Two Particles in a Box: Similar Velocities, Momentum, or Kinetic Energy?

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Discussion Overview

The discussion revolves around the behavior of two interacting particles in a box, specifically whether they are likely to have similar velocities, momenta, or kinetic energies. The scope includes theoretical considerations of particle interactions and the implications of physical collisions.

Discussion Character

  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant suggests that the question cannot be answered generally and proposes that, with physical interactions, the momenta of the two particles may approach equality over many interactions.
  • Another participant questions the reasoning behind the idea that momenta would approach equality.
  • A different participant speculates that in realistic collisions, the difference in momentum would decrease after collisions.
  • Another participant introduces a thermodynamic theorem stating that identical particles will, on average, have equal kinetic energies, suggesting a different perspective on the question.

Areas of Agreement / Disagreement

Participants express differing views on whether momenta, velocities, or kinetic energies will be similar, indicating that multiple competing perspectives remain without consensus.

Contextual Notes

The discussion highlights assumptions about the nature of interactions and the conditions under which the behavior of the particles is considered, but these assumptions are not fully explored or resolved.

nonequilibrium
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Hello,

Say you have a box with two (interacting) particles in them. If you had to venture a guess, what would be most reasonable: that they both have similar velocities, similar momenta or similar kinetic energies? (Or perhaps none of the above?)

NB: with "similar" I mean "of comparable magnitude"
 
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I'd say that this question can't probably be answered in a general way. If the interaction involved is physical (like the two particles bouncing into each other), my guess would be that over an arbitrarily large amount of interactions the two particles momenta would approach equality. I would say neither of the other two values would approach each other, and that this situation would change if the interaction changed.
 


And why would you think that the momenta would approach equality?
 


More a guess than anything, it seems to be than in realistic collisions between two bodies the difference in momentum would be less after the collision.
 


Actually, there is a theorem in Thermodynamics that states that on average, available energy is distributed equally between available degrees of freedom. That means that if particles are identical, they will, on average, have equal kinetic energies.
 

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