Gauss's Law
Gauss's law, one of Maxwell's equations, gives the relation between the electric or gravitational flux flowing out a closed surface and, respectively, the Electric Charge
or mass
enclosed in the surface. It is applicable whenever the inverse-square law holds, the most prominent examples being electrostatics and Newtonian gravitation.
If the system in question lacks symmetry, then Gauss's law is inapplicable, and integration using Coulomb's law
is necessary.
In its integral form, Gauss's law is
where
is electric flux,
is some closed surface with outward normals,
is the Electric Field,
is a differential area element,
is the permittivity of free space,
is the charge enclosed by
, and
is the volume
enclosed by
.
In its differential form, Gauss's law is
where
is the divergence
operator, and
is the charge density.
When dielectrics or other polarizable media enter the system, we must modify Gauss's law accordingly. However, we rescind the mathematical perfection of the above formulation of Gauss's law in favor of a more accurate approximation of the real world.
Polarizable media can contain two types of charge - free and bound. Free charge can move around, while bound charge results from the induced dipoles within the dielectric. Replacing the electric field with the electric displacement field, and the charge density with, specifically, the free charge density, we have a new form of Gauss's Law:
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