1. Not finding help here? Sign up for a free 30min 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!

What is the maximum charge on plates when dielectric removed

  1. Mar 11, 2016 #1
    1. The problem statement, all variables and given/known data
    A capacitor with two parallel plates of area 88.5 cm2 and nylon (dielectric strength 14 MV/m) inserted between them breaks down when a potential difference of 5.80 kV is applied. What is the maximum charge the plates will hold when the nylon is removed and the space filled with air (dielectric strength 3 MV/m)?

    2. Relevant equations
    Q = CΔV
    ΔV = Ed
    Co = εoA/d
    Coκ = εoAκ/d
    E = Eo/κ


    3. The attempt at a solution
    The maximum electric field with the nylon inserted is 14 MV/m. ΔV = Ed and we have 5800 V applied.
    Solving for d gives d = 0.00041429 m.

    The maximum electric field once the nylon removed is 3 MV/m. ΔV = Ed.
    Solving for ΔV gives ΔV = 1242.87 V.

    C = εoA/d gives C= 1.89*10^-8 F.

    Q = CΔV = (1.89*10^-8 F)(1242.87 V) = 0.0000235 C.

    The online assignment page is rejecting my answer. Where am I going wrong?
     
    Last edited: Mar 11, 2016
  2. jcsd
  3. Mar 11, 2016 #2

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Welcome to PF!

    Did you correctly convert cm2 to m2?

    Why did 10-8 change to 10-12 for C in the calculation of Q?
     
  4. Mar 11, 2016 #3
    Sorry the 10^-12 was a typo. Indeed when I did the calculation I used 10^-8 for C. Also I converted cm^2 to m^2 and converted MV and kV to volts.
     
  5. Mar 11, 2016 #4

    TSny

    User Avatar
    Homework Helper
    Gold Member

    What did you get for the area after you converted to m2?
     
  6. Mar 11, 2016 #5
    88.5 cm^2 = 0.885 m^2
     
  7. Mar 11, 2016 #6

    TSny

    User Avatar
    Homework Helper
    Gold Member

    This is not correct. Note that you are converting square centimeters.
     
  8. Mar 11, 2016 #7
    Wow. That flew right over my head. That was my problem. Thanks so much for your help!
     
  9. Mar 11, 2016 #8

    TSny

    User Avatar
    Homework Helper
    Gold Member

    Good work.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted