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Homework Help: Capacitor Charge Density

  1. Feb 12, 2017 #1
    1. The problem statement, all variables and given/known data
    Two 10-cm-diameter metal disks separated by a 0.64-mm-thick piece of Pyrex glass are charged to a potential difference of 1100 V .
    What is the surface charge density on the disks?
    What is the surface charge density on the glass?

    2. Relevant equations
    charge density=Q/A

    3. The attempt at a solution

    charge density= ((e_0A)/d)*V)/A
    charge density=(((8.85*10^-12)(pi*.05^2)*1100/.00064)/(pi*.05^2)

    I have also tried multiplying the dielectric constant for pyrex (google gave me 4.7) by e_0 and nothing has worked.

    I am not sure what the difference between the surface charge density on the glass vs. the disks would be, or how to find it.
  2. jcsd
  3. Feb 13, 2017 #2


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    2017 Award

    Staff: Mentor

    What do you mean by "nothing has worked"? What were the results you got?
    Where is the formula taking the dielectric constant into account?
    Imagine adding a very small air gap between glass and conductor. What would be the field strength there? How is the difference between that and the field strength in the glass related to the surface charge density of the glass?
  4. Feb 13, 2017 #3
    Hey, so apparently the answer was in microfarads but I was entering it in farads, but thanks for the help!
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