Thermal properties

touqra

Please help me with this one:

A group of students drove from their school at sea level up into the mountains. Upon arriving at the slopes, they discovered that the bags of potato chips they had brought had all burst open. What caused this to happen?

Pengwuino

What do you think this might have involved?

lightgrav

You've probably NOT seen many physically significant quantities yet ...
mass, momentum, Force, Energy, Torque, angular momentum ...
Which do you think might be the culprit?

touqra

Well, I have never experienced this before. But this is what I understand...
On high altitudes, the air is thinner, so the pressure is lower than at sea level. If the pressure is lower, the air inside my potato chip bags is higher. So, the air will come out and burst, out they go.
But then, the air in the bags should be cold since we are on the mountains. So the pressure drops in the bag...How then can it burst?

I have went up mountains before. And, instead of chips, I took with me a bottle of fizzy drink. The funny thing is that, when I open it, in the mountains, the gas never comes out. There is no fizzing sounds.

SpaceTiger

Let's suppose that the contents of the bag reaches thermal equilibrium with the environment. Are there any other remaining differences between the air in the bag and the air outside?

touqra

Well, if that is so, then, the pressure was higher in the bag than the mountains. So, which means the molecules have higher energy in the bag than the air molecules outside the bag. And, now, the molecules in the bag are stronger and will be able to push its way and the outside air molecules aren't able to fight back. So, the bag burst open.
In other words, pressure, P is inversely proportional to V.
Is that it?

SpaceTiger

Actually, when it reaches thermal equilibrium the average energy per molecule will be the same inside the bag as outside. Take another look at the ideal gas law. If I draw an imaginary box in the air at the bottom of the mountain, how would the contents of that box differ from the contents of an identical imaginary box at the top of the mountain?

Cyrus

Thats interesting, as potatoe chip bags dont burst when your in an airplane, granted there not at atmospheric pressure, but they are pressurized below normal atmospheric pressure. I wonder how it would compare to the elevation of the mountain they were on.

touqra

Ah, sorry. It's the high pressure in the bag versus the lower pressure outside.

SpaceTiger

Well, yes, but now I fear we're back where we started. You were right, of course, that pressure goes as one over volume, but go back to the imaginary boxes that I drew at sea level and at the top of the mountain. The volume is the same in both cases and you've already said that the temperature is different. Is there anything else that differs between the boxes?

touqra

Hmm...The number of molecules inside the sea level box is higher than the number of molecules inside the mountain box.

SpaceTiger

Right. This is the same as saying that the densities are different. All other things the same, a bag of molecules taken to the top of a mountain will want to expand to the same density as the air surrounding it -- this is when it's in pressure balance. Although there would also be a temperature difference at the top of the mountain, I suspect that its fractional change would be small relative to the density difference.