A question in thermodynamics mass and volume flow rates

[ehabmozart]

Hello everyone,

I have a small question regarding mass and volume flow rates. It is shown in my book that there is a difference between path functions and point functions. I can understand why work or heat for example are path functions. However, I don't understand why mass transfer is also a path function. Specifically, the following equation is my problem:

-------------- δ[PLAIN]http://upload.wikimedia.org/math/2/0/5/2056619d3cf80e2d000509b6110c57ec.png= ρV(normal)dA -----
They followed by discussing that for a given radius, the Area is known but is not the case for the integral of δm and we cannot say [PLAIN]http://upload.wikimedia.org/math/2/0/5/2056619d3cf80e2d000509b6110c57ec.png2- [PLAIN]http://upload.wikimedia.org/math/2/0/5/2056619d3cf80e2d000509b6110c57ec.png1... I want to understand why is it so. We can clearly define the mass flow at a particular position. Can't we?

Thanks to whoever contributes.

 

[timthereaper]

Thermodynamic mass transfer usually happens by diffusion or convection. The idea is that mass is moving from areas of high concentration to low concentration, similar to how heat transfers from high temperature to low temperature. Given a state of the system of where the mass is, there is no way to say how it got to that state (or how much work was done to get it into that state). That's why it's a path function.