- #1

Edison Bias

- 105

- 5

I wonder what pressure is when there are no walls.

The reason for me wondering this is that gas pressure actually is defined by change of particle momentum, like

[tex]p=\frac{F}{S}=\frac{1}{S}\frac{dp}{dt}=\frac{1}{S}\frac{d2mv}{dt}[/tex]

So how can there be pressure without change of momentum?

It's like Heisenberg almost because the pressure is only there when you measure it, when the walls are gone there is no pressure.

In my amateur world I would say that this wall-less pressure is made of intermolecular collissions, it has to be because there can be no pressure without collisions.

This equation can be shown to be derived from particles hitting a surface:

[tex]p=\frac{2}{3}nEk_p[/tex]

where Ekp is the particle kinetic energy and n the particle density.

It is a very interesting equation, not to mention the fact that it is derived from particles actually hitting a surface.

It says that pressure actually is proportional to kinetic energy density, it is a fascinating description of pressure which's I have never understood but right now I think I understand because pressure is the sum of all the particle's kinetic energy in a certain volume where the quadratic velocities of the different particles is directly reversely proportional to the mass, due to

[tex]Ek_p=\frac{3}{2}kT[/tex]

While the common gas law may be written

[tex]pV=n'RT=\frac{N}{N_A}RT=N\frac{R}{N_A}T=NkT[/tex]

This is the equation many physisists rely on but remember these things:

1) Normal gases do interact/collide, Dalton's Law is a joke

2) Boyle's law relies on isothermal changes that are very smal with respect to To

3) The thermal expansion koefficient of volume and pressure may not be the same (at a larger scale).

Edison