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Homework Help: Sheet with Hole

  1. Sep 15, 2013 #1
    What is the electric field at a point P, a distance h = 29.9 cm above an infinite sheet of charge, with a charge distribution of 2.29 C/m^2 and a hole of radius r = 4.49 cm with P directly above the center of the hole, as shown in the figure?

    Equations used:
    Edisk= (-σ/2ε)[1-(z/sqrt(z^2 + r^2))]
    Esheet= +σ/2ε
    Enet= ƩE

    I converted all the cm into m and plugged the numbers in. I got 3.64e10 N/C for the disk and 1.30e11 N/C for the sheet. I added the two together but my answer was wrong so i tried subtracting and still didn't get the right answer. I thought adding them would've been correct. Am I missing something?
  2. jcsd
  3. Sep 15, 2013 #2


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    Gold Member

    Yes, adding your expressions together is correct. You are essentially considering the E field at the point P due to the sheet with surface charge density +σ and subtracting the E field due to the imaginary disk of negative surface charge density -σ.

    Alternatively, just subtract the E field due to a disk from the E field due to the plane.
    In your end expression, take the limit z>>R, and see if your result makes sense.
  4. Sep 15, 2013 #3


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    2017 Award

    Staff: Mentor

    Disk and sheet should have opposite signs (as removing stuff from a sheet reduces the electric field), so your formulas look right and one of your resulting values should have a minus sign. Add both, and you get a value smaller than for the sheet alone.

    Did you check your values with WolframAlpha, Matlab or something similar?
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