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Question about parallel plate stuff

  1. May 21, 2016 #1
    1. The problem statement, all variables and given/known data
    A cell membrane consists of an inner and outer wall separated by a distance of approximately 10 nm. Assume that the walls act like a parallel plate capacitor, each with a charge density of 10^−5C/m^2, and the outer wall is positively charged. Although unrealistic, assume that the space between cell walls is filled with air.

    Part A
    What is the magnitude of the electric field between the membranes? Answer: 1*10^6 N/C

    Part B
    What is the magnitude of the force on a K+ ion between the cell walls? Answer: 2*10^-13 N

    Part C
    What is the potential difference between the cell walls? Answer: 1*10^-2 V

    Question : If released from the inner wall, what would be the kinetic energy of a 7 fC charge at the outer wall? 1fC = 10^−15C.

    2. Relevant equations
    KE = q*Delta(v)

    3. The attempt at a solution
    I have solved each of the other parts of this question. I found that Delta V was 1×10^−2V. The question states that 1fC is 10^-15 C. I plugged in for KE = (7*10^-15)(0.01V), and keep getting 7*10^-17. However the answer is 8*10^-17. Can anyone catch my mistake, or am i using the wrong formula?
     
    Last edited: May 21, 2016
  2. jcsd
  3. May 21, 2016 #2

    NascentOxygen

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    Hi johnknee. :welcome:

    There is a figure to accompany this problem, I believe?
     
  4. May 22, 2016 #3

    rude man

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    How did you compute part A? Your answer is low by about 11.3%.
     
  5. May 23, 2016 #4
    For part A, I used the equation E = charge density/Permittivity of free space = 10^-5/(8.85*10^-12) = 1*10^6 N/C
     
  6. May 23, 2016 #5

    NascentOxygen

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    = 1.13×106 N/C
     
  7. May 23, 2016 #6

    rude man

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    Your calculator needs to show more decimal points!
     
  8. May 23, 2016 #7
    Yeah I have many decimal points haha, it's just that the answer given by masteringphysics is in 1 sig fig.
     
  9. May 23, 2016 #8

    rude man

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    Well, yer' not likely to master it with that! LOL
     
  10. May 24, 2016 #9

    haruspex

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    It's ok to round the answer to 1 sig fig to match the precision of the inputs, but you should keep more digits throughout the calculation in between.
     
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