[orgo814]

Calculate the diffusion constant of nitrogen at 25 Celsius and 10 Pa. If a pressure gradient of 0.20 bar m^-1 is established in a pipe, what is the flow of gas due to diffusion?

I won't go through all the work but I calculated the diffusion constant, D. The flux due to diffusion is -D dC/dZ which is further derived to -D/RT dP/dZ. When they do the calculation for this problem they drop the negative sign so that it ends up being D/RT x Pressure Gradient. Why do they drop the negative sign?

[mjc123]

Perhaps because they are simply interested in the magnitude of the flow - the direction is obvious, it flows from high to low pressure. Or maybe they define "pressure gradient" as -dP/dz, so the statement "a pressure gradient of 0.20 bar m^-1 is established in a pipe" means that as you move 1m along the pipe (note direction not specified!) the pressure falls by 0.2 bar.

[orgo814]

The second argument you made was what I was thinking... Maybe it's implied in the term pressure gradient.

[orgo814]

Well actually I'm still confused cause negative sign is present in original definition before we derive it to dP/dZ. I know that the negative sign indicates flow from high to low pressure but how do we just exclude it?

[mjc123]

Wilful blindness

[orgo814]

?