What is the significance of calculating the average value of cos?

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Discussion Overview

The discussion revolves around the significance of calculating the average value of cos(θ) in the context of kinetic theory, particularly regarding the behavior of molecules impacting a surface. Participants explore the mathematical reasoning behind the average value being 2/3 and question its implications and relevance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why the expected value of cos(θ) would not be zero, given the nature of the distribution of angles.
  • Others suggest checking the average value of |cos(θ)| as a potential clarification.
  • There is a discussion about the range of values for θ, with some participants noting it is from 0 to π/2 and measured with respect to the normal of the wall.
  • One participant acknowledges the mathematical correctness of the average value but seeks to understand its significance in practical terms.
  • Concerns are raised about the implications of considering molecules hitting from one side versus both sides of the normal.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the significance of the average value of cos(θ) and whether the expected value should be zero. Multiple competing views remain on the relevance and implications of this average calculation.

Contextual Notes

Participants have not fully resolved the implications of the average value, and there are ongoing questions about the assumptions regarding the range of angles and the directionality of molecular impacts.

srhelfrich
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In kinetic theory, the number of molecules hitting a unit area of a surface per unit time with speeds between v and v + dv and angles between \theta and \theta + d \theta is found to be a function of sin(theta) and cos(theta). There will often be a practice problem asking to show that the average value of cos(theta) for these molecules is 2/3.

My question is why the expected value wouldn't be zero instead of 2/3? What is the significance of finding this average value?
 
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Check the average value of |cos(θ)|.
 
srhelfrich said:
My question is why the expected value wouldn't be zero instead of 2/3?
What is the range of values of θ? Are you considering molecules that hit one side or both sides of the wall?
 
I understand that it works out mathematically. I also figured out that theta is measured with respect to the normal of the wall so now I see why it shouldn't be zero.

But what is the significance of finding the average value? Why is this something we would be interested in calculating?
 
jtbell said:
What is the range of values of θ? Are you considering molecules that hit one side or both sides of the wall?
Yeah, I just realized the range is from 0 to pi/2. The angle is measured wrt the normal of the wall.
 
Svein said:
Check the average value of |cos(θ)|.
Sure, the math works.

But why is it significant?
 
srhelfrich said:
Yeah, I just realized the range is from 0 to pi/2. The angle is measured wrt the normal of the wall.
What about molecules coming in from one side of the normal, versus coming in from the other side of the normal?
 

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