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

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10/14/11 1 Chapter 5 - 1 1 ANNOUNCEMENTS #1 Reading: Chapter 5, all sections except section 5.6 Week 5 Recitations: Quiz #2 on Chapter 4 material. Return and review Midterm 1 Chapter 5 Problems: 5.3, 5.7, 5.13, 5.16, 5.22, 5.25 Chapter 12 Problems: 12.2, 12.3, 12.5, 12.15, 12.19 Need to read ahead for Chapter 12 topic. Chapter 5 - 1 2 ANNOUNCEMENTS - 2 ± Midterm #1: Average: TBD; Hi: TBD; Lo: TBD Return & review in Week 5 recitations Remember RI, TA office hours as help opportunities If you have Qs about how your midterm was graded, or find that the points were added up incorrectly, here is what to do: Append a note (indicate your Name , Section # etc.,) to your exam indicating the issue (e.g. “Incorrect total score”, or “Please re-grade Q1”) and return the exam & note to your RI Your Q will then be referred to the appropriate grader to review, revised if necessary, revised score recorded, and then returned in recitation the following week In each case, the grader will re-grade the entire page/question , so, depending on what they find, the score could go up, or down oversights can go either way ;-)
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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
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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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