A parallel-plate capacitor is formed with the bottom plate at z = 0, an insulati

Question

A parallel-plate capacitor is formed with the bottom plate at z = 0, an insulating material
?1 in the region 0 < z < d1, a second insulating material ?2 in the region d1 < z < d1 + d2, and a
top plate at z = d1 + d2.
a. Draw a diagram of the of the physical con?guration.
b. Express the capacitance C in terms of the area A, d1, d2, ?1, and ?2.
c. Let ?1 = 3?0, ?2 = 5?0, d1 = 0.3 mm, d2 = 0.3 mm, and A = 20 mm2
. The voltage across the
capacitor is 300 V . Find the total stored electric energy in the capacitor.

Explanation / Answer

(a) Electric field strength (E) = 145 V/m Now formula for the energy density is u = (1/2)eoE2 = (1/2)(8.85*10-12C2/N.m2)(145V/m)2 = 9.30*10-8 J/m3 (b) Formula for the charge stored on the parallel plate capacitor is Q = AEeo = (0.112m2)(145V/m)(8.85*10-12C2/N.m2) = 143.72*10-12C

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