A simple question with no clear answer for me: Helium is lighter than air and lighter air rises. That's it!?
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I) A helium atom is approx. 4 times as light as an an air molecule. With 4 times less mass helium should be less attracted by gravity of the Earth. But its inertia is equally to its gravitational pull. Do masses in vacuum not have the same attraction and speed? Can it be said that for air molecules the atmosphere is a vacuum? So that all together helium should have the same attraction towards Earth as air molecules?
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II) Because helium atoms are much lighter, perhaps they could have a higher speed than for example $O_2$ or $N_2$? Ok, but those helium atoms are in a balloon so they push at all sides of the balloon equally so the balloon shouldn't move at all?
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III) When a balloon starts ascending from the ground there is more air (pressure) above it than beneath. So the air pressure above it should push the balloon to the ground?
Perhaps there are more influences, but considering the three effects mentioned, helium balloons shouldn't ascend. But they do! So what is wrong or forgotten?
Best Answer
No. Their weights are different, so they are not "attracted" / pulled in by gravity equally.
Think of this: If you find 100 heavy perfectly round stones, and you put 5 plastic balls full of air with exactly the same size in the basket with them, what will then happen when you shake them a bit? Will the lighter plastic balls fall to the bottom or "float" to the top?
They will float to the top.
The point simply is that it is easier for helium atoms to move up than for air molecules. If you shake the basket violently, the stones might jump a bit while the plastic balls can jump much higher. So on average, the helium atoms will move much higher upwards, and as soon as they do that, some oxygen molecules will take their previous location. Now they have a new location higher up, and the same happens.
Overall this causes the effect of buoyancy, sometimes called updrift, which is the force that this lighter material is pushed up with. And this upwards force is exactly the same as the force, with which the heavier materials pulls downwards - in other words, the lighter material is pushed up with the weight of the displaced heavier material, which now pushes to come back in place.
This was Archimedes' discovery.
Now to your other sub-questions:
Well, no, a vacuum is a vacuum. If there are molecules present, it isn't vacuum, and the atmosphere isn't a vacuum.
No, their "attraction" to Earth are different, because that "attraction" must be weight. And the helium atoms weight is lower.
Mass (or weight) doesn't influence possible speed. It only influences how hard it is to make them reach the speed.
If only the balloon with helium was present, and no gravity or outside atmosphere, then you are completely correct. The inside pressure cannot make the balloon move. But with gravity present, the whole thing is pulled downwards, and with the atmosphere present, there is a buoyancy force upwards as discussed above. Which-ever of these forces is greater, makes the balloon move.
Incorrect. You actually said it yourself just before: Inside the balloon, the pressure equalizes throughout so the push at any point on the balloon is the same. Same goes for this air column: All the air in the column above presses down, but the tiny bit of air below pushes up with the same force to balance out the pressure.