Why Does a Helium Balloon Rise?

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A helium balloon rises because helium is less dense than air, which contains oxygen and nitrogen. Helium is a lighter and smaller molecule than both oxygen and nitrogen. Compare a helium balloon rising to a piece of wood floating in water with information from a math and science teacher in this free video science lesson.

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Video Transcript

Hi I'm Steve Jones and I am going to tell you why a helium balloon rises in air of course. So first of all there is a principle and we call it Archimedes Principle but we don't have to remember things like that. What we are interested in is things that float. Why do they float? Well first of all if I put a piece of wood into some water then obviously I know the wood will float. Now the question is why? The answer is that water pushes up on the wood because the wood pushes aside the water. If the wood pushes aside the water it has to apply a force and that force forces the water out but the water therefore tries to force its way in and the balance of these forces is what makes something float. Obviously if there is a very heavy object the force from the heavy object is much greater and the water isn't able to push hard enough and therefore cannot keep the iron afloat. So what we see however, is this can be described in terms of density. A less dense object wood density is between about .4 and .6 depending on the wood. I would put it at .5 here so wood's density is less than the density of water so the water pushes harder than the wood does. Therefore the wood floats on the water. Now if I apply the same reasoning to my diagram here air, you can't see it but it's there and helium you can't see that either but that is there too. The helium is inside the balloon. The air is outside the balloon so the weight of this object, this is less because helium is less dense than air. Air contains oxygen and nitrogen and these are big molecules and they take a lot of space but they also are much heavier so they are much more dense than are the helium molecules. Helium is a very small molecule and a very light molecule so here we have got a very light molecule. You could also say hydrogen but hydrogen is a bit dangerous because hydrogen and helium are the two light gases, helium is not reactive, hydrogen explodes if you put a match to it. So here we have the solution, air is much more dense than helium so the helium acts just like the wood and floats up in the air. Of course there is no surface to the air because the air goes on more or less to the edge of space. So that is why a helium balloon rises.

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