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Homework Help: Diagram of electric potential difference in plate capacitor

  1. Nov 13, 2016 #1
    1. The problem statement, all variables and given/known data
    Plate capacitor is filled with two dielectrics of constants 3 and 4, and with a metal as seen in the picture. If the el. field in the first dielectric is 30 kV/cm, do:

    a) diagram of el. potential difference with calculated characteristic values using point C as a reference point.

    b) voltage UBF

    2. The attempt at a solution

    There's no way φd can be 0 and I don't know any other way of getting to it.
  2. jcsd
  3. Nov 13, 2016 #2


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    Homework Helper
    Gold Member

    What does this tell you about the voltage across the first capacitor?
    What does this tell you about the ratio of two capacitances?
    Edit: I just saw your attempt below the question. It wasn't clearly visible on my phone earlier.
  4. Nov 13, 2016 #3


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    Why? What can you say about the E-field between C and D?
  5. Nov 13, 2016 #4
    Okay, so φd actually is 0. I tried something (see picture underneath), but I'm not sure if it's correct as I don't have correct answers.

  6. Nov 13, 2016 #5


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    Staff: Mentor

    The question says to use the point C as the reference point, so presumably you must set that as the zero reference for potential. That means at the very least you need to shift your graph downwards by 120 kV.

    How do you explain the dip in potential from D to E? Does the field change directions at E? I don't see a charge there that would account for it.
  7. Nov 15, 2016 #6
    I solved it, thank you all for the help!
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