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?

  • NightFantom@slrpnk.net
    19·
    6 天前

    The very short answer is that gas pressure is mostly proportional to the amount of particles per volume.

    So a balloon filled with helium has X particles per cubic cm, while the air around it has the same amount (instead of getting crushed). But because helium is a lot lighter per particle than standard air, this makes the balloon lighter than air, and like trying to push an air-filled balloon underwater, this helium-filled balloon floats to the higher layers of air, until other smaller forces also start to matter and the balance is restored.

    So a “vacuum-filled” balloon has nothing to give counter-pressure, but a balloon filled with helium definitely does.

    • Dharma Curious (he/him)@slrpnk.net
      4·
      6 天前

      I’ll post this as a thread if it’s beyond your scope to answer, but you seem like a smart person who might know a thing I have absently wondered about many times.

      Balloons eventually fall back to the ground after the helium escapes. But let’s say you made a theoretical balloon that couldn’t allow the helium to escape, it’s perfectly sealed. Now, let’s say that the material it is made of is also as light as a regular balloon, but our near-magical material is also capable of withstanding extreme cold (like, outer space cold). Would it just float into space? I know that when we shoot rockets up there’s a lot of heat from the friction, but if something is moving slowly, that wouldn’t happen, I don’t think? But can a slow moving thing escape earth?

      Tell me, oh great and wise Heliumancer, tell me the secrets of the light-gas!

      • wewbull@feddit.ukEnglish
        6·
        6 天前

        Lets start by assuming the balloon stays the same size as it rises in the air column and we’ll ignore the temperature drop. The pressure and density of the gas inside the balloon remains the same, but at some point the air density outside the baloon will drop to match the density of helium inside the baloon. At that point the balloon would stop rising as the weight of the atmosphere it displaces is the same as weight of the helium filled balloon. It’s like a little boat on a sea of air.

        However, balloons don’t stay the same size. As they float up the atmospheric pressure drops. The balloon will expand because the pressure inside the balloon is higher that the pressure outside. It still has a bouyant force on it because the weight of atmosphere it displaces is still larger than it’s own weight, so it continues to go up. Outside pressure continues to drop. Balloon continues to grow. Eventually the balloon bursts.

      • NightFantom@slrpnk.net
        3·
        6 天前

        Close, but now you come into contact with the atmosphere not actually being the same density (in weight/volume as well as in particles/volume) throughout, but instead gets thinner as you get away from the earth.

        For simplicity, assume space is actually empty, and the atmosphere gets thinner linearly up until x kilometers above sea level it’s completely empty. Then the density will also decrease with height, and the helium balloon will eventually find a spot that matches its density, and stop there.

        Again there’s so much more to it but as a simplified model this works 😅

        Rockets mostly need to fight speed (of the earth revolving around the sun), and indeed in our atmosphere speed means friction, but in space rockets still need a lot of propellant to change their trajectory. As always there’s a relevant xkcd: https://what-if.xkcd.com/58/

    • skisnow@lemmy.caEnglish
      2·
      6 天前

      That’s a great point about the water. In OP’s mental model, an air-filled ballon should be almost completely crushed in even the shallowest water.