Step 1: Capacitance of the capacitor(C)
Capacitance of parallel plate capacitor is given by:
C=∈0Ad ....(1)
Step 2: Electrostatic energy stored by the capacitor(U)
U=12CV2=12A∈0dV2 ....(2)
=12×90×10−4 m2×8.85×10−12×(400V)2(2.5×10−3 m)
=2.55×10−6J
Step 3: Energy per unit volume(u)
Energy per unit volume : u=UV′=UAd ....(3)
where V′= Volume of the capacitor
∴u=2.55×10−6J90×10−4 m2×2.5×10−3 m=0.113Jm−3
Step 4: Relation between u and electric field (E)
From eq (2) and (3)
u=UV′=12A∈0dV2Ad
⇒u=12∈0(Vd)2
Vd= Electric field ntensity =E
∴u=12∈0E2