The Paradox of Gas Density: Why Is the Center Not Higher?

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Discussion Overview

The discussion revolves around the behavior of gas molecules in a closed container, specifically addressing the paradox of why the density and pressure of an ideal gas do not appear to be higher at the center despite claims of higher probability for molecules to be found there. The conversation includes theoretical considerations and interpretations of simulations related to particle motion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the probability of a gas molecule being in the center of the container is higher than near the sides, leading to the conclusion that density and pressure should also be higher at the center.
  • Another participant challenges this by arguing that if the probability density is uniform, there would actually be a higher chance of finding particles near the sides due to the larger volume of that region.
  • A different viewpoint is presented, stating that when considering interactions between particles, there may be a higher probability of a particle being found near the center, referencing a simulation of Brownian motion.
  • One participant notes that gravity would influence the distribution of particles, making them more likely to be found near the bottom of a standing container.
  • There is a critique of the simulation, emphasizing that the presence of a specific particle (the red particle) is equally likely to be anywhere, and the uniform distribution of other particles (gray dots) supports this argument.
  • Another participant expresses skepticism about the simulation's assumptions, suggesting that the behavior of the red dot does not align with the uniform motion of other particles.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the probability distribution of gas molecules and the implications for density and pressure. The discussion remains unresolved, with no consensus on the interpretations of the simulation or the initial claims.

Contextual Notes

The discussion includes assumptions about uniform probability distributions and the effects of particle interactions, which are not fully explored or agreed upon. The implications of gravity on particle distribution are also mentioned but not elaborated upon.

ramollari
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Suppose you have an ideal gas in a closed container. The gas molecules will be moving at random at very high speeds.
Now I've figured out that the probability of a gas molecule for being in the center of the container is higher than the probability of being near the sides. Then should we also conclude that the density (and pressure) of the gas at the center is higher than that near the sides? This conclusion doesn't seem to hold.
 
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How's the probability of a particle being near the center higher?
If the probability density of a particle's position is uniform, then you'd have more chance of finding it near the sides, since the volume of a region with the sides as an outer boundary is generally much larger than a volume that is kept near the center. This doesn't affect the density, though.
 
Take the particle isolated. If it is hit by other particles, then it has more probability to be found near the centre.
Take a look at the following simulation with Brownian motion, by setting m1/m2 = 1. http://home.wanadoo.nl/perpetual/brownian.html The container area colours blue (particle's path) near the center much faster.
 
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On a side note, the effects of gravity would lead to a particle overall being more likely to be found near the bottom of a standing container. Aside from that, I have not enough experience with physics probability to comment further.
 
ramollari said:
Take the particle isolated. If it is hit by other particles, then it has more probability to be found near the centre.
Take a look at the following simulation with Brownian motion, by setting m1/m2 = 1. http://home.wanadoo.nl/perpetual/brownian.html The container area colours blue (particle's path) near the center much faster.
That's a nice link.

But it's not correct to say that the probability of finding a particle near the center is higher because the blue line crosses near the center more often. What you need to look at is the position of the red particle. It's presence in the box at any instance is equally likely to be anywhere.
Look at all the gray dots. They are uniformly distributed over the box too.
 
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Galileo said:
That's a nice link.
Seems a bit cranky to me. :rolleyes:
But it's not correct to say that the probability of finding a particle near the center is higher because the blue line crosses near the center more often. What you need to look at is the position of the red particle. It's presence in the box at any instance is equally likely to be anywhere.
Look at all the gray dots. They are uniformly distributed over the box too.
Right. I think the animation was probably written to illustrate brownian motion and has several built-in assumptions which break down when you put m1/m2 = 1. If it were accurate, then for m1/m2 = 1, the red dot would move exactly like any other dot. But it doesn't. While the gray dots are uniformly distributed and move at some average speed, the red dot zooms all over the place. I don't think so.
 
put n=200 and m1/m2 = 1.
 

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