1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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,

  2. jcsd
  3. Aug 28, 2012 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook