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Electric field magnitude between two charged disks

  1. Aug 28, 2012 #1
    1. The problem statement, all variables and given/known data

    Two 10-cm-diameter charged disks face each other, 17 cm apart. The left disk is charged to +40 nC and the right disk is charged to -40 nC.

    a)What is the electric field, both magnitude and direction, at the midpoint between the two disks?
    b) What is the force on a -1.0 nC charge placed at the midpoint?

    So I have been trying this problem for hours now and am already maxed out on tries for a. I know how to do b but need the answer for a to get b.

    Up until now I have been using E=(charge density of disk)/2(epsilon naught) however I am thinking now that this equation is only used for finding the electric field of an infinitely charged plane. So what equation am I supposed to use instead?

    Actively idle,

    Matt
     
  2. jcsd
  3. Aug 28, 2012 #2

    TSny

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    Homework Helper
    Gold Member

    Hello, Matt.
    That's right. You'll need the equation for a finite-sized disk.
    This is a standard problem that requires integration. If you are using one of the standard textbooks, check to see if it's worked out there. Otherwise, we can guide you a bit in working it through.
     
  4. Aug 29, 2012 #3
    okay I did some reading and found E=η/2ε(1-(z/sqrt(z^2-R^2)))

    now tell me if any of these assumptions are wrong
    z=the distance of point charge from plate, so .085m
    R=radius disk, so .05m
    η=density charge, 5.092E-6 N/m^2

    i solved using those values and still didnt come out right. do i need to multiply the E by 2 because of the two discs?
     
  5. Aug 29, 2012 #4
    The total electric field is the sum of all the individuals. Since you have a parallel plate capacitor with a positive charge (outward field) on one plate and a negative charge (inward field) on the other plate what do you think the field will look like exactly in between the two?
     
  6. Aug 29, 2012 #5
    Thanks I ended up getting it. Turns out I suck at working a calculator.
     
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