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# Behavior of gasses. (AS physics) watch

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1. I just don't get what all this stuff means...

What is the Boltzmann constant?

How would I work out these:

What is the average kinetic energy of a molecule of nitrogen, with relative molecular mass Mr = 28, at 300K?

Calculate the mean square speed <c²> for hydrogen at 300K, given that its molar mass is 0.002kg?

Why is <c²> so important? How can you link this random thing to all these calculations with temperature and pressure? Why mean square speed, rather than just speed... surely the speed represents the kinetic energy, not the speed squared...

Thanks for any help

edit - also when I try to check the answers they use equations I've never seen before... as if they've derived them but not explained how. So I'm so confused about all this stuff and how they all relate..
2. Have you ever been shown how the equationd PV = 1/3.Nm<c^2> is derived? If not PM me and I'll show you (our teacher showed all the derivation which I think is interesting and makes the rest make sense, although I imagine it's simpler than @ Uni obviously!).

Firstly, the Boltzmann constant "k" = R/Na where R = The Uinversal Gas constant (8.3J/mol/K) and Na = Avogadro's number (the a is usually written as subscript but obv can't do that on here!)

The "mean square speed" is used as the actual speeds of molecules are not the same and they follow a distribution (simpler to Normal distn but slightly skewed). The mean is obviously just the average of all of these different speeds and I think the square is something to do with cancelling out the difference in the direction of the motion (ie. although it says "speed" it really means velocity). I'm not entirely sure of that, sorry.

To work out the Mean EK (=1/2.m<c^2> and not 1/2.mc^2) the mean EK=3/2.KT (where K=boltzman constant and T=temp in kelvins). Thus in answer to your question, to calculate the EK of those molecules of N2 just substitute values into equation I've just given. I'm afraid can't do it for you as although I can rem avogardo's number I can't remember boltzmann constant of top of my head and I'm too lazy to go and find a constant list! lol

I hope that helps. If you want me to derive all of the equations and write out full explanations I'm quite happy to do that, but you may have to wait til weekend as v.busy at moment (I know your exams on tues rite? 2 of my m8s resittin thts y i kno!). G2g - supposed 2 be doing chem paper! oops!
3. Erm, this isnt OCR Physics is it?
4. (Original post by SunGod)
Erm, this isnt OCR Physics is it?
Not OCR PHYSICS A.

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Updated: June 10, 2004
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