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Parallel Disk Capacitor E field

  1. Jan 27, 2017 #1
    1. The problem statement, all variables and given/known data
    Two 2.4-cm-diameter disks face each other, 1.9 mm apart. They are charged to ± 12 nC .
    What is the electric field strength at the midpoint between the centers of the disks?

    2. Relevant equations
    ppcap5.gif

    3. The attempt at a solution
    Q=12*10^-9
    A= Pi * r^2 = pi*(.012)^2
    Q/A= (12*10^-9)/(pi*(.012)^2)
    E=(Q/A)/epsilon naught (8.85*10^-12)
    E= 3*10^6

    I am not sure where I go wrong, but I have one attempt left and keep getting the same answer

    [Edit] answer: 2.7*10^6 but I would appreciate if someone could explain why
     
    Last edited: Jan 27, 2017
  2. jcsd
  3. Jan 27, 2017 #2

    Student100

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    Are you checking sig figs and units if entering online?
     
  4. Jan 27, 2017 #3
    Hey student100, I am checking sig figs. It tells me to use 2
     
  5. Jan 27, 2017 #4

    Student100

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    So you're entering 3.0?
     
  6. Jan 27, 2017 #5
    Yup. I ended up guessing correctly, in an attempt to figure out the second part of the problem and the correct answer was 2.7*10^6 which I do not understand. Any ideas on how it would work out to be that?
     
  7. Jan 27, 2017 #6

    Student100

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    Only thing that comes to mind is that they wanted you to use the percise formula for disks and not approximate them as infinite sheets of charge. Have you tried that?

    There size compared to distance of seperation leads me to be skeptical of that however. Are all the numbers presented correct?
     
  8. Jan 27, 2017 #7

    TSny

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    Following Student100's advice, you should find that it does make a difference if you don't approximate the disks as infinite. However, I get 2.8 × 106 N/C rather than 2.7 × 106 N/C.
     
  9. Jan 28, 2017 #8
    All numbers presented are correct.
    I am not sure what you both mean by using the precise formula vs. approximating as an infinite sheet.
     
  10. Jan 28, 2017 #9

    TSny

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    If you are taking a "calculus-based" physics course, then a standard example is the electric field of a circular disk.
    http://www.phys.uri.edu/gerhard/PHY204/tsl36.pdf
     
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