I just don’t understand what’s happening to them. Everything I’ve read says the lower external pressure causes the helium inside the balloon to expand. Does this just mean the pressure inside the balloon is rising as if I blew up a balloon to far till it bursts? Why does the balloon expand?
In: Physics
A balloon keeps its size because there is a balance between the pressure inside and the pressure outside. If the pressure inside is less than the pressure outside, the balloon shrinks until the gas inside is so compressed that its pressure is the same as outside (this happens if you put the balloon under water). Likewise if the pressure is greater inside than outside then it will expand until the two pressures match. Of course there’s a limit how much the elastic can stretch before it breaks amd bursts.
The reason why the balloon changes its size is not because it ‘wants to’ match the pressures of course. The prevailing explanation is that the gas in the balloon has energy which makes the gas particles vibrate and push against each other, making the body of gas expand. The gas outside the balloon also has energy and pushes against the balloon, keeping it from expanding indefinitely. Gravity squashes the air close to the ground more than the air higher up, resulting in less pressure higher up. You can reduce the energy of the gas inside a balloon by freezing it, making the gas lose energy and thus reducing its pressure and shrinking it.
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