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### Topic: which equation do I use?  (Read 9586 times)

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• Guest
##### which equation do I use?
« on: November 30, 2004, 07:16:12 PM »
I am unsure which equation to use in this problem:

At an altitude of 60.00 km, the average atmospheric temperature is essentially 0 degrees C. What is the average number of air molecules per cubic centimeter of air at this altitude?

^I was thinking that I could just solve the equation for the pressure as a function of height and temperature and from this use the ideal gas equation to calculate the volume of air. My dilemna is that I cannot find the equation for pressure as a function of height. Am I supposed to get this from a table or something?

#### Demotivator

• Guest
##### Re:which equation do I use?
« Reply #1 on: November 30, 2004, 11:15:30 PM »
Well there are tables.

There is also a hydrostatic equation
dp/dz = -dens(g)
dens is average density of air at sea level. (it happens to be about 1.3 kg/m3 at normal temp)
g is gravitational acceleration (9.8 m/sec^2)

The way I see it,
dens can be calculated from n/V = P/RT
where P is about 1 atm at sea level and T = 273K (0 C). (Although temp really isn't on average zero C at the surface, setting it at 0C now makes it unnecessary to readjust P for 0 C at altitude.)
n/V is related to density through a costant (weighted average MW of air).

The equation can be integrated to solve for P at a delta z.
Of course, you have to be careful with the units.

• Guest
##### Re:which equation do I use?
« Reply #2 on: December 02, 2004, 07:42:44 PM »

I have looked in my chem and physics books, but have found no tables. If you know of a table of this sort available online, let me know. In the meanwhile, I will try and calc it.

#### Demotivator

• Guest
##### Re:which equation do I use?
« Reply #3 on: December 02, 2004, 09:01:46 PM »
http://www.sablesys.com/baro-altitude.html

btw,  60.0 km?  seems too high a number to me.

#### billnotgatez

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##### Re:which equation do I use?
« Reply #4 on: December 05, 2004, 05:47:40 AM »
Let us know if the density is near 0.0003 kg/m3