Helium balloons float because helium is lighter than air. The density difference between helium and air is small, but balloons are thin and light. Larger balloons can carry more weight, and zeppelins used to use hydrogen, which is flammable.
Helium balloons are any type of balloon that uses helium as the main gas to keep them aloft. Tethered balloons are what most people think of when they think of helium balloons, which are the smallest party-sized balloons that people tie on strings to have float as an ornament.
Helium balloons float in the air because the gas inside them is lighter than the air outside. When one gas is lighter than another, it tends to float on it. You can think of balloons floating in the air as looking like a plastic bottle filled with air in a pool of water. Because the air in the bottle is lighter than the water itself, it floats to the top, even if you place it deep under water. Likewise, helium balloons will float on top of heavier air. How fast that bottle of air floats on the surface of the water is determined by the difference in their densities, which is pretty extreme: a liter of water weighs 1kg, a liter of air weighs 1g.
Helium balloons don’t float as hard as that air bottle under water, because the density difference between helium and air is a bit smaller. A liter of helium weighs approximately 0.18 g, while a liter of air weighs approximately 1.25 g, depending on its exact composition. That’s a pretty small absolute difference, so if we were to fill our own plastic bottle with helium, it wouldn’t float in the air, because the bottle itself would weigh more than the difference between the gases. Helium balloons, however, weigh next to nothing, because they are very thin pieces of plastic. This means that even small enough balloons will float.
As helium balloons get bigger, they are able to carry more and more weight. Because their volume increases at a faster rate than their surface area, if you build balloons big enough, you can actually start carrying people and vehicles. Zeppelins work on this principle, but they must be large enough to carry passengers and compensate for the increased weight of the thicker skins and frames they have as balloons. To give you an idea of how size affects it, an average party balloon with a diameter of one foot (30cm) can carry around 14g in weight. A huge balloon, six feet (1.8 m) in diameter can carry about seven pounds (3.1 kg). And truly gigantic balloons, with a diameter of 100 feet (30 m) could carry over 16 tons (15,000 kg).
Helium balloons are starting to receive some attention in these larger sizes as possible vehicles for remote air travel. In the past, there was some attention to these zeppelins and airships. Unfortunately, some of them relied on hydrogen gas instead of helium, because hydrogen weighs less than half as much as helium, making it more efficient as a gas to float a large amount of weight. The downside, however, was that hydrogen was incredibly flammable. After the hydrogen used caused a huge disaster, people stopped trusting zeppelins completely, even though helium is not flammable at all.
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