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Old Oct 5th 2017, 10:33 AM   #1
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Join Date: Oct 2017
Location: west bengal,India
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Question kinetic theory of gases

I am confused in the collision frequency(Z).
somewhere it is:
Z={rms speed/mean free path}
and some where
Z={average speed/mean free path}
which is correct and why?
where collision frequency defines as the collisions made by a molecule in unit time.
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Old Oct 7th 2017, 10:22 AM   #2
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Location: England
Posts: 850
This link:
<Wikipedia:Collision Theory>
Relates Z to the average velocity.

However I think the main problem with your 2 equations is the equals sign,
I think it should be a proportional sign.

The speed distribution of the molecules in a gas means that the RMS and the average are closely related.
See this link:
<Wikipedia:kinetic theory of gases:speed of molecules>
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Old Oct 8th 2017, 06:12 AM   #3
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The term "average" can mean a number of different things:
The "mean" where you add up the values and divide by the number of them: the "mean" of {1, 3, 3, 3, 6, 8} is (1+ 3+ 3+ 3+ 6+ 8)/6= 24/6= 4.

The "median" which is the "middle" of all the numbers. Here, there are 6 numbers. The "middle" is between the third and fourth numbers. Since both third and fourth numbers are 3, that is 3.

The "mode" which is the most common number. Here, that is 3.

The "midrange" which is simply half way between the smallest and largest numbers. Here the "range" is "1 to 8" so the "midrange" is (1+ 8)/2= 4.5.

The "root mean square" which is the square root of the mean of all the square. The mean of the squares is (1+ 9+ 9+ 9+ 36+ 64)/6= 128/6= 21.3. The square root of that is 4.6.

The "average speed" could be any one of those, though most likely the "mean" as that is the most commonly used "average". The "root mean speed" specifically uses the last.
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