The standard answer to this question is that helium balloons rise because they are ‘lighter than air’. And this is true, but it is in some ways an unsatisfactory answer. When something experiences a force we can usually ascribe that force to some agency, but in the case of the helium balloon, the agent responsible appears to be rather mysterious. Gravity still pulls the helium balloon ‘downwards’ so what is it that pulls (or pushes) the balloon upwards?
The answer is pressure. The pressure of air above a helium balloon is less than the pressure of air below it. The difference in pressure gives rise to a net force on the balloon which lifts it upwards. This buoyancy force is present on all objects, but is so small we normally don’t notice it. Now most people’s response to this fact is simple disbelief. They will acknowledge that the pressure is indeed lower, but will argue that the difference is tiny. They are right, but then the forces lifting up a balloon are not that strong. I have done the maths on this, and amazingly the pressure change is just exactly sufficient to yield Archimedes law. But yesterday I went one step further.
We have a very sensitive barometer in the laboratory (a Druck DPI150 if you are interested) and normally it sits trapped in its rack, but a colleague asked to borrow it the other day (data on that escapade soon I hope). And since it was sitting on the bench I switched in on and lifted it up and despite the fact that I expected it to happen, I was amazed to see the pressure fall by around 10 Pascals as I lifted it up above my head, and then to increase again as I put it back on the bench. Now this fall is only 0.01% of atmospheric pressure and would not be noticed by most people or most pressure measuring equipment. And I thought this was an example where the ability to simply measure something directly really gave an extra insight into the way the world worked.
Sometimes, I really enjoy my work 🙂