Expansion and change in pressure

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AtreyiMitra2L
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Expansion and change in pressure

Postby AtreyiMitra2L » Fri Jan 12, 2018 11:05 am

So I was working on a problem that had to do with expansion and a subsequent lower internal energy. The question asked if the pressure of the gas would be higher or lower when these changes are completed. I was under the assumption that because there is an inverse relationship between pressure and volume, because there is a higher volume because of expansion, there would be a lower pressure. However this is what the problem answered: "Since the internal energy decreased, the temperture of the gas must decrease and the pressure of the gas would be lower". Can someone please explain to me the flaws in my logic as well as the correctness of the actual answer? Thank you :)

Julia Cheng 2J
Posts: 31
Joined: Thu Jul 13, 2017 3:00 am

Re: Expansion and change in pressure

Postby Julia Cheng 2J » Fri Jan 12, 2018 11:23 am

Since the internal energy of the gas is less, the molecules have less energy and experience fewer collisions with each other and the walls of the container. So, the temperature and pressure are less.
I'm not sure, but with your answer, I believe that the flaw is that the pressure could be greater even if the volume increases (for example, if the internal energy increases through the addition of heat then the molecules will move at greater velocities and experience more collisions, resulting in greater pressure).

Wenxin Fan 1J
Posts: 53
Joined: Thu Jul 13, 2017 3:00 am

Re: Expansion and change in pressure

Postby Wenxin Fan 1J » Fri Jan 12, 2018 12:57 pm

When the internal heat of the system decreases because work is done on the surroundings by the system (heat released), the molecules in the system has less kinetic energy, resulting in less movement and less pressure. Volume and pressure are inversely related if heat is not a factor.

Yixin Angela Wang 2H
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Joined: Thu Jul 27, 2017 3:00 am

Re: Expansion and change in pressure

Postby Yixin Angela Wang 2H » Fri Jan 12, 2018 7:04 pm

I think the answer is saying that with the expansion, the molecules have more room to move around and thus are less likely to collide with each other or the container. Thus their kinetic energy decreases. Collisions with the container create the pressure exerted on the container, so with less collisions the pressure also decreases. This slowing of movement is also why temperature will drop. Another example would be H20 molecules moving faster in liquid form and slower in solid form.

Joanne Guan 1B
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Joined: Sat Jul 22, 2017 3:01 am

Re: Expansion and change in pressure

Postby Joanne Guan 1B » Sun Jan 14, 2018 10:32 am

Volume and pressure are inversely related only if there are no other factors in play. However, because of the temperature change, you cannot only consider volume change when trying to figure out whether pressure increases or decreases. The answer given makes sense because as temperature drops, molecules slow down and collide less often. Therefore, pressure decreases.


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