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Diagram of electric potential difference in plate capacitor

1. Homework Statement
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
scan0008.jpg


There's no way φd can be 0 and I don't know any other way of getting to it.
 

cnh1995

Homework Helper
Gold Member
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If the el. field in the first dielectric is 30 kV/cm,
What does this tell you about the voltage across the first capacitor?
two dielectrics of constants 3 and 4,
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.
 

cnh1995

Homework Helper
Gold Member
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Why? What can you say about the E-field between C and D?
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.

scan0009.jpg
 

gneill

Mentor
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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.
 
I solved it, thank you all for the help!
upload_2016-11-16_1-38-17.png
 

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