Why is the most probable energy different from the speed?

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Discussion Overview

The discussion centers around the relationship between the most probable energy and the most probable speed of particles as described by the Maxwell-Boltzmann distribution. Participants explore the physical implications of these concepts, questioning why the most probable energy does not correspond directly to the most probable speed and examining the underlying mathematical formulations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants argue that the most probable energy level, given by ##E_{max} = \frac{k_BT}{2}##, does not equal the most probable speed squared multiplied by ##\frac{1}{2}m##, leading to confusion about the physical interpretation.
  • Others question the source of the formulas for ##E_{max}## and ##v_{max}##, seeking clarification on their derivation.
  • It is noted that averages of speed and energy are not equivalent, with some participants emphasizing that the mean velocity differs from the root-mean-square speed.
  • Some participants propose that energy levels can encompass a range of speeds, suggesting that an infinitesimally small energy change covers a broader range of speed values than an infinitesimally small speed change.
  • A few participants provide examples and mathematical reasoning to illustrate how distributions can lead to different most probable values for energy and speed.
  • There is a discussion about the physical implications of particle distributions and how they relate to energy and speed, with examples provided to clarify the concepts.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between most probable energy and speed, with no consensus reached. Some agree on the mathematical differences but struggle to reconcile these with physical interpretations.

Contextual Notes

Participants highlight that the definitions of most probable speed and average speed differ, which complicates the discussion. The mathematical transformations between velocity and energy distributions are acknowledged but not fully resolved.

  • #31
The complete solution is in #13. I still don't know, what else you need.
 
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  • #32
vanhees71 said:
The complete solution is in #13. I still don't know, what else you need.

Never mind, I figured it out, thanks. My calcuations in post #30 are wrong, I didn't pay attention when doing them. I was overlooking the fact that the relation between ##dE## and ##dv## is a function of ##v## while the probability itself is also a function of ##v##. If the relation between ##dE## and ##dv## were a constant, then then ##E_{max}## would correspond to ##v_{max}## .

Is there actually a mathematical way to explain why the maximum probability density in terms of energy ##E_{max}## is exactly half the energy that corresponds to ##v_{max}##?
 
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  • #33
JohnnyGui said:
I figured it out, thanks.

It doesn't look like it, since you come right back with the same question:

JohnnyGui said:
Is there a mathematical way to explain why the maximum probability density in terms of energy ##E_{max}## is exactly half the energy that corresponds to ##v_{max}##?

The math has already been discussed quite sufficiently in this thread. I think the discussion has run its course.

Thread closed.
 
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