Physics Quiz Question

"Clark" wrote in message
...
"d&tm" wrote in
:


"Clark" wrote in message
...
A minor nitpick on a previous post from Jim:

snip

It was stated that n/V was equal to density. While the stated equality
isn't,
the jist of what was said was ok. The equality would be

(Mass/Molecular Weight)/V = density

Since the molecular weight of air is a constant it can be combined with
the
other constant in the ideal gas law when used for atmospheric calcs.


a minor nitpick on your minor nitpick is that the molecular weight of
air is not constant. It is affected by humidity.
density is better expressed as d = PM/RT
where d will be denity in units of mass/volume
(M is molecular weight), R the gas constant ,P and T press and Temp .
For us SI people R=8.31, P in Pa and T in Kelvin gives density in kg
/m3 ( using M in kg/mole)Terry


I use standard dry lab air in all my calculations. :-)

I vote we call this one a sub minor nit pick on my minor nitpick. Just
what
is the likely range of average molecular weights? Water vapor content
varies from what? pretty darn dry to about 4%? And keep those SI units on
your side of the border, or pond, or whatever socio-economic,
nation-state,
or geographic boundry that thankfully separates us.

4% water could make the density altitude ( the important thing to us pilots)
10 to 20% higher than if the air was dry. Thats not insignificant to me.
But then I know a lot of enjuneers who would be happy with that. ( mostly
the ones who cant spell enjuneer) . and wots wrong with SI units Clark ?
God gave us 10 fingers for a reason.( at least we have 10 fingers on this
side of the pond as you call it, not so sure about you murkins :)
terry

I seem to have come late to this scrum...

Anyway, the last poster missed a thing or three... Like Euler, like
Boyles law, like the equation for lift, like Bernouille, etc..

The biggest thing he missed is if you heat a gas and keep its pressure
the same then it has to expand (That old Boyle guy again)
Expanded gas means the molecules are farther apart... Molecules being
farther apart means less density, less density means less lift...

Ya know in the ancient times of BI (before internet) you had to truck
down to the library and dig through piles of books to find this
information... Today, in 0.002 seconds your web browser will pull up
the articles and equations on this subject... It is highly recommended
by me...

denny

The statement (taking into account the later added context, i.e.
Jeppesen's text) is technically true, although not very useful in the
aviation environment.

As Jim L. pointed out, the presure is proportional to gas density and
temperature. That's all there is to it (the pressure is NOT the weight
of the the column of air). So when Jeppesen says "all else being
equal" while analyzing temperature and pressure, that "else" has to be
density. If density stays constant, higher temperature will result in
higher pressure.

But airplanes don't fly in an enclosed container where one can keep
the density constant. For example: higher temperature (all else being
equal :-) ) will tend to deacrease the density. And then there are
winds, ...

So the statement (although technically correct) is at best pointless,
and at worst (dangerously ?) misleading.

- Tom

On Mon, 6 Aug 2007 12:05:17 -0500, Dallas
wrote:

Brought over from RAS:


Assuming that all other variables remain constant:

An increase in temperature will result in a higher atmospheric pressure - a
higher temperature speeds up the movement of the air molecules, thereby
raising the pressure they exert on the surrounding atmosphere.

A) True
B) False

On Mon, 6 Aug 2007 12:05:17 -0500, Dallas wrote:

An increase in temperature will result in a higher atmospheric pressure - a
higher temperature speeds up the movement of the air molecules, thereby
raising the pressure they exert on the surrounding atmosphere.

Most respondents discounted this statement as incorrect or a flawed
problem. And yet it appears very simply stated in this form as a test
question on the private pilot written:

How do variations in temperature affect the altimeter?

A. Higher temperatures expand the pressure levels and the indicated
altitude is higher than true altitude.
B. Lower temperatures lower the pressure levels and the indicated altitude
is lower than true altitude.
C. Pressure levels are raised on warm days and the indicated altitude is
lower than true altitude.


Answer (C) is Correct - On warm days, the atmospheric pressure levels are
higher than on cold days. Your altimeter will indicate a lower than true
altitude. Remember, "Low to high, clear the sky."

:-/

--
Dallas

On Mon, 13 Aug 2007 17:26:59 -0500, Dallas
wrote:

On Mon, 6 Aug 2007 12:05:17 -0500, Dallas wrote:

An increase in temperature will result in a higher atmospheric pressure - a
higher temperature speeds up the movement of the air molecules, thereby
raising the pressure they exert on the surrounding atmosphere.

Most respondents discounted this statement as incorrect or a flawed
problem. And yet it appears very simply stated in this form as a test
question on the private pilot written:

How do variations in temperature affect the altimeter?

A. Higher temperatures expand the pressure levels and the indicated
altitude is higher than true altitude.
B. Lower temperatures lower the pressure levels and the indicated altitude
is lower than true altitude.
C. Pressure levels are raised on warm days and the indicated altitude is
lower than true altitude.


Answer (C) is Correct - On warm days, the atmospheric pressure levels are
higher than on cold days. Your altimeter will indicate a lower than true
altitude. Remember, "Low to high, clear the sky."

:-/

"Everything else being equal". But it rearely is.

Consider the following example. The numbers are temperatures and
pressures as reported by six airports in San Francisco Bay area about
an hour ago. Flying from SFO to RHV one flies over (or close to) SQL,
PAO, NUQ, and SJC, in that order. The total distance from SFO to RHV
is a little over 30 nm.
Check the numbers:

SFO 21 30.01
SQL 25 29.99
PAO 24 29.99
NUQ 23 30.00
SJC 26 29.98
RHV 28 29.99

The temperature *increases* significantly, white the pressure
*decreases" (or stays the same if we assume that 0.1% variation is
within measurement tolerances).

We can also look at temperatures and pressures in one place changing
over time, for example at SFO over the last eight hours:

15Z 14 30.07
16Z 16 30.08
17Z 17 30.08
18Z 18 30.07
19Z 19 30.06
20Z 21 30.05
21Z 21 30.03
22Z 21 30.02
23Z 21 30.01

The numbers wouldn't make FAA question designers proud.

- Tom