What keeps air inside the area we consider the atmosphere?
I would assume the answer is gravity, but I just wanted to verify. What does keep it in? Space is a vacuum which means there’s negative pressure which must constantly be compensated for, right? Why do we still have an Ozone layer? Shouldn’t that be one of the first things to go? How can the puny gravity of Earth possibly outrank the vast nothingness of space?
Assume that you have a tube a few miles wide, completely unbreakable that stretches upwards thousands of miles. Yeah yeah yeah, it’s impossible, just play along m’kay? So what happens if you stuck part of it into the ocean? Could you actually force the Atlantic upwards and shoot it into outer space? If not, is there a way to do it?
A lot more than two questions, I realize, but I’m just trying to work out a few concepts for an idea I’m thinking about.
And no, in case you’re wondering, I’m not trying to destroy the world.
Even if you evacuated the tube completely, the water would only rise a certain height (I think it’s 32 feet). The thickness of the tube doesn’t matter. The amount of water which entered the tube would weigh the same as the air that would be in the tube otherwise. The reason it would only be this much is that the thing pushing the water up is the atmospheric pressure of the air outside the tube pushing down on the ocean and as a result pushing up into the tube. And it can only push as much as it weighs.
Thanks Achernar. But does it matter if the other end of the tube is way out past the confines of the atmosphere? One end in the ocean, the other end in a complete vacuum. Would gravity still stop the constant pull and keep the ocean intact?
Building a wall around part of the ocean+atmosphere doesn’t change anything. Gravity is working on the air and water inside the walls just the same as outside the walls.
Well I think the best way to think of this is not to think of vacuum as “pulling” anything, but rather “not pushing”. In fact, that’s the most physically accurate way of thinking about it. The only reason vacuum ever seems to pull is because there’s something on the other side pushing.
Ah, a common misconception here. There is no such thing as negative pressure or sucking if you will. When you “suck” on a straw you are merely evacuating the ambient air pressure within the straw and allowing the heavy atmosphere pushing down on the surface of your liquid drink, to push through the liquid up the straw… The water though is about 800 times heavier than the adjacent air so that the atmosphere can only push the liquid up the tube by 32 feet max.
A vacuum isn’t “negative pressure” it is zero pressure. The force pulling the atmospheric molecules toward the surface is the gravitational attraction of the earth. The only thing forcing the molecules outward away from the earth results from the daily rotation of the earth and its atmosphere. This outward force isn’t enough to overcome the gravity of the earth but does cause the atmosphere to bulge outward somewhat, mostly near the equator.
Ozone is present in the atmosphere because it is continually being created by high energy radiation from the sun, like ultraviolet of the correct wavelengths. It is also being continually depleted by ultraviolet of different wavelengths that destroy the O[sub]3[/sub]. In this process the ultraviolit from the sun is mostly absorbed and the amount reaching the surface of the earth is greatly reduced. This formation-depletion cycle results in an equilibrium amount of ozone provided there is no other disturbance that alters the respective rates of formation and destruction. There often are those other disturbing effects but that is another story and has nothing to do with your question.
A column of air the height of the atmosphere having a cross sectional area of 1 in[sup]2[/sup] weighs 14.7 lb. Thus the atmospheric pressure at the surface is 14.7 lb/in[sup]2[/sup]. If the vertical tube has a vacuum at the top end and surface atmospheric pressure at the bottom the water will rise about 33 ft. The tube has atmospheric pressure at the bottom because the atmosphere is pressing down on the ocean with the surface atmospheric pressure. This force from the pressure is sufficient to lift the water 33 ft. since the only thing pushing down is the weight of the water, there being a vacuum at the top.
This is true, but I think we should also point out that atmosphere molecules can escape a planet’s gravitational pull because of plain old kinetic energy.
Thank you all for your answers. I’ve got a clearer grasp on what’s involved now.
I had an idea of a science fiction story where a society, possibly Earth, tries to create a space elevator using a method similar to what’s described above. Once turned on, however, they’re unable to shut off the flow and it pulls their oceans (or perhaps just their atmosphere), out into space, killing them all.
It may take some revisions, but thank you for pointing out that the idea, as it stands now, is unworkable.