- #1
rogerk8
- 288
- 1
Hi!
I wonder what gas pressure really is.
This I have learned from Wikipedia:
1) Pressure is a scalar (which means it has no direction).
2) Since a system under pressure has potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume.
3) Gauge Pressure is relative to normal air pressure (atmospheric pressure).
4) In a static gas, the gas as a whole does not appear to move. The individual molecules of the gas, however, are in constant random motion.
The most fascinating quality I think is 2).
A more seldom use for pressure unit is J/m^3 (even though the SI-unit is N/m^2).
I think this fact is really interesting.
A gas has an energy density.
Within one cubic meter it has Joule as unit.
So more specific, how can a unit volume of gas contain pure energy?
And what is this energy, really?
Yes, we could say that the gas has the "potential of perfoming work" (like in the Otto engine f.i)
Yes, we know that.
But what is the mechanism?
We know that the internal Ek is around
[tex]Ek=\frac{mv^2}{2}\propto kT [J][/tex]
So speed of the molecules are closely related to temperature.
This is one property.
But how about the other called pressure
[tex]p=nkT [J/m^3][/tex]
where n is the particle density.
This just says that pressure is related to density and temperature.
But what is pressure, really?
If we can say that speed of the particles is closely related to temperature according to the first formula, what is then pressure?
If it's not speed it has to be something other.
And I simply don't understand what.
A mental experiment I did not so long ago made me come to this conclusion:
[tex]P=\rho_s\frac{dv}{dt} [N/m^2][/tex]
where rho simply is the surface density of the gas.
This is nothing other than the use of Newton's first law applied on a gas.
So a gas has the ability to generate a force on f.i a window by "rushing" to it with a certain surface density (and acceleration).
This along with 2) above gives me some kind of clue of what gas pressure is.
The only enigma here is the (de)acceleration of the surface density.
Because how can we know that?
How far from the truth am I?
Roger
I wonder what gas pressure really is.
This I have learned from Wikipedia:
1) Pressure is a scalar (which means it has no direction).
2) Since a system under pressure has potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume.
3) Gauge Pressure is relative to normal air pressure (atmospheric pressure).
4) In a static gas, the gas as a whole does not appear to move. The individual molecules of the gas, however, are in constant random motion.
The most fascinating quality I think is 2).
A more seldom use for pressure unit is J/m^3 (even though the SI-unit is N/m^2).
I think this fact is really interesting.
A gas has an energy density.
Within one cubic meter it has Joule as unit.
So more specific, how can a unit volume of gas contain pure energy?
And what is this energy, really?
Yes, we could say that the gas has the "potential of perfoming work" (like in the Otto engine f.i)
Yes, we know that.
But what is the mechanism?
We know that the internal Ek is around
[tex]Ek=\frac{mv^2}{2}\propto kT [J][/tex]
So speed of the molecules are closely related to temperature.
This is one property.
But how about the other called pressure
[tex]p=nkT [J/m^3][/tex]
where n is the particle density.
This just says that pressure is related to density and temperature.
But what is pressure, really?
If we can say that speed of the particles is closely related to temperature according to the first formula, what is then pressure?
If it's not speed it has to be something other.
And I simply don't understand what.
A mental experiment I did not so long ago made me come to this conclusion:
[tex]P=\rho_s\frac{dv}{dt} [N/m^2][/tex]
where rho simply is the surface density of the gas.
This is nothing other than the use of Newton's first law applied on a gas.
So a gas has the ability to generate a force on f.i a window by "rushing" to it with a certain surface density (and acceleration).
This along with 2) above gives me some kind of clue of what gas pressure is.
The only enigma here is the (de)acceleration of the surface density.
Because how can we know that?
How far from the truth am I?
Roger