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Why do helium-filled balloons float? (lemmy.ca)

submitted 4 days ago by SethranKada@lemmy.ca to c/asklemmy@lemmy.ml

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Shouldn't the air pressure crush them until the density inside equals the density outside? Why does helium balloon behave so differently from a vacuum-filled balloon?

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[–] solrize@lemmy.ml 29 points 4 days ago* (last edited 4 days ago)

The pressure is the same inside and outside, but the helium is lighter so the balloon floats. From the ideal gas law PV=NRT and plugging in the constants you get the well known rule that at standard temperature and pressure, the volume of 1 mole of gas is about 22.7 liters. It's weight depends solely on the gas's molecular weight.

The molecular weight of helium is 4 while for air (a mix of N2 and O2) it's a little over 28. That should let you calculate the lifting force for a given size of balloon.