Electric flux density is defined as the number of tubes of force per unit area emanated normally from the surface. Eclectic flux density s represented by D and is measured in coulmbs/m2.

Let there be a  point charge of Q coulombs placed at the centre of an imaginary sphere of radius r metres. The number of lines of force per unit area of the surface of the sphere,

D =  Q
    4πr²

If the surrounding medium has a relative permittivity ε_r then the electric field intensity normal to the sphere surface is       Q       where ε_o is the permittivity of evacuated space or air.
                      4πε_oε_r r²
                 

Hence     D = ε_oε_r E

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