So when I'm on an airplane and I have a bag of chips, at some point when the pressure in the cabin lowers, the bag of chips will bulge up.

Here’s how I’d explain this on a molecular level:

There are air molecules inside the back of chips hitting the bag on the inside. There’s also air molecules outside of the bag of chips hitting the outside. Now, if the outside has a lot less of movement per area, the inside molecules kind of win this push contest and the bag thus blows up.

I assume that’s kind of correct? Or is it completely wrong?

I assume the same happens with an ear drum when f.i. driving a car in high altitude?

Now what I do not understand correctly is, what happens to things inside the bag? Do they also experience a change, because of the way the molecules inside move differently when they win against the outside molecules?

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    $\begingroup$ You are pretty much correct so far? So, now, what do you think happens to things inside the bag? $\endgroup$ – Chet Miller Jul 20 '19 at 16:27
  • $\begingroup$ I thought because the back expands, the pressure inside the bag lowers - until equilibrium. That might have a cascading effect on things inside the bag $\endgroup$ – buddhabrot Jul 20 '19 at 16:53
  • $\begingroup$ The pressure inside the bag would drop a little. It would depend on the ratio of the initial volume to the final volume. That's about all. $\endgroup$ – Chet Miller Jul 20 '19 at 18:05

Since the pressure inside the bag remains unchanged, things inside the bag feels no different.

What we experience in a plane is pretty much identical to this. The cabin of the plane is analogous to the bag in your question. Although the cabin pressure is controlled in real life and changes slightly with altitude, the change inside the cabin is nowhere near to the change outside. That is why we feel only slight effects of pressure change.

If there was no pressure change inside the cabin, the people would not feel anything at all.

  • $\begingroup$ Thx, does it remain unchanged? The bag expands in volume so there’s less molecules per unit of space that bang into things inside the bag? $\endgroup$ – buddhabrot Jul 20 '19 at 16:54
  • $\begingroup$ @buddhabrot Yes, the bag expands in response to the lower air pressure on its outside, so the pressure inside the bag decreases. $\endgroup$ – PM 2Ring Jul 20 '19 at 19:03

The difference between the air inside the bag and the air outside the bag is the air density. That is, the number of air molecules per unit volume. The air density drops exponentially with altitude. Although the cabin systems compensate almost entirely for that drop, there is still a slight difference in the air density with elevation. Assuming the temperature of air in and out of bag is the same (temperature and density together determine the pressure) each collision of the air molecule with the walls will transfer the same amount of momentum on average on both sides. The difference is that there are more collisions per second happening from the inside because there are more particles per unit volume there. Thus, more momentum in total passes to the walls from the inside and we call that higher pressure.


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