Question

Calculate the Energy Stored in the Electric Flux Density and the Potential Gradient
A capacitor consists of two metal plates each of area 50 cm^2 and spaced 0.2 mm apart in air, connected across a 120 v supply. Calculate the energy stored in the electric flux density and the potential gradient?

Answer 1

Parallel plate capacitor.

A = 50 cm^2 = 5 * 10^-3 m^2
d = 0.2 mm = 2 * 10^-4 m
V = 120 V
1/4πε  = 9 * 10⁹ units
C = capacitance = ε A/ d = 5 * 10⁻³ / [4π * 9 * 10⁹* 2*10⁻⁴]  F
    = 2.21 * 10⁻¹⁰ F

Energy stored in the capacitor after it is fully charged: 
  = U = 1/2 * C * V^2
         = 1/2 * 2.21 * 10⁻¹⁰  * 120²
          = 1.59 * 10⁻⁸ J

Potential gradient = E = V/d = 120/0.0002  = 600,000 Volts/meter