How much stored energy do the plates have?

[ikihi]

Homework Statement

Two parallel circular plates with radius 7.0 mm carrying equal magnitude surface charge of Q = 4.618 x 10^-10 C are separated by a distance of d = 1.0 mm (0.001 m). How much stored energy do the plates have?

Homework Equations

A_circle = π r²
C = ∈₀⋅A / d
PE = 1/2 ⋅ Q² / C
PE = 1/2 ⋅ [(Q²⋅d) / (∈₀⋅A)]
∈₀ = 8.85 x 10^^-12

The Attempt at a Solution

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Area = π ⋅ (0.007)²
Area = 1.539 x 10^-2 m²

PE = 1/2 ⋅ [((4.618x10^-10)²⋅0.001) / (8.85x10^-12 ⋅1.539 x 10^-4)]
PE = 7.829 x 10^-8 J

PE ≈ 78 nJ (is this correct?)

 

[gneill]

C = 1/2 ⋅ [((4.618x10-10)2⋅0.001) / (8.85x10-12 ⋅1.539 x 10-4)]
C = 7.829 x 10-8 J

C ≈ 78 nJ (is this correct?)

Other than using the variable C for energy your result looks good.