Suppose there is a parallel plate capacitor of area $S$ and separation $d$. Two different dielectrics fill-up the separation between the capacitors as shown below: 
; where $\epsilon_1=2$ and $\epsilon_2=4$ are relative permittivities.
To find its capacitance one can break up this capacitor in two ways: 
The respective circuit diagrams:
The first break-up gives capacitance $C = \frac{12\epsilon_0 S}{5d}$, whereas the the second break-up gives $C = \frac{7\epsilon_0 S}{3d}$. I do not understand which break-up is correct and which one is wrong and why?
Answer
When you have a dividing line which is between two dielectrics parallel to the plates you have to ask yourself; is the dividing line an equipotential?
That is relatively easy for diagram B as there is no change of dielectric on either side of the dividing line.
In diagram A there is a change of dielectric on either side of the dividing line and so in all probability there is a change in the potential along that line.
To check what I have written use diagram B find the the potential between the two dielectric.
Now imagine continuing that dividing line into the bottom dielectric and find the potential along that line.
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