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Topic: Guidance on this problem?  (Read 2867 times)

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Offline Loael

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Guidance on this problem?
« on: January 15, 2015, 12:43:48 AM »
So I'm going through the book and stumbled on a question that seems blatantly obvious to do, but I keep messing up somewhere. I know Guy-Lusaac's law won't work, and I'm unsure if ln(T2/T1)=ln(P2/P1)^(R/Cp) will work either. Can someone fill me in on the obvious piece I'm missing?

The temperature of Earth's atmosphere drops 5 deg C every 1000m of elevation above Earth's surface. If the ground temperature is 15 deg C with pressure = 760 mmHg, at what elevation is the pressure = 380 mmHg. Assume the air is ideal.

Offline curiouscat

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Re: Guidance on this problem?
« Reply #1 on: January 15, 2015, 01:22:18 AM »
You already know T=fn(h)

P=fn(T) is the ideal gas law.

You are asked to find h=fn(P)

Offline mjc123

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Re: Guidance on this problem?
« Reply #2 on: January 15, 2015, 06:33:45 PM »
The trouble is that P is not only a function of T. P = (n/V)*RT, and both n/V and T vary with altitude. In fact I think the decrease in pressure with elevation is more to do with the attenuation of the atmosphere than with the falling temperature - P decreases much faster than would be caused solely by a drop of 5K per 1000m. See http://en.wikipedia.org/wiki/Barometric_formula.

Offline Corribus

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Re: Guidance on this problem?
« Reply #3 on: January 15, 2015, 08:36:27 PM »
In fact I think the decrease in pressure with elevation is more to do with the attenuation of the atmosphere than with the falling temperature - P decreases much faster than would be caused solely by a drop of 5K per 1000m.
Certainly true, since the temperature actually increases as elevation increases around the stratopause.

http://en.wikipedia.org/wiki/Atmospheric_temperature
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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