The *electric flux density* **D** is given by

**div D**=ρ_{f}

where ρ

_{f} is the volume density of

*free* charges.

**D** is related to the electric field **E** and the polarization **P** via

**D**=ε_{O}**E**+**P**

The time

derivative of

**D** is known as the

**displacement current** which appears in one of

Maxwell's equations.

**curl H** = **J** + d**D**/dt

The equivalent magnetic quantity to **D** is the magnetic flux density, **B**