A vector quantity that allows Gauss's law to be written in terms of free charge 
alone as
in a dielectric having a bound charge
where P is the electric polarization.D is sometimes called the electric displacement.
In cgs, D is given by
where
is the electric susceptibility and 
is the dielectric constant (Purcell 1985, p. 381). In MKS,
where
is the permittivity of free space and 
is the dielectric constant.
It should be emphasized that unlike
for electrostatic fields,
is not, in general, equal to
. Therefore, it is not possible to write D as the gradient of a scalar potential. However, in cases of spherical, infinite cylindrical, or planar symmetry, it turns out that 
(Griffiths 1987, p. 156).
alone as (1)
in a dielectric having a bound charge
(2)
where P is the electric polarization.D is sometimes called the electric displacement.
In cgs, D is given by
(3)
(4)
(5)
where
is the electric susceptibility and is the dielectric constant (Purcell 1985, p. 381). In MKS,(6)
(7)
(8)
where
is the permittivity of free space and is the dielectric constant.It should be emphasized that unlike
for electrostatic fields, (9)
is not, in general, equal to
. Therefore, it is not possible to write D as the gradient of a scalar potential. However, in cases of spherical, infinite cylindrical, or planar symmetry, it turns out that (Griffiths 1987, p. 156). 
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