Question

A parallel plate capacitor has an electrode area of 1000 mm^2, with a spacing of 0.1 mm between the electrodes. The dielectric between the plates is air with a permittivity of 8.85∗10^-12 F/m. The charge on the capacitor is 100 v. The stored energy in the capacitor is ____________

(a) 44.3 J

(b) 444.3 nJ

(c) 88.6 nJ

(d) 44.3 nJ

I got this question in an internship interview.

This interesting question is from Singly Excited Electric Field Systems in section Basic Concepts in Rotating Machines of Electrical Machines

Answer 1

The correct choice is (d) 44.3 nJ

For explanation: Wfld = 1/2 q^2/C(x)

C(x) = ε0A/x = 8.85∗10^-12∗100∗(10^-3)^2/0.1∗10^-3 = 8.85∗10^-12 F

Wfld = 1/2∗Cv = 1/2∗100^2∗8.85∗10^-12 = 44.3nJ