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guitarstorm
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Homework Statement
At which minimum height atoms or molecules are able to leave Earth’s
atmosphere to space? To obtain an approximate estimate use the assumption that molecules can leave if the mean free path, λ, is larger than the scale height H. Write λ and H as a function of height and obtain the minimum “escape” height.
Obtain the minimum escape velocity and compare it to the molecular
velocity of the H-atom and the N2 molecule for conditions of the above
derived escape height.
Use σ=3 10-20 m2, the temperature profile of the US standard atmosphere,
and assume T=1000 K for heights > 100 km. (The real escape height is
about 400 km for the H atom because of the necessary kinetic energy.)
Homework Equations
Two equations I started with:
Mean free path: [itex]\lambda=\frac{1}{\sqrt{2}\sigma n_{v}}[/itex]
Scale height: [itex]H=\frac{RT}{g_{0}}[/itex]
The Attempt at a Solution
I'm assuming I have to set the two equal to each other, so looking through my textbook I found other equations to plug into try to get similar terms in both equations. I've only attempted the escape height part first, since I can't do the rest until I get that...
[itex]n_{v}=\frac{p}{kt}=\frac{pN_{a}}{R^{*}T}[/itex]
So then the mean free path becomes:
[itex]\lambda=\frac{R^{*}T}{\sqrt{2}\sigma pN_{a}}[/itex]
But then we need to find the height (z), so I figured I'd try to plug in [itex]p=\rho gz[/itex].
And then I set [itex]\lambda =H[/itex] as:
[itex]\frac{R^{*}T}{\sqrt{2}\sigma \rho gzN_{a}}=\frac{RT}{g_{0}}[/itex]
I solved for z and got [itex]z=\frac{1}{\sqrt{2}\sigma \rho N_{a}}[/itex], which doesn't really help me...
When I plug in the numbers, I get a very low number as well (around 10^-5), which is obviously not right. I'll try to play around with formulas some more but right now I feel overwhelmed by this problem!