Friday, 17 May 2019

thermodynamics - How to work out the relation between the "mean relative speed" and the "mean speed"?


I'm a freshman and am taking the general physics course. I just learned intro thermodynamics. One problem that really puzzles me is the calculation of "collision mean-free path", where calculating the mean relative velocity between gas molecules is needed. Our textbook simply gives a result $$\langle |v_r|\rangle =\sqrt2\langle |v|\rangle $$ without further explanation.
Here I am using angle brackets ($\langle \rangle $) to represent the "mean value" of what's inside. And note that all the velocities here are vectors, so I am using the absolute value symbols to get the "speed".

My professor has provided an explanation as follows:
Suppose that we select an arbitrary molecule A, with the velocity $v$ to the "stationary", as the reference frame. And suppose another arbitrarily selected molecule B has the velocity $v'$ to the "stationary". Therefore, in the reference frame A, B's velocity will be $(v'-v)$, which is just $v_r$, denoting B's "relative velocity" to A.
So we have: $$v_r=v'-v$$ Square both sides, $$|v_r|^2=|v'|^2+|v|^2-2v'\centerdot v$$ Now that we want to obtain the "mean value" of $v_r$ of an immense group of such "molecule B"s in a statistical sense,so my professor tried to work out the "mean value" of both sides: $$\langle |v_r|^2\rangle =\langle |v'|^2\rangle +\langle |v|^2\rangle -2\langle v'\centerdot v\rangle $$ It is plain to see (although there may be a lack of rigorousness) that, statistically $$\langle v'\centerdot v\rangle =0$$ and that $$\langle |v'|^2\rangle =\langle |v|^2\rangle $$ Therefore $$\langle |v_r|^2\rangle =2\langle |v|^2\rangle $$ Here comes the key part.From the above equation my professor concluded that $$\langle |v_r|\rangle =\sqrt2\langle |v|\rangle $$

However, I do not think this plausible step holds water. Because I think that for a statistical variable $x$, $\langle x\rangle ^2$ and $\langle x^2\rangle $ are not necessarily equal. (especially when I later learned something about Maxwell velocity distribution and found that for gas molecules the mean speed $|v|$ is actually smaller than the root mean square speed $\sqrt{\langle |v|^2\rangle }$.) So I think, instead of getting the result we want, the last step in fact gives $$\sqrt{\langle |v_r|^2\rangle }=\sqrt{2}\sqrt{\langle |v|^2\rangle }$$

This problem has been bothering me for several weeks and I want it fully explained, in an explicit and rigor way. I think only by using the knowledge of probability can a mathematically-convincing explanation be achieved. Unluckily I haven't learned much about probability and knows very little about relevant theories. Would anybody help me about this? Merci.




No comments:

Post a Comment

Understanding Stagnation point in pitot fluid

What is stagnation point in fluid mechanics. At the open end of the pitot tube the velocity of the fluid becomes zero.But that should result...