Electric displacement field

Summary

The electric displacement field $D$ is the vector field $D = \varepsilon_0 E + P$, built from the electric field $E$ and the polarization density $P$. Its divergence is the density of free charge only, with the bound charge of the material removed. This makes Gauss's law in matter depend only on the charges one controls. In a linear medium $D$ is proportional to $E$, and the factor is the permittivity.