Gauss' law (electrostatics)

In physics, more specifically in electrostatics, Gauss' law is a theorem concerning a surface integral of an electric field E. In vacuum Gauss' law takes the form:
 * $$ \iint_{\mathrm{closed}\atop\mathrm{surface}} \mathbf{E}(\mathbf{r})\cdot d \mathbf{S} =kQ_\mathrm{tot}, $$

with k = 1/&epsilon;0 in SI units and k = 4&pi; in Gaussian units. The vector dS has length dS, the area of an infinitesimal surface element on the closed surface, and direction perpendicular to the surface element dS, pointing outward. The vector E(r) is the electric field at the position dS, the dot indicates a dot product between the vectors E and dS. The double integral is over a closed surface that envelops a total electric charge Qtot, which may be the sum over one or more point charges, or an integral over a charge distribution. If the closed surface does not envelop any charge then the integral is zero. The constant &epsilon;0 is the electric constant.

The law is called after the German mathematician Carl Friedrich Gauss.

Application to spherical symmetric charge distribution
Gauss' law can be applied to compute the electric field due to a spherical-symmetric charge distribution. A point charge is a spherical-symmetric charge distribution. A charged, conducting, spherical shell bounded by radii r0 < r1, with its charge distributed homogeneously over the shell, is another well-known example of a spherical symmetric charge distribution.

Take the origin of a spherical polar coordinate system in the center of symmetry of the charge distribution&mdash;the position of the point charge, or the center of the spherical shell, respectively. Because of symmetry, E has a radial component only (parallel to the unit vector er). Moreover, this component does not depend on the polar angles,
 * $$ \mathbf{E} = \Big( E_r(r), \quad E_\theta =0, \quad E_\phi = 0 \Big). $$

Take the surface of a sphere of radius r to integrate over (in the case that we are considering a spherical shell of charge density, r must be larger than the largest radius r1 of the shell); the surface element is
 * $$ d\mathbf{S} =  r^2\, \sin\theta \, d\theta\, d\phi \, \mathbf{e}_r\quad \hbox{and}\quad \mathbf{E}\cdot d \mathbf{S} = E_r(r) r^2 \sin\theta d\theta d\phi $$

Then,
 * $$ \begin{align} k Q_\mathrm{tot} &= E_r(r)\, r^2 \iint_{\mathrm{surface}\atop\mathrm{of\, sphere}} \sin\theta \, d\theta\, d\phi = E_r(r)\, r^2 \int_{0}^{\pi}\int_{0}^{2\pi}  \sin\theta \, d\theta\, d\phi = E_r(r)\, r^2\,4\pi\\ &\Longrightarrow\, E_r(r) = \frac{k Q_\mathrm{tot}}{4\pi r^2 }, \end{align} $$

where r is the distance of the field-point to the origin. In the case of a point charge we have proved here Coulomb's law from Gauss' law. In the case of a charged spherical shell, we find that the electric field outside the shell is such that it seems that the total charge on the shell is concentrated in the center of the shell. Coulomb's law applies to the charge seemingly concentrated in the center of the shell.