- #1
daviddeakin
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Consider a thin parallel plate capacitor of an area of of 2.0x10^-4 metres squared, with
a thickness of 1.2x10^-4 metres. The top and bottom thirds of the capacitor are filled with a dielectric material with a relative dielectric permittivity εr1=4, and the central third, with another material with εr2=8 (so it's a triple layer sandwich) .The voltage on the capacitor is 3V.
Calculate the electric flux densities and the electric field magnitudes in all three parts of the device. State any assumptions used.
Answer:
We can treat this as three capacitors in series, each with a plate separation of 4x10^-5 m. The upper and lower capacitors are:
C=εA/d = 4 x 8.85x10^-12 / 4x10^-5 = 117pF
The middle capacitor is:
C=εA/d = 8 x 8.85x10^-12 / 4x10^-5 = 354pF
But I don't know how to complete the question. For a single capacitor I would find the charge: Q=CV, then find the electric field from: E = D/ε = Q/Aε
But in this case I don't know what the voltage on each capacitor is. Is it simply one third of the voltage across the whole device? Also, I must be going about this in the wrong way, because the NEXT part of the question is:
Calculate the charge on the plates and the capacitance of the device.
The total capacitance is clearly:
C = [1/C1 + 1/C2 + 1/C3]^-1 = 50.2pF
But the way I'm attempting it already gives the charge before the second part of the question!
(Also, is there a way to do superscripts on this forum?)
a thickness of 1.2x10^-4 metres. The top and bottom thirds of the capacitor are filled with a dielectric material with a relative dielectric permittivity εr1=4, and the central third, with another material with εr2=8 (so it's a triple layer sandwich) .The voltage on the capacitor is 3V.
Calculate the electric flux densities and the electric field magnitudes in all three parts of the device. State any assumptions used.
Answer:
We can treat this as three capacitors in series, each with a plate separation of 4x10^-5 m. The upper and lower capacitors are:
C=εA/d = 4 x 8.85x10^-12 / 4x10^-5 = 117pF
The middle capacitor is:
C=εA/d = 8 x 8.85x10^-12 / 4x10^-5 = 354pF
But I don't know how to complete the question. For a single capacitor I would find the charge: Q=CV, then find the electric field from: E = D/ε = Q/Aε
But in this case I don't know what the voltage on each capacitor is. Is it simply one third of the voltage across the whole device? Also, I must be going about this in the wrong way, because the NEXT part of the question is:
Calculate the charge on the plates and the capacitance of the device.
The total capacitance is clearly:
C = [1/C1 + 1/C2 + 1/C3]^-1 = 50.2pF
But the way I'm attempting it already gives the charge before the second part of the question!
(Also, is there a way to do superscripts on this forum?)