If you tried to drink the tea in a vacuum, you could not do it with a straw because you could not suck greater than vacuum.
So a siphon requires both gravity and atmosphere. Remember you need to suck on the siphon to start it (ie raise the height of the column of liquid)...once the liquid passes the high spot, it starts falling down the other side, which continues the sucking by itself.
Usually what you will get is not boiling gas, but gas going around the liquid on the down side if the low end is not submerged and the liquid does not “stick” to the tube all the way around. If it does stick to the tube, the tensile strength of the liquid will allow it to be raised higher because the viscosity will hold it up.
I hope this physicist is not under the impression that todays kids ever pick up the dictionary.
I knew a chick who could siphon a golf ball through a garden hose. Not sure if she was a physicist.
Ah, you say,the tube must be bent. Fine, run the tube around the earth until the sky end is below the equator.
courtesy of Fractured Flickers, science division.
Wrong on all counts. see 41
The pressure cannot be responsible for pushing the water up the small leg because the same pressure is applied to the water up the large leg. It cancels out in equilibrium. The siphon is 100% gravity.
The tube would have to be very small to use tensile strength. A way to illustrate this would be to make a siphon out of cloth. It takes a while, but it will eventually suck the water out of the reservoir, provided the free end of the cloth is lower than the surface level of the reservoir.
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