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Old August 18th 03, 10:19 PM
Roy Smith
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"Wayne" wrote:
So what the hell is P, V, and T? And nR for that matter.

Seems like total crap, put it in english would ya


PV = nRT is the "Ideal gas law". The letters stand for:

P - Pressure
V - Volume
n - number of moles (fancy way of saying "number of molecules")
R - some constant (some long-forgotten neuron in the back of
my brain is saying 0.82)
T - Temperature (in degrees Kelvin, which IIRC, is Celcius + 273)

It's called the "Ideal gas law" because it describes how an "ideal" gas
reacts to changes in pressure, temperature, and volume. What's an ideal
gas? Well, basicly it's a gas which acts the way the idea gas law says
it should act :-) I know that's a bit dumb, but go with it for the
moment.

Ideal gas theory thinks of a gas molecule as a little point of matter
vibrating in space. The higher the temperature, the faster it vibrates.
If you've got a bunch of these molecules confined in some space (say, a
jar), as they vibrate, they hit the walls of the container (and each
other) and bounce off.

Each time a molecule bounces off the wall, it pushes on a wall with a
little force. Take enough molecules, and all those little forces add
up. This is what's called pressure. If the temperature goes up, each
molecule is vibrating faster, so it gives the wall a bigger push each
time it bounces off. You see that as increased pressure.

The nifty thing, is that none of this depends on what kind of gas you've
got. If I've got a container filled with nitrogen at a given
temperature, pressure, and volume, I can replace nitrogen molecules with
carbon dioxide molecules at the same temperature, and the pressure won't
change. The density of the gas will increase, because each individual
CO2 molecule weighs more the N2 molecule it replaced, but the pressure
will stay the same.

The ideal gas law is the underlying principle which goes into computing
density and pressure altitude, turbocharger efficiency, and pretty much
anything that has to do with the physical behavior of gasses in any way.
It is one of the fundamental bedrock foundations on which aerodynamics
is based.