# Diffusion of linearly distributed gas particles

Discussion in 'Physics' started by boks, Feb 14, 2009.

1. ### boks Thread Starter Active Member

Oct 10, 2008
218
0
If the gas particles in a box are uniformly distributed in the y and z directions, and linearly distributed in the x direction, is it true that the concentration won't change with time, according to the diffusion equation? I find this very unintuitive.

2. ### KL7AJ Senior Member

Nov 4, 2008
2,013
273
This is an interesting one. I don't know if you could actually DO this physically. Under longitudinal pressure waves, you could solve this along the X axis using the one dimensional wave equation...but that would involve a SINUSOIDAL, not a LINEAR density distribution. Once the disturbing force that CREATED this distribution is gone, equilibrium would tend to set in, once the initial oscillation is damped.

eric

3. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
Presumably you are studying Ficks law, which states:

'The rate of change of concentration is proportional to the second derivative of concentration with respect to distance.'

The second derivative of any linear function is zero so the rate of change of concentration is zero.
And of course the second derivatives in the directions of constant concentration are also zero.

4. ### KL7AJ Senior Member

Nov 4, 2008
2,013
273

The operative phrase here is "with respect to distance". However, this is an unstable condition with respect to TIME.

eric

5. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
Nope, Ficks law connects distance and time.

'The (time) rate of change of concentration.........(my addition in brackets)

see Fick's second law

http://en.wikipedia.org/wiki/Diffusion_coefficient