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1 last infinite series, power series

  1. Sep 21, 2010 #1
    1. The problem statement, all variables and given/known data

    suppose a large number of particles are bouncing back and forth between x=0 and x=1, except that at each endpoint some escape. Let r be the fraction of particles reflected, so then you can assume (1-r) is the number of particles that escape at each wall. Suppose particles start at x=0 and head towards x=1; eventually all particles escape. Write and infinite series for the fraction at which escape at x=1 and x=0. Sum both series. What is the largest fraction of the particles which can escape at x=o



    2. Relevant equations

    sn-rsn = a(1-r^n)/(1-r)

    0<r<1

    3. The attempt at a solution


    x=1 (1-r) + (1-r)^2... (1-r)^n

    and same for x=0

    sum

    2(1-r) + 2(1-r)^2... + 2(1-r)^n
     
  2. jcsd
  3. Sep 21, 2010 #2

    gabbagabbahey

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    You need to thinkl this through more carefully... the first impact is at x=1, what fraction of the original number of particles escapes? what fraction of the original number of particles doesn't? The next impact is at x=0 (not x=1) the first impact is at x=1, what fraction of the original number of particles escapes? what fraction of the original number of particles doesn't? ...and so on
     
  4. Sep 21, 2010 #3
    at x=1 (1-r) escape and r is reflected... at x=0 (1-2r) escape and r/2 reflected? honestly this is supposed to be the easiest stuff, yet i have no problem with the apparent hard stuff...
     
  5. Sep 21, 2010 #4

    gabbagabbahey

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    Not quite...the point is that the first impact with x=0 happens after the first impact with x=1. If there is a fraction [itex]r[/itex] left after the 1st impact with x=1 and a fraction (1-r) of those espaces, doesn't that mean that [itex]r(1-r)[/itex] escape the 1st impact at x=0, and [itex]r^2[/itex] are reflected?
     
  6. Sep 21, 2010 #5
    i did write that initially on paper, but i couldn't see it working. but i guess i could try again

    so r^n(1-r) or (r^n - r^(n+1))
     
  7. Sep 21, 2010 #6

    gabbagabbahey

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    I suggest making a table for the first few impacts at x=0 and at x=1, with the fraction (of the intial number of particles) that is reflected and the fraction that escapes.
     
  8. Sep 21, 2010 #7
    is r changing with each term, or is it a constant fraction?
     
  9. Sep 22, 2010 #8

    gabbagabbahey

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    From the way the question is written, it is constant
     
  10. Sep 22, 2010 #9
    if that is true then

    x=1 would be (1-r) + r^2(r-1) +... r^2n(r-1)

    x=0 would be r(1-r) + r^3(r-1) +... r^(2n+1)(r-1)

    so added together you get

    (1-r) + r(1-r) + r^2(1-r) +... r^n(1-r)

    Sn = ((1-r)(1-r^n))/(1+r)

    well not sure about ^^^ because someone said that the max electrons leaving at x=1 as n approaches infinity is 1/2
     
  11. Sep 22, 2010 #10
    can anyone help me real quick, i just want to get my assignment done
     
  12. Sep 22, 2010 #11

    gabbagabbahey

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    That looks more like the number that are reflected....I thought you were supposed to find the fraction that escape
     
  13. Sep 22, 2010 #12
    aww crap, that is what i thought...


    but what i thought i was doing initially was, taking the reflected particles and subtracting another fraction of what escapes by finding a new reflected amount say r^2 are reflected from r
     
  14. Sep 23, 2010 #13

    gabbagabbahey

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    Hint: The total fraction of particles that escape at each side will be the fraction that impact there initially minus the total fraction that are reflected off that side
     
  15. Sep 25, 2010 #14
    well w.e it is to late anyway, i had to hand in an incomplete assignment...
     
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