Ch5part2

# Ch5part2 - 10/14/11 ANNOUNCEMENTS #1 Reading: Chapter 5,...

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10/14/11 2 Chapter 5 - 1 3 Diffusion J How do we quantify the amount, or rate, of diffusion? J Measured empirically: – Make thin film (membrane) of known surface area A – Impose a concentration gradient ( driving force ) Measure mass M of atoms or molecules diffusing through the membrane in given time t ( )( ) s m kg or s cm mol time area surface diffusing mass) (or moles Flux 2 2 = ! ! J J = M At = 1 A dM dt M = mass diffused time J slope Chapter 5 - 1 4 Diffusion - 2 Cases J Steady-State Diffusion: – Concentration profile IS NOT a function of time J Non-steady State (or Transient ) Diffusion: – Concentration profile IS a function of time
10/14/11 3 Chapter 5 - 1 5 f04_05_pg113 Diffusion - Consider the Following S-S diffusion across thin plate Now let’s plot C vs. distance x Slope = concentration gradient: Conc. grad. = dC/dx Chapter 5 - 1 6 Steady-State Diffusion dx dC D J ! = Fick’s First Law of diffusion C 1 C 2 x C 1 C 2 x 1 x 2 D diffusion coefficient (proportionality constant) Rate of diffusion is independent of time Flux J proportional to concentration gradient = dx dC 1 2 1 2 linear if x x C C x C dx dC ! ! = " " #

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10/14/11 4 Chapter 5 - 1 7 A Few Words About Units If the units of Flux J are (something - e.g. mol or kg)/m 2 s Then the units of concentration C HAVE to be (something)/m 3 and vice versa! We do not care what that “something” is: Cats, albatrosses, people, atoms, moles, kg all fine! The units of
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## This note was uploaded on 11/11/2011 for the course ENGR 231 taught by Professor Olehtretiak during the Fall '10 term at Drexel.

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Ch5part2 - 10/14/11 ANNOUNCEMENTS #1 Reading: Chapter 5,...

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