Outward mechanical force experienced by unit area of the charged surface} = p = ε₀ E² /2
Mechanical force on the area dS = F = (ε₀ E² /2) dS
Suppose the area dS is moved normally through a distance dx against F. Then, the work W = Force x distance = (ε₀ E² /2 ds)dx
Increase in volume of the field = dS dx
Work done in producing unit volume of the field} = ε₀ E² /2
The work done is stored as potential energy in the medium.
. : Energy stored per unit volume of the medium} = ε₀ E² /2
If the conductor lies in a medium of relative permittivity εr,
Energy stored per unit volume the medium} = ε_r ε₀ E² /2

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