Make the following substitutions for
**E** and
**B**:

E |
= | (3.9) | |

B |
= | (3.10) |

These substitutions satisfy the Faraday law and the

The Gauss' law and the Ampère-Maxwell law are satisfied assuming
that:

Equation (3.11) is called the

Electro- and Magento-static systems are described by Poisson equation,
i.e., a Laplace equation with a nonzero right hand side. The right
hand sides of equations (3.12) and (3.13) are
referred to as *sources*.