Saturday, April 27, 2019

thermodynamics - What is precisely the reason that a helium balloon ascends?


A simple question with no clear answer for me: Helium is lighter than air and lighter air rises. That's it!?




  • 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?




  • 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?





  • 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?




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