On Feb 11, 9:07*am, Mxsmanic wrote:
terry writes:
Did I say the pressure would rise if you increased the temperature?

If it follows the combined laws, it will. *But in the case of the atmosphere,
it doesn't, because the volume of the atmosphere is not constrained, and the
source of atmospheric pressure is gravity, not the random kinetic energy of
air molecules.

For everyone else following this thread, dont worry I am not crazy, I
am just testing a theory that it is possible to teach Msx something.
So far I must admit I am a litte discouraged.
But I will just persevere for a little longer to test my teaching
skills. . Now Mxs before you take off in your simulated Baron,
do you check the takeoff performance figures in the simulated flight
manual versus the lenght of runway at your simulated departure
airport? Have you noticed they are a function of a thing called
density altitude.? Do you know how to work out what your density
altitude is ? Real pilots do this if there is any doubt they might
not have enough distance to clear the runway or any obstacles, and
since you are so obsessed with manufacturing pretended reality I am
sure you would want to be doing this also. If you did this what
you should have realised is that you are relying on the fact that the
density of the atmosphere at the particular point you are at (ie
your simulated airfield at the particular simulated temperature and
pressure conditions ) is determined only by 3 things. 1. the
temperature, 2 the pressure, 3 the chemical compositon of the
atmosphere ( ie the average molecular weight which is generally
assumed to 28.84 except for the extra dilligent who will correct for
humidity ). And the relationship between them? I think I have
already told you about 3 times density = PM/RT and where does this
equation come from? directly from the universal gas law PV=nRT by
substituting m/M for n .
Since you dont have access to a real airplane ( thank god) you could
even calculate the density of the air in your cockpit ( apartment)
with a thermometer and a barometer using the above equation. I
assure you if you do it carefully you can get a very accurate value of
the air density. ( if you must you could even open your window to
simulate an infinite volume! ).
If your answer is not something like 1.22 kg/m3 its probably because
you have underestimated M by not allowing for the methane percentage
in your apartment atmosphere.

You see your probelm is that you have this mistaken belief that the
gas law is only useful to describe a given bunch of molecule in a
balloon, probably something you picked up in grade school. but what
it really does is illustrate the point that gas molecules occupy a
volume of space that is dependant only on their number , pressure and
temperature. The forces that lead to whatever pressure and
temperature conditions exist is another subject, called meterology.
Instead of thinking that increasing the temperature should increase
the pressure ( as it would if you were talking about a fixed number of
molcules in a rigid tank) you could just as easily think that
increasing the temperature reduces the number of molecules per unit
volume, in an atmosphere where the air can expand and thus the
pressure stays relatively constant.. Either example can use the same
equation PV=nRT to describe what is happening.
Terry
PPL Downunder